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Anne's picks of the June literature: Fluvial Geomorphology and Landscape Evolution

ResearchBlogging.orgA post by Anne JeffersonHow do rivers erode bedrock streams, during big floods, and in the presence of groundwater? Laboratory and accidental experiments are providing some cool new insights.

Johnson, J., & Whipple, K. (2010). Evaluating the controls of shear stress, sediment supply, alluvial cover, and channel morphology on experimental bedrock incision rate Journal of Geophysical Research, 115 (F2) DOI: 10.1029/2009JF001335

Take a moment to contemplate the title of this paper…experimental bedrock incision rate….how do you measure something like bedrock incision in an experimental setting? how do you measure it in time scales than can be accomplished in the laboratory? Johnson and Whipple figured out how to do it – building a weak concrete streambed in a flume at the National Center for Earth-surface Dynamics and then conducting a series of experiments to isolate each of the variables. Their study is related to question of the role of loose sediment in controlling the rates of bedrock river erosion. When does sediment act as a “tool” for erosion by banging into the river bed and abrading it, and when does sediment act as a “cover” for the river bed, protecting it from just such abrasion? Do these two effects create a trade-off suggesting that at some optimal level of sediment abundance, erosion rates are maximized? Johnson and Whipple’s experiments showed that erosion rates increased linearly with sediment flux , but decreased linearly with the extent of sediment cover. They also demonstrated that the extent of sediment cover was function of the ratio of sediment flux to sediment transport capacity, although it was sensitive to local topographic roughness. Their experiments also showed some interesting patterns of how bed roughness develops from focused erosion in interconnected topographically low areas (e.g., @colo_kea’s great video of the Skagway River), but that this development was muted by variations in discharge and sediment flux.* Also note that Johnson, Whipple, and L. Sklar have another new paper out, contrasting rates of bedrock incision from snowmelt and flash floods in Utah’s Henry Mountains. That paper is in GSA Bulletin.

Lamb, M., & Fonstad, M. (2010). Rapid formation of a modern bedrock canyon by a single flood event Nature Geoscience, 3 (7), 477-481 DOI: 10.1038/ngeo894

In 2002, a dam overspill in Texas created a 7 m deep, 1 km long gorge in jointed bedrock and this article by Lamb and Fonstad examines the mechanics of gorge formation and the importance of plucking as erosional mechanism. Brian Romans (Clastic Detritus) has written a nice post on this article and how it links to ideas of uniformitarianism and Kyle House posted before and after photos at Pathological Geomorphology.

Pornprommin, A., & Izumi, N. (2010). Inception of stream incision by seepage erosion Journal of Geophysical Research, 115 (F2) DOI: 10.1029/2009JF001369

An experimental study in layered sediment showed that seepage-drive scarp retreat was a function of the discharge per unit area and “a diffusion-like function that describes the incision edge shapes.” That diffusion-like function was then related to the weight of the failure block and hydraulic pressure. This paper potentially has some insights for thinking about landscape evolution in groundwater-rich areas (like I tend to do) and for those interested in slope stability analyses.*

Anne's picks of the June literature: Watershed Hydrology

ResearchBlogging.orgIt starts when when a water molecule in precipitation lands on the ground, and it ends when that same water molecule leaves the watershed as streamflow. In between, that molecule may move over the land surface, through the soil in big holes (macropores) or in tiny spaces between grains in the soil, through the bedrock as groundwater, or any combination of those pathways. How long it takes for the water molecule to make its journey, what hydrologists call the transit time, depends on the flow paths that it takes. And that transit time, in turn, affects biogeochemical processing and contaminant persistence. Inversely, if hydrologists can measure the distribution of transit times for a particular watershed, they can infer things about the storage, flowpaths, and sources of water in the watershed. Thus, transit time distributions help us peek into the hidden inner workings of the watershed….if we understand what we are really measuring and what those measurements are really telling us. And that topic is one of lots of active research in the community of watershed hydrologists, and its the subject of a number of recently published papers.

In what seems to be an annual tradition, Hydrological Processes has devoted their June issue to topics relating to catchment hydrology and flowpath tracers. This year, the focus is Preferential Flowpaths and Residence Time Distributions and it’s edited by Keith Beven. It’s the sort of issue that makes me want to go over to the library stacks and spend the day in a comfy chair reading and enjoying the journal from cover to cover. While all of the articles in this special issue make my pulse race a little, here are a couple that really strike my fancy:

McDonnell, J., McGuire, K., Aggarwal, P., Beven, K., Biondi, D., Destouni, G., Dunn, S., James, A., Kirchner, J., Kraft, P., Lyon, S., Maloszewski, P., Newman, B., Pfister, L., Rinaldo, A., Rodhe, A., Sayama, T., Seibert, J., Solomon, K., Soulsby, C., Stewart, M., Tetzlaff, D., Tobin, C., Troch, P., Weiler, M., Western, A., Wörman, A., & Wrede, S. (2010). How old is streamwater? Open questions in catchment transit time conceptualization, modelling and analysis Hydrological Processes, 24 (12), 1745-1754 DOI: 10.1002/hyp.7796

In this invited commentary, McDonnell and 28 colleagues lay out the definition of transit time and the current limits of our understanding on its controls in watersheds and its relationship to hydrograph characteristics, groundwater, and biogeochemical processing. They then provide their research vision for pushing past these limits, through a combination of field research and advances in modeling.

