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rivers

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: 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.

Braided river meets mountain gorge: The Snake River escapes Jackson Hole

Though I don’t think anything can top Kyle’s pathologically misdirected RYNHO, I recently had cause to contemplate a river that everyone has heard of – the Snake River of the northwestern United States. Now, the Snake River has a famous gorge, a famous lava plain, and it’s had a famously big flood or two, but the upper reaches of the Snake are pretty scenic too. The Snake originates in Yellowstone National Park and flows through Grand Teton National Park and the Jackson Hole valley. Throughout the broad, flat valley, the Snake is beautifully braided (with some gorgeous terraces too).Then it runs into some mountains – the Wyoming Range – and it runs out of room to braid, becoming constricted into a narrow mountain gorge. Interestingly, after heading south from Yellowstone and through Jackson Hole, the river turns west through the mountains and then quite abruptly turns north towards Idaho’s Snake River Plain.

I’d love to know how and why the river started along this path and how intensely the river’s course is geologically controlled. I think the gorge is south of the Teton block, and it’s possible that it’s in an narrow zone that hasn’t seen as much uplift as other mountain blocks in the Basin and Range, but I’m just speculating here. If anyone has any good ideas or citations, please drop them in the comments.

The images below are from a mix of Flash Earth (permalink here) and Google Earth. The first is a large scale view of the braided-gorge transition, while the second and third are close-ups of typical braided and gorge reaches, respectively.

Posted via web from Pathological Geomorphology

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

ResearchBlogging.org

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

When it rains a lot and the mountains fall down

Cross-posted at Highly Allochthonous

2006 debris flow deposit in the Eliot Glacier drainage, north flank of Mount Hood (Photo by Anne Jefferson)

The geo-image bonanza of this month’s Accretionary Wedge gives me a good reason to make good on a promise I made a few months ago. I promised to write about what can happen on the flanks of Pacific Northwest volcanoes when a warm, heavy rainfall hits glacial ice at the end of a long melt season. The image above shows the result…warm heavy rainfall + glaciers + steep mountain flanks + exposed unconsolidated sediments are a recipe for debris flows in the Cascades. Let me tell you the story of this one.

It was the first week of November 2006, and a “pineapple express” (warm, wet air from the tropic Pacific) had moved into the Pacific Northwest. This warm front increased temperatures and brought rain to the Cascades…a lot of rain. In the vicinity of Mt. Hood, there was more than 34 cm in 6 days, and that’s at elevations where we have rain gages. Higher on the mountain, there may even have been more rain…and because it was warm, it was *all* rain. Normally, at this time of year, the high mountain areas would only get snow.

While it was raining, my collaborators and I were sitting in our cozy, dry offices in Corvallis, planning a really cool project to look at the impact of climate change on glacial meltwater contributions to the agriculturally-important Hood River valley. Outside, nature was opting to make our on-next field season a bit more tricky. We planned to install stream gages at the toe of the Eliot and Coe glaciers on the north flank of Mt. Hood, as well as farther downstream where water is diverted for irrigation. But instead of nice, neat, stable stream channels, when we went out to scout field sites the following spring, we were greeted by scenes like the one above.

Because sometime on 6 or 7 November, the mountain flank below Eliot Glacier gave way…triggering a massive debris flow that roared down Eliot Creek, bulking up with sediment along the way and completely obliterating any signs of the pre-existing stream channel. By the time the flow reached the area where the irrigation diversion occur, it had traveled 7 km in length and 1000 m in elevation, and it had finally reached the point where the valley opens up and the slope decreases. So the sediment began to drop out. And debris flows can carry some big stuff (like the picture below) and like the bridge that was washed out, carried downstream 100 m and turned sideways.

2006 Eliot Glacier debris flow deposit (photo by Anne Jefferson)

2006 Eliot Glacier debris flow deposit (photo by Anne Jefferson)

In this area, the deposit is at least 300 m wide and at least a few meters deep.

Eliot Creek, April 2007 (photo by Anne Jefferson)

Eliot Creek, April 2007 (photo by Anne Jefferson)

With all the big debris settling out, farther downstream the river was content to just flood…

[youtube=http://www.youtube.com/watch?v=J4eduMJU710]
Youtube video from dankleinsmith of the Hood River flooding at the Farmers Irrigation Headgates

and flood…

West Fork Hood River flood, November 2006 from http://elskablog.wordpress.com/2006/11

West Fork Hood River flood, November 2006 from http://elskablog.wordpress.com/2006/11/. For the same view during normal flows, take a look at my picture from April 2007: http://scienceblogs.com/highlyallochthonous/upload/2009/10/IMG_1108.JPG.

and create a new delta where Hood River enters the Columbia.

Hood River delta created in November 2006 (photo found at http://www.city-data.com/picfilesc/picc30876.php)

Hood River delta created in November 2006 (photo found at http://www.city-data.com/picfilesc/picc30876.php

And it wasn’t just Mt. Hood’s Eliot Glacier drainage that took a beating in this event. Of the 11 drainages on Mt. Hood, seven experienced debris flows, including a rather spectacular one at White River that closed the main access to a popular ski resort. And every major volcano from Mt. Jefferson to Mt. Rainier experienced debris flows, with repercussions ranging from downstream turbidity affecting the water supply for the city of Salem to the destruction of popular trails, roads, and campgrounds in Mt. Rainier National Park (pdf, but very cool photos).

In the end, our project on climate change and glacial meltwater was funded, we managed to collect some neat data in the Eliot and Coe watersheds in the summer of 2007, and the resulting paper is wending its way through review. The November 2006 debris flows triggered at least two MS thesis projects and some serious public attention to debris flow hazards in the Pacific Northwest. They also gave me some really cool pictures.

Conference presentation: Effects of river management & sediment supply on island evolution in Pool 6 of the Upper Mississippi River, southeast Minnesota

Watershed Hydrogeology Lab graduate student Brock Freyer has spent the last two years learning deeply about the hydrology, geomorphology, and sedimentology of the Upper Mississippi River System, as well as learning to use some sophisticated GIS techniques for 3-D analysis of topographic data. This week he is presenting the results of his work: “Effects of river management & sediment supply on island evolution in Pool 6 of the Upper Mississippi River, southeast Minnesota” at the Upper Midwest Stream Restoration Symposium. Brock is speaking in a session on Large River Restoration. Brock will be defending his M.S. thesis sometime in late spring.

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.

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.