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Ralph McGee and Cameron Moore will graduate next week!

Major congratulations to two Watershed Hydrogeology Lab graduate students who have finished writing their MS theses and will defend them next week. Ralph McGee and Cameron Moore both started in our MS in Earth Science program in August 2009, and less than two years later they have each completed impressive MS projects on headwater streams in Redlair Forest of the North Carolina Piedmont.

Ralph McGee will present his research on “Hydrogeomorphic processes influencing ephemeral streams in forested watersheds of the southeastern Piedmont U.S.A.” on Thursday, May 12th at 10:00 am in McEniry Hall, room 111 on the UNC Charlotte campus.

The unofficial title for Ralph’s work is “Tiny Torrents Tell Tall Tales.” Watch the video below to see why.
[youtube=http://www.youtube.com/watch?v=PjINxXuy5Aw&w=640&h=390]

Cameron Moore will present his research on “Surface/Groundwater Interactions and Sediment Characteristics of Headwater Streams in the Piedmont of North Carolina” on Friday, May 13th at 9:00 am in McEniry Hall, room 111 on the UNC Charlotte campus.

When Cameron started working on this project, I had thought that the story would focus on how fractured bedrock contributed to groundwater upwelling in the streams, but it turns out the small debris jams (like the one below) are the dominant driver of groundwater/stream interactions and spatial variability of channel morphology.

Debris jam in Deep Creek

Looking upstream at a debris jam in Deep Creek


Faculty, students, and the public are encouraged to attend the presentations and ask Ralph and Cameron any questions they may have.

A continental divide that runs through a valley

Now that’s pathological.

Parts of the Upper Midwest are disappearing under spring floods. The Red River of the North is at major flood stage, again, and the Minnesota River flood crest is moving downstream. It’s a pretty frequent occurrence in both of these river systems, and in part, flooding is a legacy of the glacial history of the area. The Red River flows to the north along the lake bed of Glacial Lake Agassiz, which is pathologically flat. The Minnesota River flows to the south along the channel of the Glacial River Warren, which was gouged out of the landscape by water draining from Lake Agassiz.

14,000 years ago there was direct connection between what is now the Red River basin and the Minnesota River basin. Today, there’s a continental divide – with the Red flowing toward Hudson Bay and the Minnesota flowing toward the Mississippi and Gulf of Mexico. But what a strange continental divide it is – for it runs through the former outlet of Lake Agassiz, in what is now known as Brown’s Valley or the Traverse Gap. This divide is not so much a high point in the landscape, but a just-not-quite-as-low area. The little community of Brown’s Valley sits between Lake Traverse (flows to the North, forming the headwaters of the Red) and Big Stone Lake (flows to the south, forming the headwaters of the Minnesota).

Here’s what it looks like on Google Earth. Note that I’ve set the terrain to 3x vertical exaggeration, so that you have some hope of seeing the subtle topography of this area.

Croppercapture12

And here’s a very, very cool oblique photo from Wikipedia. It shows the divide looking from north to south — mostly covered by floodwaters in 2007. It’s not every day you get to see a continental divide covered in water.

800px-browns_valley_flood_07

Why does the Red River of the North have so many floods?

Cross-posted at Highly Allochthonous

Communities along the Minnesota-North Dakota border are watching the water levels, listening to the weather forecasts, and preparing for another season of flooding. It must be a disconcertingly familiar routine, as this will be the third year in a row in which the Red River of the North reaches major flooding levels. But this isn’t merely a run of bad luck for residents in the Red River Valley, major floods are to be expected in a place with an unfortunate combination of extremely low relief and a river at the whim of snowmelt and ice jams.

The Red River of the North begins in Minnesota, near the border with North and South Dakota, and it flows northward through Fargo/Moorhead, Grand Forks, and Winnipeg before emptying into Lake Winnipeg, Manitoba. The landscape around the Red River is excruciatingly flat (Figure 1), for the Red River Valley isn’t a stream-formed feature at all, but is the remnant landscape of Glacial Lake Agassiz, which held meltwaters from the Laurentide Ice Sheet for more than 5000 years. The modern Red River has barely managed to incise into this flat, flat surface, because it slopes only very gently to the north (~17 cm/km). Instead, the river tightly meanders across the old lake bed, slowly carrying its water to the north. Topographically, this is a pretty bad setting for a flood, because floodwaters spread out over large areas and take a long time to drain away.

