Revealing the eco-geomorphic dynamics of emergent reservoir landscapes using remote sensing data
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As the world's reservoirs experience drawdowns from reduced precipitation and increased water demand, vast expanses of sediment are being exposed and colonized by vegetation. Within Lake Powell, the second largest reservoir in the U.S., drought in the Colorado River Basin has led to a rapid decrease in water level since ~2000. Here we use multispectral imagery and airborne lidar to quantify vegetation dynamics across more than 200 km2 of backwater sediment along the geomorphically-distinct Colorado and San Juan Rivers between 2000-present. Our findings indicate that emergent sediment deposits, which are up to 50 m thick, are colonized rapidly by mesic riparian herbaceous plants and woody vegetation. Plant communities consist of both non-native species including tamarisk (Tamarix spp.) and Russian olive (Eleagnus angustifolia), along with native shrubs and trees including willow (Salix spp.) and cottonwood (Populus spp). With continued decline in the reservoir-influenced water table, stranding of vegetation leads to widespread stress and mortality on the highest exposure surfaces. However, native vegetation continues to colonize and persist at greater rates than non-native plants on channel-proximal surfaces. Divergence in valley gradient and width, driven by contrasting geologic setting, has led to more pronounced and rapid de-coupling of upland surfaces from the water table along the Colorado River versus the San Juan River. These physiographic differences have manifested as widespread plant mortality in response to water level fall in the steep and narrow Colorado River valley. In contrast, vegetation communities persist over decadal timescales in the wider, flatter San Juan River valley, where field observations indicate that native woody species are preferentially colonizing exposed sediment deposits. Quantifying the dynamics of sediment exposure and vegetation, and controls on these processes, provides opportunities for reservoir management to encourage the maintenance of native and/or culturally-valued ecosystems. The views expressed in this abstract are those of the authors and do not necessarily represent the views or policies of the U.S. Government.