Temporal and Spatial Variation in Tributary and Mainstem Suspended Sediment Fluxes in Big Creek, a Recently Burned Sub-Alpine Idaho Catchment

We analyze the spatial and temporal patterns of suspended sediment flux through the tributaries and mainstem of Big Creek, a 1445 km2 tributary to the Middle Fork Salmon River in Idaho. A series of fires of varying severity over the last 100 years have influenced the volume and timing of suspended sediment delivery. The persistence of elevated sediment delivery following forest fires exert direct pressure on aquatic ecosystems and affects the viability of floodplains and other riparian environments. During the Spring and Summer of 2008, we measured suspended sediment concentrations in 10 tributaries or various burn ages and within the mainstem. Roughly 3 catchments have experienced severe recent burns, 3 have had recent but less severe burns and 3 have not been burned in over 20 years. Samples from the mainstem were collected using both automated pump samplers and a tethered isokinetic suspended sediment sampler deployed from the Taylor Ranch Field Station Bridge. A hand held isokinetic sampler was used in tributaries. Isokinetic samples from the mainstem and tributaries were collected roughly every 3 days during stable weather and at higher frequency during large discharge events associated with spring runoff or late summer precipitation. The automated sampler collected samples from the mainstem 4 times a day to provide a more complete temporal record of the sediment flux. For the 246 water samples collected, we measured the mass of both organic and inorganic sediment. Stage height in the 10 tributaries and mainstem channel was measured continuously using pressure or radar sensors and calibrated to discharge using either an acoustic Doppler velocimeter or profiler depending on flow level. Temporal and spatial variations in sediment discharge are analyzed to reveal the dynamic dependence of sediment flux on season, storm severity and burn severity. Hysteresis in sediment flux within the mainstem and select tributaries is also analyzed with regard to storm and burn severity. Sediment rating curves developed for the tributaries and mainstem channel suggest that the three burn severities do not create drastic differences in baseflow sediment discharge but are distinct at higher flows. This is essential information to consider when quantifying habitat quality, water quality, and the consequences of forest fire on riparian and aquatic ecosystems.