Kirchner, J., Tetzlaff, D., & Soulsby, C. (2010). Comparing chloride and water isotopes as hydrological tracers in two Scottish catchments Hydrological Processes, 24 (12), 1631-1645 DOI: 10.1002/hyp.7676

Oxygen isotopes of water and chloride concentrations have been widely used to estimate watershed travel times. They are generally regarded as conservative tracers, but they are not perfect. Here Kirchner et al. compare the time series of the two tracers for a pair of Scottish catchments and show that while both tracers exhibit strongly damped signals relative to precipitation, the travel times calculated using oxygen isotopes were 2-3 times longer than for chloride. So it seems that both tracers are telling us similar things about the ways that catchments move and store water, but that quantitative estimates of travel time are going to be tricky to compare across tracers.

Stewart, M., Morgenstern, U., & McDonnell, J. (2010). Truncation of stream residence time: how the use of stable isotopes has skewed our concept of streamwater age and origin Hydrological Processes, 24 (12), 1646-1659 DOI: 10.1002/hyp.7576

The stable isotopes of water have a shelf life of about 5 years or less. It’s not that they break down (they are stable isotopes, after all); it’s that seasonal input signals get damped over time, so that ages greater than 5 years can’t be resolved. In contrast, tritium (the unstable isotope of hydrogen) has a half life of ~12.4 years. A few decades ago, water ages were estimated using tritium, which conveniently had a bomb peak that made a handy marker of recharge in the early 1960s. These days, water ages are usually estimated by the stable isotopes alone. In this paper, Stewart et al suggest that we are missing part of the story when we use just stable isotopes, because we effectively discount any contributions from water >5 years since it feel from the sky. Incidentally, those contributions that we have been neglecting? That’s the bedrock groundwater and it might be quite important to explaining the behavior of streams. Stewart et al. suggest that we return to embracing tritium as part of a “dual isotope framework” so that we can more accurately quantify groundwater contributions to streamflow. The issue of the shape of travel time distributions (are they exponential or fractal?) is explored in more detail in a paper by Godsey et al. in the same issue and Soulsby et al. explore how relationships between transit times and hydrograph and watershed characteristics might be used to estimate streamflows in data-sparse mountain watersheds.

Anne's picks of the June literature: Humans as Agents of Hydrologic Change

ResearchBlogging.orgHow the world’s biggest river basins are going to respond to mid-century climate change…and how large reservoirs affect our measurements of global sea level rise.

Immerzeel, W., van Beek, L., & Bierkens, M. (2010). Climate Change Will Affect the Asian Water Towers Science, 328 (5984), 1382-1385 DOI: 10.1126/science.1183188

Where do 1 in 4 people live? Where do those people get their water? 1.4 billion people live in five river basins (Indus, Ganges, Brahmaputra, Yangtze, and Yellow) and those mighty rivers source some of their water in the Himalayas, where on-going climate change will have a big impact on glacier melt and seasonal precipitation. In this paper, Immerzeel and colleagues used the SRM hydrologic model and GCM outputs to simulate the years 2046-2065 under two different glacier extent scenarios, a “best-guess” and an extreme case where all glacier cover had disappeared. The five basins all behaved quite differently from each other, because each basin has a different topographic distribution. The Brahmaputra and Indus have the highest percent of glacier-covered area, and these two rivers will be the most severely impacted by projected climate change via decreases in late spring and summer streamflow, as reduced glacier melt is only partially offset by increased spring rains. Between these two basins, the authors estimate that the hydrologic changes will reduce the number of people who can be fed by 60 million people! On the other hand, basins with less reliance on meltwater will not be as bad off – in fact, the Yellow River is likely to experience an increase in spring streamflow and may be able to feed 3 million more people. To me this paper emphasizes the fact that the consequences of climate change are not going to be evenly dispensed across the world’s population and that we’ve really got an urgent task of figuring out how regional climate changes will cascade through hydrology, ecology, food security, disease, and almost every other aspect of the world on which we depend.

Fiedler, J., & Conrad, C. (2010). Spatial variability of sea level rise due to water impoundment behind dams Geophysical Research Letters, 37 (12) DOI: 10.1029/2010GL043462

Global reservoirs trap ~10,800 cubic kilometers of water – enough volume to reduce sea level by ~30 mm. But when large reservoirs are filled, the water weight locally depresses the Earth’s surface and increases local relative sea level. Thus, tide gages that are close to large reservoirs don’t record the true sea level effects of water impoundment – instead recording only about 60% of the true drop. This creates an added wrinkle in the estimation of global sea level rise over the last century, and Fiedler and Conrad compute that these reservoir effects on the geoid have caused an ~10% over-estimation in rates of sea level rise. The largest effects on sea level rise records are places where tide gages are near big reservoirs – like the east coast of North America. *

* Please note that I can’t read the full article of AGU publications (including WRR, JGR, and GRL) until July 2010 or the print issue arrives in my institution’s library. Summaries of those articles are based on the abstract only.

New publication: Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA

How does a landscape go from looking like this…

<2000 year old landscape on basaltic lava with no surface drainage

~1500 year old basaltic lava landscape with no surface drainage

to looking like this?

2 Million year old landscape on basaltic lava

2 Million year old landscape on basaltic lava. Note steep slopes and incised valleys

Find out in my new paper in Earth Surface Processes and Landforms.

