Stream restoration design discharge for snowmelt dominated streams

Dominant (or 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. It is often assumed in stream restoration design that the bankfull discharge is the channel forming discharge, with a return period of about 1-2 years. It is further assumed that watershed characteristics are similar between reference and design reaches, and little regard is given to how they impact channel form and channel forming discharge. Natural stream discharge in general is a strong function of watershed runoff characteristics including, geology, soil type, land use, and basin topography. The determination of an appropriate channel forming discharge must, therefore, take into account watershed characteristics as well as the erosion potential of the stream. The purpose of this paper is to present the results of an investigation into channel forming discharge and watershed characteristics for snow-melt dominated streams in Wyoming and Colorado. The study provides relationships between bankfull/effective discharge and watershed and hydrologic characteristics, and determines which watershed characteristics are important to the channel forming discharge and should be considered in design. Relationships between bankfull channel geometry and watershed characteristics are also presented to show which characteristics are important to shaping the channel. The principal goal of this research investigation is to offer the stream restoration community technical guidance on the selection of an appropriate design discharge for stream restoration projects based on given watershed conditions and hydrologic regimes.