Examining geomorphic effects of flow diversions on low-gradient mountain streams in the Routt National Forest, Colorado

2012 Fall.Includes bibliographical references.The western United States is faced with an increasing human demand for water, coupled with a decreasing supply. Resource managers are looking for ways to meet the demands of both anthropogenic use and the needs of instream flows to maintain channel characteristics for water quality as well as riparian and aquatic ecosystems. In the Routt National Forest in northern Colorado, ditches typically divert flows from headwater streams to supply the land below the mountains for agricultural purposes. Many studies have focused on the biotic response to streamflow diversions, but relatively little research has been done to quantify the physical effects of ditch diversions. The purpose of this study was to contribute to the understanding of geomorphic effects of flow diversions in the Routt National Forest, and to inform management decisions related to water on the Routt by supplying localized data. Thirteen streams were surveyed during the summer of 2011, yielding 11 control reaches, located upstream of a diversion point, and 11 diverted reaches, which were downstream of a diversion point. Reach lengths were spaced approximately 20 times bankfull width. Four cross sections per reach were surveyed to collect width and depth information using reference discharge indicators approximating bankfull flow. Pebble counts of 100 clasts per reach were evenly spaced between riffles, and pools were avoided. Riparian vegetation, lithology, and valley characteristics were qualitatively and quantitatively assessed at the reach sites and using US Forest Service geospatial data. Statistical analyses conducted using the collected data included both t-tests and non-parametric Wilcoxon tests, as the small sample size limited the ability to reject assumptions of normality and conduct multivariate analyses. Univariate mixed-effects models were developed to compare reach response variables between diverted and control reaches while including the effects of unevenly-paired reaches, valley characteristics, lithology, and riparian vegetation. T-tests and Wilcoxon tests found only sinuosity to be significant, with the possibility of riparian vegetation types (willow or grass/sedge) having an effect on variables related to bank stability (width, depth). The mixed effects models found width, width-to-depth ratio, sinuosity, and cross-sectional area to be significant. Because the mixed models included the effects of valley characteristics, riparian vegetation types, lithology, and drainage basin size, these are considered to be more representative of the downstream response to flow diversions than the t-tests and Wilcoxon tests. This study provides some evidence for the downstream alteration of channels due to diversions. Two channels were noted to have been completely dewatered at the time of surveying in late July-to-August, and several variables were significantly different in statistical tests. For management purposes, it is recommended that high flows periodically enter diverted reaches to help offset the morphology and water quality effects of diversions during dry years. This study stresses the importance of further research to more accurately constrain and quantify physical effects of diversions