Hint: Using a chronosequence of watersheds in the Oregon Cascades, we argue that the rates and processes of landscape evolution are driven by whether the water sinks into the lava flows and moves slowly toward springs with steady hydrographs or whether the water moves quickly through the shallow subsurface and creates streams with flashy hydrographs. Further, we suggest that this water routing is controlled by an elusive landscape-scale permeability which decreases over time as processes like chemical weathering create soil and clog up pores in the rock. And as a bonus, because of the high initial permeability of basaltic landscapes, the formation of stream networks and the dissection of the landscape appears to take far longer than in places with less permeable lithologies.

Jefferson, A., Grant, G., Lewis, S., & Lancaster, S. (2010). Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA Earth Surface Processes and Landforms, 35 (7), 803-816 DOI: 10.1002/esp.1976

Urban streams with green walls

ResearchBlogging.orgWill Dalen Rice and a friendNote: This post is a collaborative effort by Anne and guest blogger Will Dalen Rice, a graduate student in the Department of Geography and Earth Sciences at UNC Charlotte. He had the misfortune of taking a couple of courses from Anne this semester and has become a certified stream junkie, going out on rainy nights to see how high Charlotte’s urban streams are running.

Most cities were started around the idea of available surface water resources. Development and misuse of our streams (ex: “dilution is the solution to pollution”) has resulted in the modern urban stream. These streams are straight and good at carrying storm water, full of sediment and pollutants, and they lack good habitat for plants and animals. Now that we are beginning to notice how degraded and trashed these city waterways are though, scientists and engineers are beginning to attempt to address the form and function of these waterways to hopefully return them to a more “natural” (or at least aesthetically pleasing) state. While there are many stream restoration techniques, they often involve mechanical manipulation of the stream channel and banks and the planting of riparian plants along the stream corridor. As the streamside ecosystem redevelops, the idea is that health of the stream will also improve (leave it to nature to clean up our messes, given the chance).

For large urban streams, the standard practices in stream and habitat restoration are sometimes not possible, often because decades of infrastructure development have pinned the stream into a narrow corridor. So other approaches need to be considered, and Robert Francis and Simon Hoggart of King’s College London discuss ways that existing artificial structures can be put to work to mitigate some of the ecological impacts of urbanization. In the specific case of the River Thames in England, habitat development has been observed on man-made structures, and furthermore, certain types of man-made structures grow life better than others. Francis and Hoggart show that indeed plants (and therefore animals) can develop in a riparian zone better when brick and wood and rougher materials are used over concrete and steel. If concrete and steel already exist, adding brick and wood can further trap sediment for habitat growth (like gluing a cup of dirt to a steel wall). They suggest that this should become standard practice when thinking of restoration efforts in large, urban waterways.

The NOAA’s Northwest Fisheries Science Center says Thornton Creek in downtown Seattle exemplifies “the challenges facing rehabilitating urban streams.” But a look at the NOAA picture below shows that this stream is also emblematic of a riparian ecosystem that has developed within the constraints of the existing structures and maybe even a spontaneous model for the sort of restoration that Francis and Hoggart envision.

Seattle urban stream from NOAA website

Francis, R., & Hoggart, S. (2008). Waste Not, Want Not: The Need to Utilize Existing Artificial Structures for Habitat Improvement Along Urban Rivers Restoration Ecology, 16 (3), 373-381 DOI: 10.1111/j.1526-100X.2008.00434.x

The hydrogeology of Yellowstone: It's all about the cold water

Cross posted at Highly Allochthonous

ResearchBlogging.orgThe Yellowstone caldera is home to thousands of geothermal springs and 75% of the world’s geysers, with kilometers-deep groundwater flow systems that tap magmatic heat sources. As that hot groundwater rises toward the surface, it interacts with shallower, cooler groundwater to produce multi-phase mixing, boiling, and a huge array of different hydrothermal features. While the deep, geothermal water is sexy and merits both the tourist and scientific attention given to it, there’s a largely untold story in the shallow groundwater, where huge volumes of cold water may advect more heat than the hydrothermal features.

Grand Prismatic Spring at Yellowstone National Park. Photo by Alaskan Dude on Flickr.
Grand Prismatic Spring. (Photo by Alaskan Dude on Flickr.)

Yellowstone is a rhyolitic caldera that has produced 6000 cubic kilometers of ash flow tuffs, rhyolites, and basalts that form a poorly-characterized, heterogeneous fractured rock aquifer, hosting both hot/deep and cold/shallow flow systems. The Yellowstone volcanics lie on top of the Rocky Mountain Cordillera, which itself is a complex hydrogeologic system, ranging from low permeability metamorphic rocks to high permeability limestones.

In a paper in the Journal of Hydrology, Gardner and colleagues (2010) use stream hydrographs and groundwater residence times to characterize the cold, shallow groundwater of the greater Yellowstone area. Stream hydrographs, or the time series of stream discharge, are useful indicators of groundwater dynamics, because in between rain or snowmelt events, streamwater is outflowing groundwater. The recession behavior of a hydrograph during periods between storms can be used to estimate aquifer volumes. In the Yellowstone region, the annual hydrograph is strongly dominated by the snowmelt peak, and Gardner et al. used the mean daily discharge record from 39 streams to characterize the recession behavior of streams on different lithologies. What they found was that streams flowing in watersheds dominated by volcanic rocks have much less variable hydrographs than those on other rock types. The figure below uses data from the USGS to illustrate these differences, which are in line with studies in the Oregon Cascades* and elsewhere which suggest that young volcanic rocks produce groundwater-fed streams with muted hydrographs.

Daily discharge for the Firehole River (USGS gage #06036905) and Soda Butte Creek (USGS gage #06187950) for the 2006-2007 water years, expressed on a unit area basis.

