Coupled Vegetation and Sediment Changes Along the San Juan River Arm of Lake Powell Reservoir
Cynthia Dott1*, Gary Gianniny2 and Alan Kasprak3
1Fort Lewis College, Dept. of Biology, Durango, CO, USA; dott_c@fortlewis.edu
2Fort Lewis College, Dept. of Geosciences, Durango, CO, USA; gianniny_g@fortlewis.edu
3Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, USA; kaspraka@oregonstate.edu
During prolonged and ongoing drought in western North America, water levels in reservoirs like Lake Powell on the Colorado River have been dropping for the last two decades and exposing new land that is open to colonization by riparian plants. Thanks to logistical support from The Returning Rapids Project, we have been able to travel into the remote and hard-to-access San Juan arm of Lake Powell Reservoir, where the ages of exposed sediments range from 1995 to 2024 (and younger). From these expeditions, we have made initial observations of several interesting patterns in the vegetation colonizing the exposed reservoir sediments, including three main take-aways that will inform more in-depth future research.
First, surprisingly, the trees and shrubs that dominate the newly formed river corridor are primarily native species (coyote willow, Fremont cottonwood, Gooddings willow) with invasive non-natives (tamarisk, Russian olive) being much less common. Seth Arens work has documented similar patterns in side canyons of the Colorado Arm of Powell Reservoir (see Arens 2024 REW abstract).
Second: however, the river channel is so dynamic as it cuts through erodible reservoir sediments that it often maroons large areas of previously moist habitat. On portions of the San Juan arm of the reservoir, a wider valley exists due to geologic fold-parallel valleys and more easily erodible bedrock strata. These wider valley segments with more extensive Dominy Formation (reservoir sediment) deposits create large areas of drought-stressed habitat. Tamarisk was especially prevalent in these areas, and is now often stressed and dying, leaving those areas open to other invasive weeds. (also see Kasprak et al. 2026 Jour. Geophysical Resch.: Biogeosci.)
Third, the plant succession story seen in the drying reservoir operates in two directions: One, older plant communities establish up-river with younger ones down river as the reservoir drops; and two, laterally, with older vegetation on high Dominy Fm. terraces and youngest along the inset channel margins.
Our baseline observations are important as we expect vegetation composition in these new habitats to continue to change over the coming decades of anticipated lowering water levels.