Downstream Effects of Diversion Dams on Riparian Vegetation Communities in the Routt National Forest, Colorado

The relationship between riparian vegetation and changes in fluvial processes as a response to flow diversion is not well understood. Water extraction affects the hydrologic flow regime (i.e., magnitude, duration, and frequency of flows) reducing peak and base-flows, which could negatively impact riparian vegetation. Vegetation communities are temporally and spatially variable and strongly interrelated with alluvial landforms and hydrograph variability. This research compares riparian community characteristics on diverted and undiverted pool-riffle channels and low gradient valleys to examine changes associated with flow diversion in the Routt National Forest (RNF).  Site-specific or cumulative effects of diversions on riparian ecosystems have not been investigated.  Systematic investigation is necessary to determine whether existing flow diversions have influenced riparian communities and, if so, which communities are most sensitive.  Methods include surveying channel cross sections randomly established per site, extending into the riparian zone at distance of two times the active channel width, with vegetation being sampled using the line-point intercept method.  Preliminary results suggest a shift in vegetation communities from typical riparian species composition to more upland vegetation.  The relative sensitivity of these responses differ depending on valley type; low- gradient, unconfined areas are less tolerant of diversion than steeper, confined reaches. Functional trait analysis shows that below diversions, species less tolerant to drought and reduced water availability were favored.  The results of this study contribute to the collective understanding of mountain headwater riparian vegetation community response to changes in flow regimes and fluvial processes related directly to water extraction by diversion dams.