Topography of the US portion of the Red River Valley from SRTM data as displayed by NASA's Earth Observatoryredriver_srtm_palette

Figure 1. Topography of the US portion of the Red River Valley from SRTM data as displayed by NASA's Earth Observatory

The climate of the Red River watershed makes it prone to flooding during the spring, usually peaking in about mid-April. The area receives about 1 m of snow between October and May, and the river freezes over. In late March to early April, the temperatures generally rise above freezing, triggering the start of snowmelt. Temperatures warm soonest in the southern, upstream end of the watershed and they get above freezing the latest near the mouth of the river. This means that snowmelt drains into the river’s upper reaches while downstream the river is still frozen, impeding flow (Figure 2). As the ice goes out, jams can temporarily occur and dam or back up the river, exacerbating local flooding problems.

Red River near Oslo, Minnesota, 3 April 2009, photo by David Willis

Figure 2. Red River near Oslo, Minnesota, 3 April 2009. Here the main river channel is still clogged with ice, while surrounding farmland is underwater. Photo by David Willis of http://www.cropnet.com/.

Together the topography and climate of the Red River watershed are a recipe for large-scale flooding, and the historical record shows that floods are a frequent occurrence on the river. Usually, hydrologists talk about rivers in terms of their flow, or discharge, which is the volume of water per second that passes a point. But, when talking about floods like those on the Red River, it’s not so much volume that matters as how high the water rises (“stage”). The National Weather Service is responsible for flood prediction in the US, and they define flood stage as “the stage at which overflow of the natural streambanks begins to cause damage in the reach in which the elevation is measured.” If the water level continues to rise, “moderate flooding” occurs when “some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary.” Further increases in water levels can bring a river to “major flooding“, when “extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.” That’s the sort of flooding that will happen in places along the Red River this spring, as it has many springs in the historical record (Figure 3).

Annual peak stage on the Red River at Grand Forks, North Dakota

Figure 3. Annual peak stage on the Red River at Grand Forks, North Dakota. Data replotted from the USGS, with local NWS flood stages shown.

Already, flood warnings are being issued for the Red River and its tributaries. As I’ll discuss in my next post, the long-range forecast for this spring’s floods on the Red is looking pretty grim. But as the communities along the river brace for the on-coming flood, it is important to remember that the geology and climate of the region make repeated major floods inevitable.

Edible debris flow

Cross-posted at Highly Allochthonous</em>

Steep hillslopes with loose sediment are at risk from debris flows triggered by heavy rain or rapid snowmelt. As water is added to the hillslope, surface runoff or positive pore water pressure catastrophically destabilizes a portion of the slope. Pulled by gravity, the water and sediment mixture moves downslope – picking up trees, boulders, gravel, and more mud and water along the way. Usually nothing stops these flowing masses of debris until they reach a relatively flat surface.

Debris flows have the power to reshape mountainsides and valley bottoms, and they can cause tragic devastation to people and property in their way. From North Carolina and California to Japan and Brazil, debris flows are a significant natural hazard and an area of active research by geoscientists and engineers.

In the spirit of the Accretionary Wedge, I decided to undertake my own research and investigate the possibilities for an edible analog for debris flows. First, I assembled sediments of a range of sizes, shapes, and natural cohesions.

Ingredients of my debris flow pilaf

Loose sediments (clockwise from top right) of the onion, rice, lentil, potato, portabella, garlic, pepper, salt, coriander, ginger, and barley varieties.

Then I added water to saturate the mixture, and placed it on a slope. Voila, debris flow pilaf! From the view below, you can see a bunch of features of debris flows.

