A nice British video explaining the connection between rivers and groundwater. I can’t get the embed to work, so you’ll have to click through to watch: http://www.groundwateruk.org/How-Rivers-Work-Role-of-Groundwater.aspx This is why I say I study rivers AND groundwater – if you want to understand how water moves through a watershed, you’ve got to …
Currently browsing category
Cross-posted at Highly Allochthonous (for obvious reasons)
Allochthonous may have some obscure usage related to rocks, but in ecology, allochthonous material is a major concept that underpins thinking about nutrient cycling and food web dynamics. In its most general definition, allochthonous material is something imported into an ecosystem from outside of it. Usually, ecologists are thinking about organic matter and the nutrients (C, N, and P) that come with it.
In streams, allochthonous material includes leaves that fall or are washed into the water and branches and trees that topple into the stream. These would both be called “coarse particulate organic matter” or “CPOM” in the lingo of stream ecologists. In headwater streams, especially in forested areas, there is a lot of CPOM, and the community of aquatic organisms has a high proportion of “shredders” – the critters that that feed on CPOM and break it up into tinier bits called “fine particulate organic matter” or FPOM. In turn, organisms called “collectors” make use of the FPOM by filtering it from the water or accessing it in the sediments. [Allochthonous material can also include dissolved organic matter (DOM) carried into the stream by overland or subsurface flow.]
As you move downstream from the headwaters toward medium-sized rivers, the stream channel becomes wider and allochthonous input from overhanging forest and riparian vegetation decreases in abundance and importance relative to primary production (or autochthonous organic mattter) driven by available sunlight. In other words, algae and aquatic plants become the most important food producers. Organisms called “grazers” who scrape algae from surfaces become an important component of the aquatic food web, and grazers become less abundant.
Farther downstream, the ecosystem shifts again, as there is so much FPOM moving with the water and sediment, that collecters far outnumber either shredders or grazers. There’s still allochthonous input from the banks and being carried in by tributaries, and there’s still primary production occurring in the stream, but upstream “system inefficiency” or “leakage” in the processing of nutrients and organic material lets large river aquatic communities be based on material washing in from upstream.
The adjustment of river ecosystems in a downstream fashion that I’ve described above is part of the “river continuum concept”, described by Vannote and colleagues in 1980 in the Canadian Journal of Fisheries and Aquatic Science, and it is one of the unifying principles of modern stream ecology. At its root, the river continuum concept is driven by the relative proportion of allochthonous to autochthonous organic matter inputs to the stream.
While I’m not an ecologist, I was raised by one and I work with them, so when I hear the word allochthonous, I pictures leaves and logs in streams, rather than anything to do with rocks. So, I’ll end this post with some nice pictures of allochthonous material.
Vannote, R., Minshall, G., Cummins, K., Sedell, J., & Cushing, C. (1980). The River Continuum Concept Canadian Journal of Fisheries and Aquatic Sciences, 37 (1), 130-137 DOI: 10.1139/f80-017
Cross-posted at Highly Allochthonous
Got flood fatigue yet? Too bad, because the wet weather and the high water keeps coming. Here is a quick round up of the notable flood-related news of the week.
- My post, ‘Levees and the illusion of flood control‘, has been nominated for the 2011 3 Quarks Daily Prize in Science.
Right now, 20 semi-finalists are being chosen by a round of public voting that ends tomorrow (June 11). If you haven’t voted yet, please take a moment to vote for me. (It’s easy; no registration required.) If my flood post didn’t float your boat, there is also a bevy of other good Earth Science blogging in the running, as summarized by Lab Lemming. Please head over to vote:
- Columbia’s Earth Institute has a must-read piece on alternative to levees as a river- and flood-management tool – highlighting the Dutch approach to “making room for the rivers” and how that might be applied in the US.
- The US Army Corps of Engineers is slowly closing the gates at the Morganza Floodway in Louisiana. Only one gate is still open, but you can still see the inundation on today’s NASA Earth Observatory image.
- Floodwaters have retreated from most of the Bird’s Point Floodway, as shown by this fantastic pair of images from NASA. Nonetheless, land owners in the floodway are suing the Corps of Engineers for damages.
- This spring’s floods have prompted some public musing about alternative ways to manage the Mississippi. America’s Waterway has been active on Twitter during the floods, asking for a basin-wide approach to river management.
- Worth a watch. Reeks with hubris but a good overview of this year’s Mississippi flood: USACE video
Update: I got voted into the semi-finals and picked by the editors for inclusion in the final round of the contest. Thanks for your support.
Heavy snowpacks in the Missouri River watershed (an areally large, but volumetrically smaller contributor to the Mississippi) have led to near-record flooding that is on-going along its whole length from Montana to Missouri. It’s not getting as much media attention as the Mississippi River, but water levels may stay above flood stage for months. Right now there are heavy rains occurring in parts of the basin, with more rain in the forecast, which will only add to flood problems.
Like the Mississippi, the Missouri is heavily managed by the Corps of Engineers, which is taking some criticism for residents in affected cities. There have also been evacuations because of seepage under levees and concerns about the possibility of failure. Like all big river/developed world flood stories, this one is a complicated mix of huge volumes of water, complicated multi-purpose river management plans, and unwise historical floodplain development.