Using a nifty technique to separate the recessions into components attributable to snowmelt versus groundwater, Gardner et al. were able to calculate a ratio of the groundwater discharge to the total discharge of each stream and to calculate the hydraulic diffusivity, which is a ratio of permeability (how easily a fluid moves through a rock) compared to the amount of water stored in the system. If hydraulic diffusivity is low, the flow in the stream decreases slowly over time, like the Firestone River in the figure above. But hydraulic diffusivity can be low either because of low permeability or large aquifer storage volumes, so being able to tease apart those two components is key to understanding the hydrograph behavior. Gardner et al. did this by looking at the ratio of groundwater discharge to maximum discharge and using that as an index of aquifer storage. Based on these ratios, Gardner et al. separated the streams in the Yellowstone area into three groups (runoff-dominated, intermediate, and groundwater-dominated) with contrasting hydrogeologic properties.

Upper Cenozoic Geologic Map, Yellowstone Plateau
Geologic map of a portion of the Yellowstone Plateau, with approximate locations of stream gages of interest noted. Modified from Christiansen (2001, USGS Prof. Pap. 729-G).

Soda Butte and Teton Creeks are runoff dominated, with low groundwater storage and middling recession behavior. Since there is little groundwater storage, in order for hydraulic diffusivity to be low, then permeability must also be low. Sure enough, Teton Creek lies on top of Precambrian gneiss and granite, and unfractured metamorphic and intrusive igneous rocks like these have the lowest possible permeabilities. The Soda Butte Creek watershed comprises Eocene Absaroka volcanics, and older volcanic rocks like these can be quite weathered to clays and relatively impermeable.

The intermediate watersheds of Tower Creek and Cache Creek have significant ratios of groundwater discharge to maximum discharge, but their hydrographs recede rapidly over the summer. This means that they have high permeabilities relative to their aquifer storage volume. The Tower Creek watershed has Eocene tuffs and glacial valleys with alluvial fill, and Cache Creek watershed has Paleozoic carbonates. These materials are known for their high permeabilities, and the low storage volumes can be explained if those layers thinly overly less conductive materials.

The Firehole River, Gibbon River, and Snake River above Jackson Lake are groundwater-dominated, with very high permeabilities but even larger aquifer storage volumes. All of those streams drain primarily Quaternary Yellowstone volcanics, and this hydrologic behavior is in keeping with other young volcanic terrains.

Not content to stop with this hydrogeologic classification of the Yellowstone area, Gardner et al. collected water samples from small, cold springs to analyze CFC and tritium concentrations, which are useful tracers of groundwater travel times. For the springs they sampled, they found an average travel time (from recharge to discharge) of ~30 years. Using those CFC-derived groundwater transit times and some back-of-the-envelope estimates of aquifer geometry, Gardner et al. estimate that the Quaternary Yellowstone volcanics have a permeability of 10-11 to 10-13 m2, which is in line with estimates of young volcanics elsewhere. They also estimated that the aquifer depth represented by these small springs was ~70 m, but speculated that deeper flowpaths might have been discharging directly into the streams, out of reach of their CFC and tritium sampling abilities.

Finally, Gardner et al. note that the cold springs they studied are actually not as cold as they should be. In fact, they appear to be what are coming to be called “slightly thermal” springs. Groundwater recharge temperature is commonly assumed to be approximately mean annual temperature, and in the Norris Geyser Basin area, that’s around 4-5 &deg C. But the cold springs in the area are around 10 &deg C. Using this temperature difference and a handy equation from Manga and Kirchner (2004), Gardner et al. are able to calculate the heat flux advected by these cool springs. Their value of ~3800 W/m2 for the springs around Norris is about 10% of the heat flux from the Norris and Gibbon Geyser Basins themselves. That number becomes even more astonishing when you consider the relative scales of the cool versus the thermal groundwater systems. Geyser basins cover ~10 km2 of the Yellowstone Plateau, whereas cool groundwater drains under the entire ~1000 km2 plateau, and could be discharging far more heat than those showy thermal springs and geysers themselves.

So if you happen to go to Yellowstone this summer, in between gawking at Old Faithful, Artist Paint Pots, and Mammoth Hot Springs, take a few moments to appreciate the waters of the less dramatic cool rivers and streams. Their waters too are profoundly shaped by the geologic history of Yellowstone, and they are taking an awful lot of heat.

*Disclaimer: My PhD research focused on the hydrology and hydrogeology of volcanic aquifers and streams of the Oregon Cascades.

Payton Gardner, W., Susong, D., Kip Solomon, D., & Heasler, H. (2010). Snowmelt hydrograph interpretation: Revealing watershed scale hydrologic characteristics of the Yellowstone volcanic plateau Journal of Hydrology, 383 (3-4), 209-222 DOI: 10.1016/j.jhydrol.2009.12.037

My picks of the December literature

Cross-posted at Highly Allochthonous

I’m a few days behind on sharing my picks from December’s journals, but Chris has been doing such a stupendous job of sharing absolutely wonderful geology posts (and of deconstructing terrible science reporting), that I hardly feel guilty waiting until he’s occupied with travels before sneaking this post onto the blog.

Without further ado, here is the odd assortment of articles that hit my email box in December that I found most intriguing. They reflect a mixture of my past, present, and future research and teaching interests and should not be considered a reflection of anyone else’s tastes in science.

