UNDERSTANDING RELATIONSHIPS AMONG STREAM INCISION, CLIMATE AND VEGETATION IN CENTRAL NEVADA WATERSHEDS

Past research suggests that climate change processes during the last several thousand years are responsible for a natural tendency of stream systems in central Nevada to incise. More recently, anthropogenic disturbances have accelerated the rate and magnitude of stream incision. Understanding stream incision is necessary to effectively manage western watersheds. This research focused on determining the timing and magnitude of recent stream incision episodes in central Nevada over the period of historical record. The influence of regional climate change (precipitation, temperature and hydrologic regime) on stream incision was studied using discriminant analyses. We used hydrologic data and cross-section measurements from USGS gaging stations, and conducted field surveys of uniform valley segments at the base of each drainage (upstream of the gage) to study stream incision in seven small watersheds. Field measurements included: stream channel cross-section surveys, pebble counts, soil sampling and tree aging to date stream terraces. We assumed that the potential for stream incision throughout a watershed would be reflected in changes in permanent channel cross-sections at USGS gaging stations and localized stream incision along basal uniform valley segments. To explore the relationship between stream incision and vegetation, climatic factors were related to riparian tree/shrub establishment episodes using discriminant analyses. Preliminary results indicate that stream incision is widespread among the study drainages. The magnitude and pattern of incision along the stream reach is not uniform, and field observations suggest stream incision is influenced by basin geomorphology and sediment transport mechanisms. Episodes of recent stream incision are correlated with flows capable of entraining channel bed sediment and dated terraces within the incised channels correspond with flood events that occurred in the mid-60s, late 70s, and early 80s. Flood events generally result from heavy winter precipitation patterns that produce large snow packs and/or combinations of heavy winter and spring precipitation.