Relationships between Watershed and Stream Characteristics and Channel Forming Discharge in Snowmelt Dominated Streams

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The selection of an appropriate design discharge that best approximates the channel forming discharge is critical to channel design and dictates the success of restoration projects in improving channel stability. Channel forming discharge is defined as the theoretical discharge that if constantly maintained in an alluvial stream over a period of time will produce the same channel geometry that is produced by the long-term hydrograph, and it is often estimated as the bankfull, effective, or recurrence interval discharge. The determination of the channel forming flow often focuses on the physical and erosional characteristics of the stream at a reach scale (analyzing sediment transport characteristics or identifying bankfull indicators). However, with wide variability in streamflow and flood timing across regions, it is critical to examine hydrologic and watershed characteristics and their relationship to channel forming flow. Obtaining a good estimate of the channel forming discharge requires looking beyond the reach scale and gaining an understanding of regional watershed hydrology and runoff characteristics (e.g., rainfall, snowfall, geology, soil type, land use, basin topography). The purpose of this work was to conduct an investigation to determine relationships between channel forming discharge and hydrologic and watershed characteristics for snow-melt dominated streams in Wyoming and Colorado. Significant relationships between bankfull/effective discharge and watershed and hydrologic characteristics were determined. In particular, it was determined that certain characteristics directly related to snowmelt hydrology and runoff (e.g., drainage area above 9000 ft and snow water equivalent) are most significant in estimating a channel forming discharge. The results of this study provide regional characteristics of snowmelt dominant systems that are important when determining a design discharge for stream restoration projects.