Burbey, T.J. (2010) Fracture characterization using Earth tide analysis, Journal of Hydrology, 380:237-246. doi:10.1016/j.jhydrol.2009.10.037

Tides are popping up all over in the geology literature these days, from the Slumgullion earthflow (atmospheric tides) to the San Andreas fault (earth tides). Here Burbey uses water-level fluctuations in fractured rock confined aquifers to quantify specific storage and secondary porosity. Fractured rock aquifers are notoriously tricky to understand, and this method gives hydrogeologists one more tool in their arsenal for understanding places like the Blue Ridge Mountains and the Piedmont. Since I’m getting interested in the fractured rocks in just those areas, this paper caught my eye.

Burnett, W.C., Peterson, R.N., Santos, I.R., and Hicks, R.W. (2010) Use of automated radon measurements for rapid assessment of groundwater flow into Florida streams Journal of Hydrology, 380:298-304. doi:10.1016/j.jhydrol.2009.11.005

Radon is a conservative tracer with concentrations several orders of magnitude higher in groundwater than surface water. That means that it can be used to evaluate the groundwater inputs into different stream reaches, though it is often used in conjunction with other tracers to get quantitative estimates. In this paper, Burnett and colleagues lay out a method for using radon as a sole tracer to quantify groundwater discharge. I’m looking around for tracers to separate overland flow, flow through the soil/saprolite, and groundwater from rock fractures, so this paper piqued my interest as radon is one candidate I’m learning more about.

Garcia-Castellanos, D., Estrada, F., Jiménez-Munt, I., Gorini, C., Fernàndez, M., Vergés, J. and De Vicente, R. 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature, 462, 778-781, doi:10.1038/nature08555.

5.6 million years ago the Mediterranean basin was nearly dry and highly saline in the midst of a period known as the Messinian salinity crisis, but 5.33 million years the Atlantic Ocean rapidly refilled the basin by overtopping and incising through the sill at the Straits of Gibraltar. How fast did that sea refill? How big was the peak discharge? And what did all that water do the straits itself? Those are the questions tackled in this paper, which combines borehole and seismic data with hydrodynamic and morphodynamic modeling. The story that Garcia-Castellanos and colleagues tell as a result of their work is truly astounding. The Atlantic Ocean overtopped the sill and slowly began to refill the Mediterranean, but as the sill eroded, discharge (and incision) increased exponentially until peak discharges on the order of 108m3/sec were reached and sea levels in the Mediterranean were increasing by up to 10 m per day.  While the beginning and the end of the flood may have stretched out for thousands of years, the modeling work suggests that the vast majority of water transfer and the incision of greater than 250 m deep canyons across the Straits of Gibraltar was done on a time scale of several months to two years. That peak discharge is ten times greater than that estimated for the Missoula Floods, themselves not trifling events, and there may have been profound paleoclimate repercussions from such a significant change in the region’s hydrological status.

Grimm, R. E., and S. L. Painter (2009), On the secular evolution of groundwater on Mars, Geophys. Res. Lett., 36, L24803, doi:10.1029/2009GL041018.

Grimm and Painter created a 2D pole-to-equator model of subsurface water and carbon dioxide transport, initiated the model by simulating sudden freezing, and then looked at the effects over geologic time scales (secular evolution). According to their abstract, their model predicts water to be found in different places on the Martian landscape than previous ideas had suggested. I guess we’ll just have to go look and see who is right.

Jiang, Xiao-Wei; Wan, Li; Wang, Xu-Sheng; Ge, Shemin; Liu, Jie Effect of exponential decay in hydraulic conductivity with depth on regional groundwater flow Geophys. Res. Lett., 36, L24402, doi:10.1029/2009GL041251.

In soils and in the Earth’s crust, hydraulic conductivity (K) generally decreases exponentially with depth. This phenomenon is the result of the compaction and compression of the overlying strata. In this paper, Jiang and colleagues examine the implications such decreases in K on local versus regional groundwater flow systems. They find that the more quickly K decreases, the less water makes into the deeper regional flow systems and local flow systems extend deeper into the subsurface. They suggest that when hydrogeologists try to interpret regional flow problems, that we need to bear in mind the effects of decreasing K on the systems.

Knight, D.B. and Davis, R.E. 2009. Contribution of tropical cyclones to extreme rainfall events in the southeastern United States. J. Geophys. Res., 114, D23102, doi:10.1029/2009JD012511.

Knight and Davis used 25 years of observational, wind-corrected, and reanalysis data for the southeastern Atlantic coastal US states and found that extreme precipitation from tropical storms and hurricanes (TCs) has increased over the study period.  They find that this increase in TC contribution to extreme precipitation is a function of increasing storm wetness and frequency, but not storm duration. If TCs are producing more precipitation, their flood hazards are also increasing, and flooding is already the leading cause of deaths associated with TCs.

Meade, R.H. and Moody, J.A. 2009. Causes for the decline of suspended-sediment discharge in the Mississippi River system, 1940-2007. Hydrological Processes. 24, 35-49. doi:10.1002/hyp.7477

Dams on the Missouri and Upper Mississippi Rivers have been blamed for trapping almost 2/3 of the sediment that used to reach the Lower Mississippi and Delta.  Here, Meade and Moody show that the dams are only trapping half of the missing sediment, while engineering practices such as bank revetments and meander cutoffs, combined with better erosion control practices in the drainage basin, probably account for the rest. Meade and Moody suggest that this river system, in the largest basin in North America, has been transformed from transport-limited to supply-limited, which is a pretty amazing fundamental shift in the behavior of the river and its ability to deliver sediments to the Gulf of Mexico. [Note that there’s another article in the same issue on “A quarter century of declining suspended sediment fluxes in the Mississippi River and the effect of the 1993 flood.” Both articles are in the public domain and not subject to US copyright laws, though there doesn’t seem to be an obvious way to take advantage of that from the Wiley website.]

