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 hydrologic drought and increasing consumptive water use, reservoirs worldwide are experiencing declines in storage. The emergent 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 river reaches to dam and reservoir operations, we know little regarding the eco-geomorphic processes that shape backwater reaches 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 kilometers of the Colorado and San Juan River backwaters within Lake Powell in Utah. Our results along both rivers 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 plant water stress. Similar patterns of landscape evolution were observed with regard to exposure age of formerly-inundated surfaces on both study rivers, wherein older surfaces were marked by bare sediment and/or stressed vegetation. Divergence in response to reservoir drawdown between systems was observed with regard to the downstream propagation rate of land cover changes. Specifically, on the steep-and-narrow Colorado River, the successional 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 in this study may guide reservoir management to promote recruitment of desirable land cover types (e.g., native and/or complex ecosystems) under shifting water supply.

Alan Kasprak

Physical Geographer, Geomorphologist, Geospatial Scientist

I study how rivers work, how we affect them, and the ways that we can restore their physical and ecological processes.