IMPACTS OF SPATIAL AND TEMPORAL VARIATIONS IN DISCHARGE ON LONG-TERM WATERSHED EVOLUTION

Over long periods of time, watersheds evolve in response to fluvial processes that are driven by a wide range of discharges. In this study, we examine the implications of spatial and temporal variations of discharge on watershed evolution through modeling exercises. Regarding spatial variations, we examine the evolution of gullies, which are pervasive geomorphic features in the western United States and other regions. It is hypothesized that the small extent of convective storms and substantial transmission losses in channels promote the downstream disappearance of gullies. The role of these factors is tested by applying a geomorphic model in which storms occur within circular portions of the simulation domain and channel flow is lost to seepage up to a specified infiltration or seepage capacity in each grid cell. These processes reduce the sediment transport capacity in the downstream direction relative to the case with an infinite storm size and no channel transmission losses. The reduced sediment capacity alters the relationship between slope and drainage area for topographies at equilibrium, and limited storm sizes can also produce disconnected areas of incision within generally depositional portions of the landscape. Regarding temporal variability, we examine the behavior of the so-called geomorphically effective event (GEE), which is defined as the discharge that contributes the most geomorphic work. A generic stream power model with a threshold is used to describe either the detachment or transport of sediment and the behavior of the GEE is examined under various conditions. The results suggest that the return period of the GEE can vary spatially in a basin. For example, the return period can be different between locations where the fluvial process is dominant and sub-dominant if the threshold is non-zero. For a detachment-limited model of erosion, the return period of the GEE is different upstream and downstream of knickpoints, and for a transport-limited model, the return period is different along channel profiles even at steady state. Spatial variations in streamflow generation also produce spatial variations in the return period of the GEE.