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fluvial geomorphology

Development of hyporheic exchange and nutrient uptake following stream restoration

Next week, the Watershed Hydrology Lab will be well represented at the CUAHSI 2014 Biennial Colloquium. We’ll be presenting four posters, so here come the abstracts…

Development of hyporheic exchange and nutrient uptake following stream restoration

Stuart Baker and Anne Jefferson

Stream restoration is a multi-million dollar industry in Ohio, with major goals of improving water quality and degraded habitat. Yet restoration often falls short of significant improvements in water quality and biodiversity. It is thus important to improve the theory and practice of stream restoration in order to achieve greater benefits per dollar spent, yet there are limited data and understanding of the physical and biogeochemical responses to restoration that constrain the potential for water quality and ecological improvements. Hyporheic exchange, the flow of water into and out of the streambed, is an important stream process that serves critical roles in naturally functioning streams, allowing for stream water to participate with the substrate in various processes. Hyporheic flowpaths can be altered by the transport of fine sediment through the stream bed and are thus susceptible to changes in sediment regime and hydraulics, as well as the changes wrought by construction of a restoration project. The goal of this research is to determine the effectiveness of restoration in enhancing hyporheic flow and associated biogeochemical processes to improve water quality. Preliminary results from Kelsey Creek, OH, a second-order stream restored in August 2013, show a decrease in average hydraulic conductivity but an increase in heterogeneity from pre-restoration (geometric mean 8.47×10-5 m/s, range 1.18×10-6-1.19×10-3) to post-restoration (geometric mean 4.41×10-5 m/s, range 2.67×10-5-3.05×10-4) in piezometer nests through large constructed riffle structures. These piezometers also indicate dominance of downwelling throughout riffle structures with only isolated locations of upwelling. Transient storage and hyporheic exchange will be measured with resazurin injections for comparison between pre-restoration and post-restoration, and nutrient injections of NH4Cl at time points following the restoration will compare the nitrogen uptake rates of the restored reach to an unrestored reach downstream. Additional sites are planned for study to include restoration projects of different ages to examine the development of hyporheic exchange and biogeochemistry after completion of restoration projects.

After the dam comes out: groundwater-stream interactions and water quality impacts of former reservoir sites

Next week, the Watershed Hydrology Lab will be well represented at the CUAHSI 2014 Biennial Colloquium. We’ll be presenting four posters, so here come the abstracts…


After the dam comes out: groundwater-stream interactions and water quality impacts of former reservoir sites

Krista Brown and Anne Jefferson

Over that past decade, dam removals have become increasingly popular, as many dams near the end of their life expectancy. With an increasing number of anticipated dam removals coming in the near future this study aims to develop an understanding of groundwater-stream interactions and water quality in former reservoir sites after dam removals have occurred. Low head dams (~2 m) were removed in 2009 from Plum Creek in Kent, Portage County, Ohio and on Kelsey Creek in Cuyahoga Falls, Summit County, Ohio. Kelsey Creek reservoir has been unaltered since the dam removal and consists of a stream channel flowing through riparian- wetland environments, while Plum Creek reservoir underwent channel restoration in 2011. At Kelsey Creek, 20 piezometers and 3 wells were installed in the stream and riparian areas. Pressure transducers were also deployed in each well and stream from November 20, 2013 to January 5, 2014. Hydraulic conductivity was calculated using the Hvorslev method. Since October 2013, hydraulic heads have been recorded semi-weekly and water samples have been taken in the wells and stream. Water quality is being evaluated with field-measured pH, temperature, specific conductance, and dissolved oxygen, and ion chromatography of chloride, bromide, nitrate, sulfate and phosphate concentrations. Plum Creek is being used to understand the water quality effects of channel restoration at former reservoir sites.
At Kelsey Creek, hydraulic conductivity ranges five magnitudes, from 10?2 to 10?6 m/s, but wells near the channel, in an off-channel wetland, and on an adjacent hillslope respond similarly during high flow events. However, the well closest to the stream shows substantial variability in specific conductance, indicating bidirectional groundwater-stream exchange. Despite the wetlands and presumed greater groundwater-stream exchange in the unrestored Kelsey Creek, stream water quality is similar to the restored Plum Creek site. This suggests that the water quality measures considered here should not determine whether to restore channels within former reservoir sites. Findings from this research may be applicable when considering options for future dam removal sites.

Abstract: Evaluating restoration effects on transient storage and hyporheic exchange in urban and forested streams

A third abstract from our group for the 2012 Geological Society of America meeting:

EVALUATING RESTORATION EFFECTS ON TRANSIENT STORAGE AND HYPORHEIC EXCHANGE IN URBAN AND FORESTED STREAMS

OSYPIAN, Mackenzie L., Civil Engineering, University of North Carolina at Charlotte, Charlotte, NC 28262, mosypian@uncc.edu, JEFFERSON, Anne J., Department of Geology, Kent State University, 221 McGilvrey Hall, Kent, OH 44240, and CLINTON, Sandra, Department of Geography and Earth Sciences, University of North Carolina at Charlotte, Charlotte, NC 28223

Millions of dollars are spent each year on restoration projects designed to improve stream habitat, but few studies have investigated effects of restoration on hyporheic exchange and transient storage. Stream water-groundwater interactions and transient storage in four second-order streams (urban/forest; restored/urestored) were studied by measuring geomorphology, streambed vertical head gradients and water fluxes, and by using conservative, impulse-loaded tracer studies along with the OTIS model. The magnitude of upwelling and down welling was observed to be greatest in the restored urban stream, which contains large step structures, while the smallest gradients were observed in the unrestored urban stream, which is incised to bedrock. OTIS results show that the 120 m unrestored urban reach with a debris dam has an average transient storage of 1.8×10^-2 m2/m and an ? of 9.5×10^-4 s^-1 while a 55m restored forested reach with log sills has an average transient storage of 8.3×10^-2 m2/m and an ? of 1.5×10^-4 s^-1. Based on these results, we conclude that restoration changes transient storage metrics, and ongoing work aims to understand how these changes affect ecosystem health.

