Decadal‐Scale Trajectories of Land Cover Change Along the Colorado and San Juan Rivers in Response to Declining Water Storage in Lake Powell Reservoir

Abstract As a result of drought and consumptive water use, reservoirs worldwide are experiencing declines in storage. The landscapes exposed by falling water levels may be colonized by novel vegetation communities in terms of species composition and succession, and/or reworked by geomorphic processes at sub‐annual to decadal timescales. Despite research on the response of downstream rivers to dam and reservoir operations, we know surprisingly little regarding the eco‐geomorphic processes that shape backwaters upstream of reservoirs. Here we use a decadal‐scale record of aerial imagery and support vector machine learning to classify land cover change along more than 140 km of the Colorado and San Juan Rivers within Lake Powell Reservoir in Utah. Our results reveal a consistent trajectory of land cover change, wherein declining water levels expose bare sediment, which is subsequently colonized by vegetation. With continued water level declines, vegetation exhibits defoliation consistent with water stress. Similar patterns of landscape evolution were observed with regard to exposure age of formerly inundated surfaces on both rivers, wherein older surfaces were marked by bare sediment and/or stressed vegetation. Divergence in response to reservoir drawdown was observed with regard to the downstream propagation rate of land cover changes. Specifically, on the steep‐and‐narrow Colorado River, the trajectory of bare sediment to green vegetation to stressed vegetation occurred more rapidly as water levels declined than on the San Juan River. The observations detailed here may guide reservoir management to promote recruitment of desirable land cover types (e.g., native and/or complex ecosystems) under shifting water supply.