At the USGS Grand Canyon Monitoring and Research Center, much of my work is focused on understanding how river regulation affects the landscape and sediment transport processes in Grand Canyon. It’s a really interesting place to work, because sediment (mostly sand) moves between the river, sandbars, and upland dune fields through a variety of different mechanisms, or pathways: transport by water (fluvial or alluvial), wind (aeolian), and by hillslope processes like sheet wash or gullying.
Pathways of sediment transport in Grand Canyon.
Since Glen Canyon Dam was completed in 1963, large floods in Grand Canyon have disappeared, as has much of the sediment that was supplied to the river prior to dam construction.
Hydrologic alteration as seen at Lee's Ferry (about 15 miles downstream of Glen Canyon Dam) from 1922-2016.
Scientists have noticed a couple major changes in the landscape: the evacuation of sand from the river channel and sandbars lining the channel banks, and also that vegetation is becoming established in much greater abundance than it was during pre-dam times.
Matched photographs from 1890 (Stanton expedition) and 2010 about 50 miles downstream from Lee's Ferry. Note increase of riparian vegetation onto previously bare surfaces along the river.
My research in Grand Canyon falls into two major focus areas:Developing Software for Big-Data Analysis of Landscape Response to River Regulation
At the moment, geomorphologists are faced with a good problem to have, but a problem nonetheless: we’re able to collect more topographic data, at higher resolutions, than we can ever hope to analyze in a timely manner. The advent of new technologies like terrestrial laser scanners and structure-from-motion photogrammetry mean that we can capture the form of landscapes at channel reach scales, but because of the time it takes to analyze those data, our ability to make fundamental statements about landscape evolution and response to human activities like land use, agricultural development, or river regulation is quite limited.
To address this, I’ve been working with Josh Caster and Joel Sankey (both at the USGS), and Sara Bangen (at Utah State University) to develop open-source software for rapidly and objectively discerning the sediment transport mechanisms that drive topographic change. These mechanisms can include pathways like fluvial or alluvial transport, mass failure, and aeolian (i.e., windblown) sediment transport. Using the software we’ve developed, we’re able to analyze repeat topographic datasets as they’re collected and quantify the relative role of sediment transport pathways in driving topographic change, in the process uncovering how those pathways change in response to anthropogenic landscape alteration or river regulation.
Inferred mechanisms of geomorphic change at a site along the Colorado River. Mechanisms were computed using original software for rapid and obective analysis of repeat topographic data.
The Relative Roles of Hydrologic Alteration and Vegetation Encroachment on Landscape Change along the Colorado River
Nearly every river and watershed that we look at is altered in multiple ways; it’s rare to find just one cause of disturbance, and that certainly applies to the Colorado River in Grand Canyon. One of my main research interests lies in disentangling the competing drivers of landscape change: for example, separating the influence of natural and anthropogenic processes in delivering sediment to rivers.
Since 1963, Glen Canyon Dam has altered the flow of the river for hydropower generation, increasing baseflows and eliminating spring snowmelt floods. At the same time, owing to the lack of geomorphically-effective floods, vegetation has colonized many surfaces that were previously composed of bare sand.
The result is that the amount of sand along the river corridor that’s available for transport by wind has been reduced due to a combination of hydrologic alteration and vegetation encroachment. Aeolian, or windblown, sediment transport was historically a vital process in Grand Canyon, as it is in many other dryland rivers, for transferring sediment from the active river channel to upland areas where it has roles in vegetation colonization, bird/animal habitat, and in the preservation of archaeological sites.
The same matched photographs as above, just to hammer home the decrease in bare sand area along the river corridor.
In this big data project, I am synthesizing multiple large-scale datasets of sand extent from disparate sources including multibeam and singlebeam sonar, total station surveys, and automated land cover mapping from aerial photography to map the complete extent of sand coverage from the channel bed to the historic high water line along 30 miles of the Colorado River from 1965 to the present. In combination with historic vegetation mapping and the hydrologic record of the Colorado River dating from 1922 to the present, I am quantifying the individual and cumulative influence of hydrologic alteration and vegetation growth on reducing bare sand cover along the Colorado River.
Alterations to exposed sand area along a ~30 mile reach of the Colorado River. Relative to pre-dam period (1922-1963), the combination of hydrologic alteration and vegetation encroachment have reduced the exposed sand area by 27%. Note the 'slow and steady' decrease in exposed sand area in the post-dam period as vegetation colonized previously bare surfaces.