The geomorphological behaviour of a snow-fed, semi-arid gully system in western Colorado

Western Colorado experiences a winter snowfall maximum and considerable snowmelt flooding in spring, followed by a summer characterised by short, high-intensity storms. An examination of local records and terrain characteristics suggests that not only are these two processes temporally distinct, but also that the environmental controls on runoff generation for each vary, so that each process affects different parts of the landscape. A gully system lying within a small (0.4 km squared) sub-catchment of the Alkali Creek basin was surveyed in 1962, 1975, and 1980. Because the watershed materials are erodible and sensitive to all watershed runoff events, it is hypothesised that the distinctive downstream erosion pattern described by the surveys may be explained in terms of the sediment transfer capabilities of the two asymmetrical hydrological process Intensity domains' routed into the channel phase. Frequency-weighted runoff models for melt and overland flow are consequently developed and tested against field discharge data. Linking these simulations to sediment rating curves allows event sediment yields for watershed sites to be estimated. The procedure shows snowmelt to be overwhelmingly effective in transporting sediment out of the basin, but that in the headwaters summer storm erosion is locally more important. A complex annual sequence of entrainment, lodging and flushing emerges which can be compared with other areas. Spatially differencing these yield data as an index of site scour and fill suggests that main channel erosion is considerable during snowmelt, and tributary junctions emerge as the most active watershed sites by a factor of at least 10. The peak power of simulated flows and the scour and fill indices work well in predicting contemporary form but less well against form change, nevertheless the methodology is generally supported by these results. Threshold behaviour is considered, and although it is argued that the sort of lodged sediments normally prone to trenching will not survive the snowmelt flushing on the main channel, there are possibilities for network Integration through fans on 'summer-sensitive' parts of the watershed. These possibilities and the large negative sediment yield budget indicate a state of chronic transience, an observation supported by evidence from an adjacent watershed