Riparian Vegetated Area in Pre‐Dam, Post‐Dam, and Environmental Flow Periods in Canyonlands National Park From 1940 to 2022

ABSTRACT The Upper Colorado River Basin is the principal water supply of the western United States and includes a series of canyons that provide habitat for disproportionate numbers of flora and fauna. Following the closing of Flaming Gorge and Blue Mesa dams in 1963 and 1966, decreases in peak flows and elevated base flows allowed extensive vegetation encroachment, channel narrowing, and channel simplification. Since 1992, reservoir releases have been modified to increase the ratio of peak to base flows for environmental reasons, including protection of endangered fish. We used remote imagery from 1940 to 2022 to examine rates of vegetation encroachment along three river reaches in Canyonlands National Park during the pre‐dam (1940–1966), post‐dam (1967–1992), and environmental flows (1993–2022) periods. We found an increase in the vegetated area along the Colorado and Green Rivers upstream of their confluence since 1940. We documented a 6.1% and 4.0% increase in vegetated area in the post‐dam period and a 19.5% and 6.5% increase in vegetated area in the environmental flows period on the Colorado and Green Rivers, respectively. The Cataract Canyon reach (Colorado River below the confluence) has been stable since 1966. All three river reaches showed the slowest period of vegetation encroachment, indicative of channel narrowing, in the last 16 years of environmental flows that included a large peakflow in 2011. Environmental flows that mimic the natural hydrograph have not reversed decreases in peak flow and channel width, due in part to decreasing runoff and increasing flow diversion. Flow alterations that reduce the spring peak could cause further narrowing.