Spring Break: tracer injection in Beaver Dam Creek

Spring Break: tracer injection in Beaver Dam Creek

Spring Break: tracer injection in Beaver Dam Creek

Some of our students are in the field this week, injecting Cl- and Br- into a restored reach and an unrestored reach in tributaries of Beaver Dam Creek. Our goal is to understand the role of wood jams versus restoration structures in promoting stream-hyporheic exchange.

In the photo are Alea, Xueying, and Mackenzie. Photo by Brittany. They’ve got it so capably handled they didn’t even need Sandra or I out there with them today, but I’m going tomorrow for an excuse to be in the field as much as anything.

Spring Break: tracer injection in Beaver Dam Creek

Some of our students are in the field this week, injecting Cl- and Br- into a restored reach and an unrestored reach in tributaries of Beaver Dam Creek. Our goal is to understand the role of wood jams versus restoration structures in promoting stream-hyporheic exchange.

In the photo are Alea, Xueying, and Mackenzie. Photo by Brittany. They’ve got it so capably handled they didn’t even need Sandra or I out there with them today, but I’m going tomorrow for an excuse to be in the field as much as anything.

Spring Break: tracer injection in Beaver Dam Creek

Some of our students are in the field this week, injecting Cl- and Br- into a restored reach and an unrestored reach in tributaries of Beaver Dam Creek. Our goal is to understand the role of wood jams versus restoration structures in promoting stream-hyporheic exchange.

In the photo are Alea, Xueying, and Mackenzie. Photo by Brittany. They’ve got it so capably handled they didn’t even need Sandra or I out there with them today, but I’m going tomorrow for an excuse to be in the field as much as anything.

GSA 2011 abstract: Spatial variability in groundwater-stream interactions in first-order North Carolina Piedmont streams

At the 2011 GSA Meeting in Minneapolis next week, I’ll be presenting the following talk in the session “Monitoring and Understanding Our Landscape for the Long Term through Small Catchment Studies I: A Tribute to the Career of Owen P. Bricker.” My talk is in Minneapolis Convention Center: Room M100FG, on Wednesday, 12 October 2011 at 9:30 am.

Spatial variability in groundwater-stream interactions in first-order North Carolina Piedmont streams

JEFFERSON, Anne J. and MOORE, Cameron, Dept. of Geography and Earth Sciences, University of North Carolina at Charlotte,

Groundwater upwelling and hyporheic exchange are spatially variable in three first-order Piedmont streams, resulting in variable discharge, water chemistry and temperature. Stream gradient, valley confinement, and woody debris jams appear to be the major controls on the distribution and size of upwelling zones. Temperature and specific conductance values at 25 m intervals on 18 dates revealed distinct zones of groundwater-stream interaction, confirmed by discharge and piezometer measurements. Baseflow accumulates unevenly along the streams, with upper reaches in confined valleys generally gaining discharge more rapidly than lower reaches. Elevated calcium concentrations occur in groundwater upwelling zones, such as in a 50 m reach in which baseflow triples. Near their mouths, where the streams reach a river floodplain, baseflow quantity and chemistry may be influenced by a larger groundwater system. At a smaller scale, spatial variability in stream chemistry and streambed hydraulic gradients are dominantly controlled by the size and position of woody debris jams. Fine sediment wedges extend 5-15 m upstream of the 0.25-1 m high jams, and strong down-welling hydraulic gradients occur in these areas. Upwelling of water with higher specific conductance and moderated temperatures occurs downstream of the jams. Nitrate concentrations decreased up to 50% immediately below large woody debris jams, while ammonium concentrations tended to be highest there.

Rapid urbanization in the Carolina Piedmont is drastically altering headwater catchments, but well-documented reference watersheds are lacking. The measurements described above are from three first-order streams in forested watersheds, with permanent protection by a land conservancy. Their hydrology and water chemistry demonstrates the rich spatial variability of Piedmont headwater streams, and we hope that long-term study of these sites provides useful understanding for stream restoration and watershed management.

Debris jam and sediment in a first order Deep Creek at Redlair. Photo by Cameron Moore.

Debris jam and sediment in a first order Deep Creek at Redlair. Photo by Cameron Moore.

Simulating river processes…ooh shiny, stream table!

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.

Em4 model at work.

Em4 model at work in promoting discussion about whether the arrow points to a good place to build a house.


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.

Steve Gough, Anne Jefferson and a research assistant in front of LRRD, May 2011

Steve Gough, Anne Jefferson and a research assistant in front of LRRD, May 2011

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.

[youtube http://www.youtube.com/watch?v=ZBoeI3ZX7us&w=480&h=390]

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?

Em4 looking downstream

Looking dowstream, I see a transition from "bedrock" to alluvial substrate, a really nice train of standing waves, meandering, a floodwall, and some sort of infrastructure project in the floodplain gone horribly wrong.

base level fall

A sudden base level fall is driving incision through an old delta. The dark red sediment is the finest grain size.

tracer test

Green dye was used to examine hyporheic flow transversely through a mid-channel bar. Now blue dye is being added to look for zones of in-channel transient storage.

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.

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.