| Spatial variability in sediment transport intensity and its relation to periphyton were investigated in three reaches of the Williams Fork River, CO. The frequency of sediment transport events was also investigated by developing a regional model to predict the frequency distribution of daily flows at ungauged locations. Sediment transport intensity and channel bed disturbance were calculated based on two-dimensional flow modeling of shear stress for varying flows and detailed measurements of the grain size distribution of the channel bed. Periphyton was monitored bi-weekly after the snowmelts of 2004, 2005, and 2007. Results indicate that, even though mean tau values for a given flow are highly variable among sites, the properties of the normalized distributions of shear stress are similar across sites. Analysis of sediment transport intensity indicate that most of the bed load is composed of intermediate grain sizes (16--45mm) carried through places in the bed that experience the highest range of shear stress. Transport intensity influences the post-flood accrual pattern of periphyton in years of both low and high peak flows, the accrual of periphyton is faster when the preceding peak flow is smaller, and the accrual rate of periphyton is not uniform, with locations at higher disturbance characterized by slower accrual than locations at lower disturbance. The regional model was derived based on the analysis of the flow record of 33 gauges in Colorado. The frequency distribution of daily flows at those locations can be reproduced with a broken power law function described by two parameters. These parameters are correlated with drainage area. Based on these correlations, a regional model capable of predicting the frequency distribution of flows at ungauged locations was formulated. According to the model, the frequency of large floods decreases downstream, meaning that there is non linearity in the snowmelt-runoff process. The tendency for high flows to become less frequent as drainage area increases can be physically interpreted as the result of the interaction between spatial variability in precipitation, flood attenuation, flow routing, and differences in the contribution of groundwater to base flow. |