Neumann, R.B.,  Ashfaque, A.N,  Badruzzaman, A. B. M.,  Ali, M.A.,  Shoemaker, J.K., and Harvey, C.F. 2010. Anthropogenic influences on groundwater arsenic concentrations in Bangladesh, Nature Geoscience 3, 46-52. doi:10.1038/ngeo685

The story of groundwater of southeast Asia’s deltas, where tens of millions of people live at risk of arsenic poisoning from their drinking water, is perhaps the most compelling contemporary scientific story of how geology, geomorphology, hydrology, and humans intertwine. It’s also an extremely complicated story, with arsenic-laden sediment from the Himalayas settling in the deltas , irrigated rice fields and ponds  changing the local groundwater flow patterns, and microbially mediated oxidation of organic carbon driving the geochemical release of the arsenic into the groundwater. This story has been being pieced together in many papers in the last several years, and in this paper Neumann et al. show that groundwater recharge from the ponds, but not the rice fields, draws the organic carbon into the shallow aquifer, and then groundwater flow modified by pumping brings the carbon to the depths with the greatest dissolved arsenic concentrations. Add some biogeochemistry data, isotope tracing of source waters, incubation experiments, and 3-D flow modeling, and this paper adds some important elements to our understanding of how this public health risk came to be – and how we might be able to mitigate the risks for the people who have little choice but to drink the water from their local wells. [Also note that the same issue of Nature Geosciences has another article on “arsenic relase from paddy soils during monsoon flooding” as well as an editorial, commentary, backstory, and news and views piece on the southeast Asia arsenic problem.]

Pritchard, D., G. G. Roberts, N. J. White, and C. N. Richardson (2009), Uplift histories from river profiles, Geophys. Res. Lett., 36, L24301, doi:10.1029/2009GL040928.

In rivers that have adjusted to their tectonic and climatic regimes, the long profile of a river is smooth and concave. The interesting places are where river profiles don’t look like that ideal. This paper interprets river longitudinal profiles as a way to understand the tectonic uplift history of the area, through a non-linear equation. They check their interpretation against an independently constrained uplift history for a river in Angola.

Stone, R. 2009. Peril in the Pamirs. Science 326(5960): 1614-1617. doi: 10.1126/science.326.5960.1614

Dave Petley at Dave’s Landslide Blog has the must-read summary of this article on the risks associated with the giant lake impounded by the world’s tallest landslide dam. This is seriously fascinating stuff. I already talked a bit about the Usoi Dam in my dam-break floods spiel in my Fluvial Processes class, and now I have more ammunition for this year’s crop of students. In the same issue of Science, Stone also summarizes some of the other water issues facing Central Asia.

Please note that I can’t read the full article of AGU publications (including WRR, JGR, and GRL) until July 2010 or the print issue arrives in my institution’s library. Summaries of those articles are based on the abstract only. UNC Charlotte also does not have access to Nature Geoscience.

Gifts for future hydrologists

Cross-posted on Highly Allochthonous

Doing some last minute shopping for the young’uns on your list? Want to inspire a love and respect for the natural world? Then take the kid outside for a hike up a mountain or splash in a stream and let them experience first-hand how amazing Earth’s landscapes can be.

But if you want to give something a bit more material, then here are a couple of water-themed books I recommend for kids. Most of these have been tested on my almost 3-year-old, so my age recommendations have only one true calibration point.

For preschoolers


“I pass through a gateway
of high stone palisades,
leaving the land behind.
Cool silver moonlight
sparkles and dances
on my waves.
I am the sea.

Thomas Locker’s Water Dance follows the water cycle with lyrical prose and beautiful paintings to accompany each store of water. Locker’s lovely paintings could also be used without the text, just as a way to point out waterfalls, storms, oceans, etc. and to spark a conversation with a young child about their experiences with rain or other hydrological phenomena.

Where the River BeginsMany preschoolers prefer listen to stories with a clear plot, and might have a hard time identifying with the sea, stream, and storm of “Water Dance.” If you think that’s the case for the preschooler on your list, I recommend another Thomas Locker book, “Where the River Begins.” In this book, two boys and their grandfather set out on a hike to find the source of the gentle, meandering river that flows past their house. They trace the river to a rapidly cascading mountain stream that begins in a quiet pond. On the way home, they get caught in a rain storm which floods their path. There’s some hydrology embedded in there, but msotly a clear narrative for the plot-driven preschooler. My daughter approves of this book.

A Drop Around the WorldFor early elementary age readers
A Drop Around the World
by Barbara McKinney is an amazing book that follows a single water molecule from raindrop on the Maine coast to glacier melt in Switzerland to a monsoon flood in India and back to the eastern U.S, with many more stops along the way This vividly colorful book uses the water molecule as narrator and has nifty little symbols for the phases and their changes. It also emphasizes the trans-cultural importance of water. Young readers can hunt for the water droplet with the smiley face hiding on each page. The last two pages provide a legend for the little symbols giving more hydrological info for adults or interested kids. There’s also an educators’ guide to go with the book. My nearly 3-year old liked looking at the pictures, but the story hasn’t drawn her in quite yet, so I’d put this book in the 4+ age range. Perhaps it’s that plot and character identification problem again…