Debris flow pilaf

Debris flow pilaf

  • At the top, there is the area of initial failure. In this case, it appears to be in the midst of a broccoli clear cut, where root strength had been weakened, reducing cohesion in the soils.
  • The debris flow then moved downslope in a somewhat confined manner. Usually the flow will move down an existing channel on the slope, but sometimes debris flows have to start from scratch and may not leave much of an erosional impression.
  • There is some evidence that the debris flow bulked up by lateral accumulation of material on its downslope track (i.e., places where sediment appears to accrete along the sides of the flow).
  • At least one large boulder has been rafted along the top of the flow, thanks to the quirks of fluid mechanics in very viscous fluids.
  • When the flow moved off the hillslope and onto the valley floor, the potential energy disappeared and the flow quickly stopped moving. Sometimes, debris flows will form a fan shape deposit at their front. But in our case, while there was some lateral spreading, it just stopped moving a short distance out onto the flat.
  • At the flow front, there is a significant accumulation of woody debris. (Amazing that it has kept its leaves on!) This debris has either been rafted on the top of the flow or been pushed along ahead of it.
  • There is a higher concentration of the coarsest grain sizes at the flow front. This sort of bouldery front is typical of debris flows where coarse material is available. You can see this better in the image below.
Overhead view of the debris flow runout.

Overhead view of the debris flow runout.

Of course, there are some limitations to using kitchen ingredients as analogs for debris flows. I highly encourage you to watch this classic USGS video on debris flows for its incredible footage of a whole range of debris flow materials and behaviors, all set to the most wonderful classical music. (Seriously, try the first 40 seconds of part 1 and see if you are not hooked.) Then, after you’ve watched the videos, I encourage you to use the comment section to make suggestions for improvements to the physical realism of future experiments with edible debris flows.

[youtube=http://www.youtube.com/watch?v=zpGP1uoCHr4&w=640&h=510]
[youtube=http://www.youtube.com/watch?v=i5nuwPVlHKU&w=640&h=510]
[youtube=http://www.youtube.com/watch?v=-rD_T8CBEVE&w=640&h=510]

If that’s not enough debris flow video goodness to satisfy your appetite, check out these USGS videos of debris flows at their experimental flume in the Oregon Cascades. The recipe for barley pilaf with lentil confetti (or as it shall always be known in my mind: “debris flow pilaf”) came from Didi Evans’ Vegetarian Planet.

Abstract: Dynamics of Ephemeral Channels in Humid, Forested Watersheds

At the 2010 Geological Society of America meeting, MS student Ralph McGee will be presenting preliminary results of his thesis work in a session on Hydrogeomorphic Processes in Hillslopes, Rivers, and Landscapes which I will be convening along with Ben Crosby and Christopher Tennant of Idaho State University.

Here’s Ralph’s abstract:

DYNAMICS OF EPHEMERAL CHANNELS IN HUMID, FORESTED WATERSHEDS

MCGEE, Ralph, Department of Geography & Earth Sciences, University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, NC 28223-0001,

In the moderate relief Piedmont of the southeastern United States, headwater watersheds are drained by dendritic ephemeral channel networks that contribute to perennial streams. Such headwater watersheds (<1km2) account for approximately 60% of the total land area in the North Carolina Piedmont, where rapid population growth is converting forest and farmland to urban land uses, but little is known about the magnitude and frequency of flow and erosion in the ephemeral channel network and their controlling factors. Using map grade GPS units, >100 forested channel heads are being mapped, to evaluate the contributing area-slope relationships and their variation with soil type and land-use history. Six ephemeral streams have been instrumented with 41 crest stage gauges to monitor the magnitude and frequency of peak flows, relative to antecedent and event moisture conditions and position within the channel network. Source areas for ephemeral channels are on the order of 0.95 ha (range= 0.8-1.5 ha) with an average length 161.1 m (range= 88.4-245.9m). Preliminary data suggest there are no significant relationships between slope and area at channel heads within or across soil types (r2< 0.53). In summer 2010, two ~2.5 cm rain events generated measurable flow in all portions of the creeks with the exception of the channel heads. These results suggest that channelized overland flow occurs during typical precipitation events over a significant portion of the flowpath from ridgeline to perennial channel. Disturbances to the ephemeral channel network, such as occur during urbanization, may have an under-appreciated impact on larger streams.

