Suspended sediment concentrations following natural watershed disturbances: temporal and spatial considerations

The High Park fire burned over 27,000 ha within the Cache la Poudre watershed (CO) in early summer 2012. Given the proximity of the burn and the implications for water quality supplied to local municipalities, there was an expressed interest for improved understanding of potential sediment loads emanating from the burn area. In response, a series of studies was established, including monitoring of discharge and suspended sediment in the South Fork of the Cache la Poudre (SFCLP). We present data on suspended sediment concentrations (SSCs) collected over three years post-fire (2013-15) at 3 sites on the SFCLP located directly downstream of burned sub-basins. Turbidity sensors were deployed at each station as a continuous, surrogate measure of SSC, along with stage and precipitation. The turbidity signals were calibrated using both grab samples and samples obtained from an automated water sampler triggered by a turbidity threshold; over 1300 samples were collected during the monitoring period. Most samples were analyzed for particle size (i.e., sand-silt split) and burned for organic matter content. The burned area was also impacted by the September 2013 Northern Colorado Flood during the monitoring period. Complex patterns of SSC emerged both temporally and spatially in response to both fire and flood disturbances. Among the highest SSCs (up to 7000 mg L-1) were those measured the first year post-fire (2013) during moderate to high intensity storms (peak 10-minute intensity > 20 mm hr-1); there was, however, little change in discharge during these events. Fortuitously, during the September 2013 flood, one of the turbidity sensors survived and recorded what appear to be reasonable values that would suggest peak SSCs on the order of 10,000 mg L-1. In the second year, three storms produced substantial increases in SSC, but these were largely under 1000 mg L-1. By the third year, maximum measured SSC during storms was less than 300 mg L-1. Spatially, there were spikes in SSCs due to inputs from the downstream burned sub-basins in the first 2 years of monitoring. By 2015, the spatial differences were considerably muted. Preliminary analysis suggests that overtime, the amount of sediment generated during higher intensity rainfall is reduced as vegetation regrows, soil erosion declines, and tributary channels stabilize.