Across the southwestern U.S., ongoing hydroclimatic drought is compounded by long-term overallocation of water for domestic and agricultural use, leading to natural and anthropogenic drought which has persisted for nearly a century on numerous river systems. In southwestern Colorado, the hydrology of the Dolores River has been altered during two distinct periods of drought: trans-basin water diversions beginning in the 1890s, which reduced summer flows, and the construction of a large dam (beginning 1983) which eliminated large floods and increased baseflows. Using remote sensing and field data on vegetation cover and channel metrics along a 12 km reach of the Dolores River, we seek to quantify the impact of these distinct periods of altered hydrology on riparian plant community composition and plant density, along with quantifying the influence of plant communities on channel planform. Our previous work has identified two distinct periods of riparian plant colonization: the first marked by invasive tamarisk recruitment (Tamarix spp.; 1950 - 1983) and the second period characterized by widespread coyote willow recruitment (Salix exigua; 1983 - present).
Results from this study suggest that on the Dolores River, colonization by tamarisk did not have a profound effect on channel planform: using aerial photographs captured in 1947 and 1981, we note that both channel sinuosity and the active channel width remained constant. However, during the period following dam construction in 1983, channel sinuosity decreased 9% and the active channel width decreased 70%, as evidenced by comparing photos from 1981 and 2019. In contrast to the early period of widespread tamarisk establishment, ongoing colonization by coyote willow during this more recent period has led to a large reduction in bare sand area within the study reach (63% decrease) and a concomitant increase in vegetation coverage (67% increase). Our work suggests that different styles or magnitudes of hydrologic alteration can drive preferential colonization by contrasting riparian species, which in turn exert starkly differing controls on resultant channel planform, with implications for in-channel habitat. Specific to the Dolores River, our results imply that the channel remained eco-geomorphically resilient during the early period of tamarisk establishment, but elimination of large floods following dam construction represented a threshold that has led to willow colonization and a highly altered riverscape.