Riparian tree growth response to drought and altered streamflow along the dolores river, colorado

In the western United States, management to sustain riparian forests along regulated rivers will be challenged by future increases in drought and human water use. This study investigated influences of streamflow regulation by McPhee Dam on the Lower Dolores River, Colorado, on the growth response of three riparian tree species (Populus angustifolia, Populus deltoides subsp. wislizenii, and Acer negundo). Standardized indices of radial growth were developed at multiple reaches along the Lower Dolores for a predam canal-diverted period (1961-1984) and a postdam regulated period (1985-2008), and at the unregulated Upper Dolores and San Miguel Rivers during the same time periods. Our results strongly suggest that diversions of the Lower Dolores River into irrigation canals and reservoir storage predisposed Populus species to drought-induced growth declines. Tree growth along the Lower Dolores River was low during severe drought years with low streamflow, and growth decreased sharply when annual streamflow was less than 2.0 * 108 m³ year^-1. Our results provide guidelines for flow releases from McPhee Dam to mitigate drought impacts on riparian tree growth along the Lower Dolores River.