I study how dams affect sediment transport and landscape processes in river corridors across the Colorado Plateau. Recently, much of my work has taken place in Grand Canyon.
Grand Canyon is an 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 Lees 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.
Mapping 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 GIS project, I am synthesizing multiple large-scale geospatial 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 115 miles of the Colorado River from 1965 to the present.
An example of mapped sand from several remote sensing techniques along a 2 km segment of the Colorado River in Grand Canyon National Park. Mapped sand extent ranges from the bottom of the river channel to the historic flood of record in Grand Canyon.
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.
In 2018, we published a paper in Progress in Physical Geography that details the results of this work along a shorter, ~30 mile reach of the Colorado River. You can read that manuscript here.
We're currently in the process of upscaling this initial ~30 mile project to 115 miles of the Colorado River in Grand Canyon using recently-collect GIS data. In addition, we're moving beyond simply quantifying how land cover change has shifted in the past, and we're now examining how the landscape of Grand Canyon will change in the future. We're analyzing 15 potential future water release patterns from Glen Canyon Dam to understand the implications of river management on land cover using models that predict vegetation growth and bare sand extent as a function of river flows. This work will enable stakeholders to forecast how altered hydrology will affect the landscape - and what we can do about it.
Projected changes in exposed sand extent over a 30-mile study reach under each of seven alternative discharge regimes from Glen Canyon Dam proposed under the 2016 Long Term Experimental and Management Plan Environmental Impact Statement process (top); bottom panel shows projected loss of exposed sand area under the selected alternative (Alt. D) over the next 20 years due to vegetation encroachment and inundation by Colorado River flows.