Geomorphic and process domain controls on riparian zones in the Colorado Front Range

Delineation of riparian zones along mountain streams is complicated by substantial longitudinal variability in gradient and valley geometry and the lack of a floodplain along many stream reaches. We propose an approach to defining and delineating riparian zones based on site-specific geomorphic features and process domains, as well as the distribution of plant species. We then (1) evaluate which basin-scale (elevation and drainage area) and reach-scale (valley geometry, gradient, and channel width) parameters correlate with riparian width along mountain streams in the Colorado Front Range, and (2) test a three-parameter method for delineating the riparian edge, using geomorphology, hydrology, and vegetation. We identify four general process domains in the Colorado Front Range based on valley geometry and elevation, which reflect glacial history and hydroclimatology: confined, low elevation (CL); unconfined, low elevation (UL); confined, high elevation (CH); and unconfined, high elevation (UH). We utilize process domains because we hypothesize that reach-scale variables correlate more strongly than basin-scale variables with riparian width. The relationships between (i) connectedness, a valley geometry metric describing the average absolute distance from the channel edge to the valley edge, (ii) process domain, and (iii) riparian width were evaluated using a multiple linear regression. Collectively, these variables explained 86% of variability in field-delineated riparian zone width. Reach-scale processes exert a greater influence than longitudinally variable basin-scale parameters on riparian width in the mountainous watersheds of the Colorado Front Range. With respect to the three-parameter method, hydrologic relationships are applicable when comparing by process domain, allowing the stage associated with the riparian edge to be constrained between flow stages. Vegetation composition is significantly different for each of the four process domains, and within each process domain, vegetation is significantly similar within the geomorphically-delineated riparian zone. Although vegetation composition commonly reflects geomorphic and hydrologic processes, many plant species are relatively short-lived and individual plants respond to local disturbances over short time scales. Consequently, we recommend that the larger scale and longer duration riparian controllers, hydrology and geomorphology, be used to delineate riparian areas.