Characterizing Controls of Riparian Width for Mountain Streams in the Colorado Front Range

High variability of mountain streams causes riparian width to vary greatly from changes in drainage, valley and channel characteristics. GIS- based models for predicting flood-prone width, valley bottoms, or riparian zones may not accurately reflect processes at the reach scale, therefore field verification and reach-specific studies are needed. Management of riparian areas often designates a generalized width, which may under- or over-estimate the true riparian width. This study examines correlations between potential control variables and riparian zone width in the Colorado Front Range. Results from this study will be used to predict the riparian zone as a proxy for flood-prone width in the semi-arid Colorado Front Range. We hypothesize that local controls interact with large- scale controls to determine floodplain processes. Large-scale controls identified are elevation, which reflects hydroclimatology and glacial history, gradient and drainage area. Local controls are entrenchment, the ratio of the valley width to channel width, connectedness, defined as the distance from the channel to valley edge, presence of colluvium, and vegetation type, affecting roughness during flooding and bank stability. We chose twenty reaches based on elevation, connectedness, gradient and drainage area using a GIS base map in anthropogenically undisturbed areas of the Colorado Front Range, which included the Cache la Poudre and North St. Vrain drainages. Riparian width was defined using a three-tiered approach: evidence of fluvial processes and presence of riparian vegetation, compared with the Q100 stage. A longitudinal and two valley and channel cross-section surveys were completed at each stream reach to determine valley and channel geometry and bed gradient. Preliminary results show significant positive correlations between drainage area, entrenchment, and connectedness and riparian width, and negative correlations between gradient and riparian width, supporting the hypothesis that controls at various scales influence floodplain processes. Statistical analyses are used to determine the relative importance of each control variable using multiple regressions and best subsets regressions using Adjusted R2 and Mallow's Cp model selection criteria. Results from these data will allow for more accurate delineations of the riparian zone for mountain streams, in addition to an understanding of the reach-scale significance of interactions between floodplain and local hillslope versus drainage basin processes.