Anne's picks of the literature: river and floodplain sediments

ResearchBlogging.orgIn July, four geomorphology papers particularly piqued my interest, and, as I started to summarize them, I realized they were loosely connected by a common theme. These four papers all attempt to understand what controls the sediments that make up the streambed and floodplain and that get preserved in the geologic record. White et al. look at how riffle positions are governed by valley width variations, while Jerolmack and Brzinski find striking similarities in grain size transitions observed in rivers and dune fields. Hart et al. examine the relationship between glacial advances and downstream sediment deposition, while Sambrook Smith et al. investigate the sedimentological record of floods.

White, J., Pasternack, G., & Moir, H. (2010). Valley width variation influences riffle–pool location and persistence on a rapidly incising gravel-bed river Geomorphology, 121 (3-4), 206-221 DOI: 10.1016/j.geomorph.2010.04.012

In gravel-bed rivers, channels commonly take the form of alternating pools and riffles. During low flows, pools have deep, slow flow, while riffles are shallow and fast. During floods, pools scour deeper, while riffles may get sediment deposited. This counter-intuitive behavior is explained by channel width variations during high flow – riffles tend to be wider than pools. As the water level rises, valley width may come into play. If the river is confined by valley walls, it will be deeper and faster and able to carry more sediment than where the river is unconfined by the valley walls. By what is known as flow convergence routing, deposition occurs where the river is least width-confined and has the lowest transport capacity. In this paper, White et al. examine the location and persistence of riffles in relation to oscillations in valley width for one reach of California’s Yuba River. Using repeat aerial photography, they show that many riffle crests are located in the widest portions of the valley, and that these riffle crests were persistent for decades. Despite being downstream of several dams, the study reach was geomorphically active – with frequent overbank flows, planform change, and rapid incision (0.16 m/yr), and yet riffles located in the widest parts of the valley remained stationary. Conversely, where riffles were created by large mid-channel gravel bars, and were not in sync with valley width oscillations, they tended to be destroyed by large floods. These results support the idea that flow convergence routing is an important control on pool-riffle channel form and stability and that the common assumption of uniform flow is invalid. Many river restoration practices are based on uniform flow assumptions, and the authors assert that without considering the implications of flow convergence routing, restoration practices are fundamentally misguided.

Jerolmack, D., & Brzinski, T. (2010). Equivalence of abrupt grain-size transitions in alluvial rivers and eolian sand seas: A hypothesis Geology, 38 (8), 719-722 DOI: 10.1130/G30922.1

Rivers generally exhibit downstream fining of sediments – in which the coarsest sediments are found near the headwaters and the finest sediments are found near the mouth. This fining trend occurs exponentially downstream – rapidly in steep rivers as boulders and cobbles give way to gravel-bed streams and much more slowly in low-relief settings where sand and silt can form the streambed for hundreds of kilometers. One interesting phenomenon is that the transition from gravel-beds to sand-beds occurs much more abruptly than might be expected, and many rivers have a deficit of sediment in the coarse sand/fine gravel size ranges (1-10 mm). Downstream fining in rivers has been attributed both to abrasion and to selective deposition of the coarse particles, but laboratory abrasion studies often show much lower rates of downstream fining than are observed in real rivers. Like rivers, wind-blown eolian sediments also exhibit an abrupt grain size transition – between sand and silty loess. Jerolmack and Brzinski (2010) examine the transport and abrasion dynamics of dunes and gravel-bed rivers to understand what mechanisms might be creating the abrupt grain size transition in both systems. Maximum geomorphic work (sediment transport times frequency of the event) in both gravel-bed streams and sand dune fields occurs when Shields’ shear stress is only less than two times greater than the stress required to mobilize the sediment. Similarly, the two systems are comparable in terms of abrasion collision dynamics, as estimated by the collision Stokes number. Abrasion produces smaller particles as big ones collide into each other and chip small pieces off. Abrasion efficiency decreases rapidly as grain size decreases, resulting in a minimum sediment size, which for rivers is in the range of ~10 mm gravel. Meanwhile, the small chipped-off pieces are sand-size (less than 2 mm) and continue to be transported downstream in suspension when the gravel settles out. As Jerolmack and Brzinski conclude “abrasion produces a bimodal grain-size distribution while sorting acts to segregate these grains to produce an abrupt transition.” While the authors acknowledge that the sediment transport in rivers and air are each subject to different constraints, they maintain that their abrasion/sorting hypothesis may explain the longitudinal sediment distribution in both environments. They also propose several additional areas where work is needed to test their hypothesis – including studies of gravel and sand source regions in multiple river systems.

