Ecological - Second Order
Debris flows and hyperconcentrated flows immediately impact streams by changing channel morphology, grain size, sediment storage and transport, amount of incision, riparian vegetation, large woody debris dynamics, and extirpating fish, amphibian, and insect populations. In central Idaho, these disturbances are commonly triggered by intense thunderstorms or rain-on-snow events, and are exacerbated by wildfires which alter basin hydrology and sediment supply by removing vegetation and creating hydrophobic soils. While the immediate effect of these flows is dramatic, the time to recovery of the physical habitat is poorly understood and the long-term significance of these disturbances to aquatic organisms is unknown. Stream temperature is a key variable of stream ecosystems and has been shown to control the distribution of salmonids in our study area of the Idaho Batholith. Previous research in 10 recently disturbed streams shows a systematic increase in stream temperature across three stream types representing progressively greater disturbance: undisturbed; burned; and those impacted by both fire and mass-wasting events. Here, we test the hypothesis that the observed pattern of warming is due to increased solar radiation loading caused by wider, shallower streams and the removal of vegetative shade by fires and mass-wasting events. We examine channel conditions across several treatment classes (undisturbed, post-fire debris flow, debris flow without fire) and time since disturbance (1964 to present). In 32 streams, 200-600 meter reaches were surveyed and upstream and downstream temperatures were monitored throughout the summer, the solar load was estimated as a function of shading (measured with hemispherical photo analysis), stream width and depth, and average velocity estimated with salt tracers. Preliminary results indicate that while recent disturbances (1995-2003) significantly increase the solar load and stream temperatures, older disturbances (1964) are similar to undisturbed streams.