- Lots of aerial photos of Missouri River Flooding near Pierre, SD
http://www.disasterrecovery.sd.gov/photogal/Pierre/gallery_Jun6.aspx(Check out the whirlpool!)
For months, China has been stricken by its most intense drought in 60 years, but right now it’s too much, not too little, water that is the problem. Flooding since the 1st of the month has affected East China’s Jiangxi Province and 12 provinces in central and southern China, and more rain is in the forecast for many areas. Intense rains over the last few days have caused the evacuation of more than 100,000 people and killed at least 54.
The Flood Observatory is also reporting on-going flooding in Colombia, the Philippines, Algeria, Haiti and the Dominican Republic, Canada, India, and Upstate New York/Vermont’s Lake Champlain area. In every one of these places, people are losing their homes and lives. While volcanoes and earthquakes shake things up spectacularly now and again, every single day, somewhere in the world, there’s a devastating flood going on.
Cross-posted at Highly AllochthonousOne week ago today (28 May 2011), I had the chance to explore the lingering flooding along the Mississippi River and its tributary Big Muddy River in southern Illinois. The area was long past its crest; it is upriver of Cairo and the Birds Point Floodway. Around Carbondale, evidence of the recent high water was still visible in all of the drainages, but the water was back well within the stream banks. Closer to the confluence with the Mississippi though, high water levels on the Mississippi were still forcing backwater flooding of the floodplain and the Big Muddy River.
Driving and hiking along the escarpment of the LaRue-Pine Research Natural Area afforded expansive views of the flooding – and the remnant landscapes of previous millenia of river activity.
Once we descended from the hills and onto the floodplain, we were immediately greeted by floodwaters.
Driving away from the hills towards the Mississippi, our road took us along the top of the levee, giving us close up views of the effects of leveeing, levee repairs, and local wildlife.
After crossing the Big Muddy River, we drove along a state highway that was not atop a levee, and only a few feet above flooded fields. Egrets and herons were everywhere in the standing water, and a pleasant breeze whipped up waves on the water. But we were reminded that this scene was normally not so watery…in the image below, you might be able to see a center pivot irrigation line in the field, standing in the flood waters.
Finally we reached the Mississippi itself, in Grand Tower, Illinois. The river was definitely high, but open for business – we watched a tow and barges go by. The town of Grand Tower is situated immediately adjacent to the Mississippi – and protected by a big levee. Near the north end of town, the levee was a few feet lower than the rest, and here a metal floodwall had been constructed atop the levee. There was also evidence that a pumping operation had been set up – to pump water from behind the levee back into the river. Whether this pumping was necessitated by seepage or localized ponding, I couldn’t tell. But here, in a sleepy little town on the Mississippi, the effects of our efforts to keep floodwaters off the floodplain were in full display.
Cross-posted at Highly Allochthonous
I’ve got a shiny new Emriver Em2 river processes simulator (i.e., stream table), thanks to departmental equipment funds and enthusiastic colleagues. I’ve been on sabbatical this semester and away from campus, so I haven’t had a chance to play with it yet, but it is enticing me to return. I’ll be teaching Fluvial Processes fall semester, so I’m sure that my students and I will get plenty of chances to explore all of the nifty ways in which we can demonstrate and experiment with fluvial geomorphology. I’m also playing with ideas for using the Emriver model in my hydrogeology class in the spring. I think it will be a perfect way to demonstrate ideas of hyporheic flow, seepage erosion, and break through curves in tracer tests. I think my colleagues are planning to use it in sedimentology, geomorphology and hydrology classes, and one colleague may take it with him when he does outreach activities. I’m sure we will come up with even more uses for it once we get started.
My appetite for experiment with the stream table was whetted by a recent visit to Carbondale, Illinois and the base of operations for Little River Research and Design (LRRD). Steve Gough is the owner of LRRD, the mastermind behind the Emriver models, and a genuinely fantastically nice person. Motivated by the idea that hands on education about stream processes is the best way to instill respect for and promote protection of streams and rivers, Steve has poured himself into making the best stream table on the market, and making it affordable enough to for people like me to get their hands on.
Personally, I’d always been somewhat underwhelmed by teaching- and demonstration-grade stream tables before seeing the Emriver ones. Partly it was because I’d seen and read about big research flumes, like those at the St. Anthony Falls Lab and Johns Hopkins. But another part of it was that every time I had a chance to play with a home-built stream table I was frustrated by what it couldn’t do. Principally, most stream tables don’t do a very good job of reproducing the meandering behavior of lowland streams. This has even been an area of active and high profile research in the fluvial geomorphology community. Steve’s use of low density plastic beads instead of quartz sand solves that problem pretty nicely, though there’s definitely still some braiding going on.
In addition to the 2-m long Em2 model that I have, LRRD also makes an extremely cool and versatile 4-m long model Em4. With beads colored by size, you can see (and measure) the sorting and selective transport of sediments. You can tilt the table laterally – simulating differential uplift/subsidence across the basin. There’s even a groundwater feed and extraction system! This model is pretty much as cool as I can imagine – at least short of the big research flumes mentioned above.
I can personally attest that this stream table model has the versatility to entrance both a PhD and a preschooler for more than two hours…and the preschooler wanted to go back the next day! Below I’ve added some shots of the Em4 in action. What geomorphic processes do you see?
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.
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.
In 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.
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.*
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.
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.