Letting Swift River GoJane Yolen’s Letting Swift River Go tells the tale of the damming of the Swift River in western Massachusetts to form the Quabbin Reservoir in the 1920s and 1930s. The story is told from the point-of-view of a young girl who watches her hometown and the surrounding farmlands and forests disappear under the rising waters. I really like this book because it integrates issues of water and society within a compelling narrator with whom children can identify. I put this book in the early elementary category, but my daughter has enjoyed listening to the story, though it verges on the long side for her attention span. I look forward to many more years of reading this story with her and the discussions I am sure it will engender as we walk in the reservoir-side parks along our local Catawba River.

tree-rings-fleck.jpgFor older kids
One book I haven’t read yet, but which I am anxious to get my hands on is John Fleck’s “The Tree Rings’ Tale: Understanding Our Changing Climate.” Fleck is an outstanding science journalist at the Albuquerque Journal and water blogger. The early reviews of his new book have been highly complimentary, and I love the idea of how he interweaves a history of the Colorado River with the science of dendrochronology and climate change.

Though not exactly a fly-fishing or white-water rafting trip, or even a walk along your local creekside greenway, the books above still make fine gifts and may even spark inspiration in a future hydrologist.

My picks of the November literature

It is not that there was no October literature to pick. My time to read articles simply disappeared in the lead-up to and excitement of the Geological Society of America meeting. This month, however, I am back on track and I will try to update this post as I move through the last few weeks of November.

Fussel, H-M. 2009. An updated assessment of the risks from climate change based on research published since the IPCC Fourth Assessment Report. Climatic Change (2009) 97:469–482. doi:10.1007/s10584-009-9648-5
The takeaway message is this: While some topics are still under debate (e.g., changes to tropical cyclones), most recent research indicates that things are looking even worse now than we thought a few years ago. Greenhouse gas emissions are rising faster than we anticipated, and we have already committed to substantial warming, which is currently somewhat masked by high aerosol concentrations. It is increasingly urgent to find mitigation and adaptation strategies. Not good.

Gardner, LR. 2009. Assessing the effect of climate change on mean annual runoff. Journal of Hydrology. 379 (3-4): 351-359. doi:10.1016/j.jhydrol.2009.10.021
This fascinating article starts by showing a strong correlation (r2 = 0.94) between mean annual runoff and a function of potential evapotranspiration and precipitation. The author then goes on to derive an equation that shows how temperature increases can be used to calculate the change in evapotranspiration, therefore solving the water budget and allowing the calculation of the change in mean annual runoff. Conversely, the same equation can be used to solve for the necessary increase in precipitation to sustain current runoff under different warming scenarios.

Schuler, T. V., and U. H. Fischer. 2009.Modeling the diurnal variation of tracer transit velocity through a subglacial channel, J. Geophys. Res., 114, F04017, doi:10.1029/2008JF001238.
The authors made multiple dye tracer injections into a glacial moulin and then measured discharge and tracer breakthrough at the proglacial channel. They found strong hysteresis in the relationship between tracer velocity and proglacial discharge and attributed this hysteresis to the adjustment of the size of a subglacial Röthlisberger channel to hydraulic conditions that change over the course of the day. Cool!

Bense, V. F., G. Ferguson, and H. Kooi (2009), Evolution of shallow groundwater flow systems in areas of degrading permafrost, Geophys. Res. Lett., 36, L22401, doi:10.1029/2009GL039225.
Warming temperatures in the Arctic and sub-arctic are lowering the permafrost table and activating shallow groundwater systems, causing increasing baseflow discharge of Arctic rivers. This paper shows how the groundwater flow conditions adjust to lowering permafrost over decades to centuries and suggests that even if air temperatures are stabilized, baseflow discharge will continue to increase for a long time.

Soulsby, Tetzlaff, and Hrachowitz. Tracers and transit times: Windows for viewing catchment scale storage. Hydrological Processes. 23(24): 3503 – 3507. doi: 10.1002/hyp.7501
In this installment of Hydrological Processes series of excellent invited commentaries, Soulsby and colleagues remind readers that although flux measurements have been the major focus of hydrologic science for decades, it is storage that is most relevant for applied water resources problems. They show that tracer-derived estimates of mean transit time combined with streamflow measurements can be used to calculate the amount of water stored in the watershed. They use their long-term study watersheds in the Scottish Highlands to illustrate how transit time and storage scale together and correlate with climate, physiography, and soils in the watersheds. Finally, they argue that while such tracer-derived storage estimates have uncertainties and are not a panacea, they do show promise across a range of scales and geographies.

Chatanantavet, P., and G. Parker (2009), Physically based modeling of bedrock incision by abrasion, plucking, and macroabrasion, J. Geophys. Res., 114, F04018, doi:10.1029/2008JF001044.
Over the past 2 decades, geomorphologists have developed much better insight into the landscape evolution of mountainous areas by developing computerized landscape evolution models. A key component of such models is the stream power rule for bedrock incision, but some have complained that is not physically based enough to describe. In this paper, the authors lay out a new model for bedrock incision based on the mechanisms of abrasion, plucking, and macroabrasion (fracturing and removal of rock by the impact of moving sediment) and incorporating the hydrology and hydraulics of mountain rivers. This could be an influential paper.

Payn, R. A., M. N. Gooseff, B. L. McGlynn, K. E. Bencala, and S. M. Wondzell (2009), Channel water balance and exchange with subsurface flow along a mountain headwater stream in Montana, United States, Water Resour. Res., 45, W11427, doi:10.1029/2008WR007644.