Hart, S., Clague, J., & Smith, D. (2010). Dendrogeomorphic reconstruction of Little Ice Age paraglacial activity in the vicinity of the Homathko Icefield, British Columbia Coast Mountains, Canada Geomorphology, 121 (3-4), 197-205 DOI: 10.1016/j.geomorph.2010.04.011

Paraglacial geomorphology refers to landscape forms and processes that occur in areas adjacent to glaciers and the movement of large amounts of sediment from valley slopes to river systems that accompanies glacial advances and retreats. How rapidly this sediment is transferred from glacial areas to paraglacial areas is of interest to geomorphologists working in alpine and polar landscapes. This paper uses dendrochronology and geomorphic mapping to investigate paraglacial geomorphology and the time lags between glacier activity and downstream sediment deposition in the southern British Columbia coastal mountains. In 1997, a moraine dam overtopped and breached, draining a proglacial lake, and flooding the rivers downstream. The floodwaters eroded through 4 m of paraglacial valley-fill units with in-situ tree stumps and woody detritus. Tree rings from the stumps indicate that they died because of rapid burial by overlying sediment (i.e., from flood deposits). Multiple valley-fill deposits indicate and provide dates for six aggradation events between 718 and 1794, and correlation of these dates with independent regional glacial chronologies suggest that all of the aggradation events occurring during periods of glacier advance. This suggests that river valleys downstream of glacier limits were affected by synchronous redistribution of sand and silt from glacial forefields, moraines, and valley slopes when climates were cold and wet and glaciers were active, and that there was little lag between glacial erosion and advance and sediment delivery to downstream areas. This is paper is cool because it provides data that speak to the rapid delivery of sediment from glacial to paraglacial areas and because it uses trees in the paraglacial deposits themselves to give a much longer dendrochronology than can usually be obtained in glaciated areas.

Sambrook Smith, G., Best, J., Ashworth, P., Lane, S., Parker, N., Lunt, I., Thomas, R., & Simpson, C. (2010). Can we distinguish flood frequency and magnitude in the sedimentological record of rivers? Geology, 38 (7), 579-582 DOI: 10.1130/G30861.1

Over time, the flows that commit the most geomorphic work are those moderately high flows that occur moderately frequently – generally every 1-2 years. But large floods – like those that occur every 50 to 500 years on average – can dramatically reshape the form of the river and floodplain. These generalizations are based on observations of modern river systems, but how do they apply to the sedimentological record that will preserve the river’s legacy for future eons? Using detailed digital elevation models (DEMs) and ground penetrating radar (GPR) surveys of the South Saskatchewan River, Sambrook Smith et al. investigated the legacy of a flood with a 1 in 40 year recurrence interval (i.e., 0.31% probability). What they found is that while there were significant erosion and deposition across the braided river, the depth of sediment scoured or deposited was not substantially greater than that observed following much smaller floods. The larger high bars steered flow around them, even during the flood, forcing channel erosion, but only facilitating less than 0.5 m of deposition on bar surfaces. While erosion and deposition was spatially extensive in the large flood, the style and scale of the deposits was similar to small floods. Thus, there was no distinct legacy of this flood event that would differentiate it from smaller events in the geologic record. More generally, the authors conclude that in rivers which can widen during floods (i.e.,are not valley confined), there may be little preserved evidence in the sedimentological record to identify low-frequency high magnitude events from run-of-the-mill annual floods. The present is the key to the past, but the past that is preserved in the geologic record loses some of the sweet details of the dynamic events that act on the earth’s surface.

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.*

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

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.