Evaluation of the integrated riparian ecosystem response to future flow regimes on semiarid rivers in Colorado, USA

Riparian ecosystems are shaped by interactions among streamflow, plants, and physical processes. Sustaining functioning riparian ecosystems in the face of climate change, growing human demands for water, and increasing water scarcity requires improved understanding of the sensitivity of riparian ecosystems to shifts in flow regimes and associated adaptive management strategies. We applied projected future flow regimes to an ecogeomorphic model of riparian and channel response to evaluate these interactions. We tested the hypothesis that components of the riparian ecosystem vary in their vulnerabilities to shifts in flow attributes and that changes in the representation of functional groups of plants result from interactions between ecological and physical drivers. Using the Yampa and Green Rivers in northwestern Colorado as our test system, we investigated ecogeomorphic response to (1) synthetic flow regimes representing continuous changes from baseline flows; and (2) future flow scenarios that incorporate changing climate, demand, and water-resource projects. For this region, we showed that riparian plant presence, composition, and cover are highly sensitive to the high flows that occur early in the growing season, but that shifts to low flows are also important, especially for determining the functional diversity of a riparian community. Future flow regimes are likely to induce vegetation encroachment on lower channel surfaces and to increase plant cover, which will be dominated by fewer functional groups. In particular, we predict a decrease in some mesic plants (shrubs and tall herbs) and an increase in presence and cover of late-seral, xeric shrubs, most of which are non-native species. Managing for high flows that occur early in the growing season must complement maintenance of adequate baseflows to maintain ecosystem functioning in the face of hydrologic alterations induced by climate change and human water demand.