Tracer tests were conducted along 13 continuous reaches of a mountain stream to quantify gross change in discharge versus net loss and net gain. Interestingly, the change in discharge over some reaches did not correspond to calculations of net loss or net gain based on tracer recovery. These results suggests that commonly used methods for estimating exchange with subsurface flow may not be representing all fluxes. Bidirectional exchange with the subsurface, like that found in this paper, is likely to be very important for nutrient processing and benthic ecology.

Please note that I can’t read the full article of AGU publications (including WRR, JGR, and GRL) until July 2010 or the print issue arrives in my institution’s library. Summaries of those articles are based on the abstract only.

How to build a meandering river in your basement

This post is cross-posted at Highly Allochthonous. Please look over there for 15+ comments on the post.

Meandering rivers are characterized by regularly spaced bends that grow and cutoff and generally march downstream in a fairly orderly fashion. Click the image below to watch a movie of meander migration on the Allier River near Chateau de Lys, France

Movie 1. Meander bend migration and cut off using aerial photos and maps from: 1945,1960,1971,1980,1982, 1992, 1995, and 1997 on the Allier River, France. Created by A. Wilbers, originally found here.

Though meandering rivers are by far the most common river form on Earth, building a meandering river in a laboratory flume eluded scientists for decades. The conditions necessary to support self-maintaining meandering rivers were not known well enough to recreate in the laboratory. Flumes, or experimental channels, are a really important tool for understanding river processes, because sediment and water influxes can be tightly controlled and high precision measurements made.

Sand and gravel, the most common sediments in river banks, have low cohesion. In flumes, channels through sand and gravel, even if initially forced into a meander form, inevitably end up as wide channels with active braid bars. Solving the bank cohesion problem, by replacing sand and gravel with silt and clay, results in flume channels that have lots of curvature (sinuousity) but do not maintain their geometry through multiple meander cut-offs. Over the last 10 years, graduate students Karen Gran and Michal Tal working with Chris Paola at the University of Minnesota figured out how to make a self-sustaining single channel in coarse sediment. The key to creating a single channel was to plant alfalfa seedlings to give the banks some cohesion. You can see the results of alfalfa growth in a Quicktime video of Tal’s experiments. (Click the image below.)

capture1.pngMovie 2. Tal and Paola’s experiments with alfalfa seedlings and channel form. More movies of these experiments here.

If you watched the video, you’ll notice that while the channel is indeed single thread and it does move around, the meanders don’t move downstream in the relatively orderly fashion of a natural river. So the insight of alfalfa sprouts from Gran and Paola (2001) and Tal and Paola (2007) got geomorphologists a long way towards understanding the controls on meander self-maintenance in coarse-bedded rivers, but they didn’t quite reach the finish line.

Now, a paper in the Proceedings of the National Academy of Sciences by UC Berkeley graudate student Christian Braudrick, his advisor Bill Dietrich and collaborators Glen Leverich and Leonard Sklar from San Francisco State University reports that they have succeeded where so many others have failed. In a 17-m long, 6.7 m wide flume, Braudrick and colleagues created a self-sustaining meandering channel. Their work was featured on National Public Radio’s Science Friday show, which produced the following video giving the basics of Braudrick’s process.

Movie 3. Science Friday’s video about Braudrick et al’s experiments.

One of the key things mentioned in the video, but not explained is why the lightweight sediment was plastic. In slimming down a river to fit within a laboratory, researchers have to take into account all of the possible scaling effects. That’s why alfalfa seedlings are used to simulate the grasses and trees of a normal riparian zone, for instance. The power of the water, or its shear stress, is a function of depth, slope, fluid density, and gravity. Since the depth of flume channels is so much smaller than real rivers, it means that the shear stress available to move sediment is much lower. This means flumes can’t move fist sizes pieces of gravel and the size of the sediment in the study must be scaled down accordingly. Gravel scales down reasonably well to coarse sand, but sand scales down to silt, and silt has much different cohesive properties than sand. This is where the plastic came in, because the researchers wanted to create meanders using the alfalfa to create cohesive banks not by adding cohesive sediment. The plastic beads were the size of very fine sand and they lacked cohesion. Thus, the researchers created laboratory conditions of that mimicked natural rivers – channel banks where there was a mixture of sizes of non-cohesive sediment held together by roots.

When the flume was turned on, the little plastic beads moved both along the channel bed and suspended within the water column, much as sand would do in a natural channel. With a small initial curvature at the upstream end of the flume, meanders propogated downstream and began to grow and cut off. In previous alfalfa-only experiments ( Tal and Paola, 2007), each time meanders were cut off, a trough was left on the upstream side of the abandoned meander. In natural systems, these troughs get plugged with fine sediment and create oxbow lakes that eventually fill in. In the alfalfa-only, the troughs persisted, opening the possibility of islands developing in the channel. In Braudrick’s alfalfa+plastic experiments, the little plastic beads moving in suspension filled in the troughs at the upstream end of the abandoned meander, blocking future flow through that old pathway.

From Braudrick and colleagues’ results, it appears that sand and fine sediment have an important role to play in reinforcing and maintaining the meandering pattern of river channels. Out in the real world, such fine sediment is often regarded as an undesirable pollutant of coarse-bedded rivers, so these results have the potential to change the goals of river restoration and management. Plus, now that geomorphologists have a way to simulate realistic meandering rivers in the flume, new insights into the controls and behavior of meandering rivers are likely to start pouring in.

This post is cross-posted at Highly Allochthonous. Please look over there for 15+ comments on the post.