SUSPENDED SEDIMENT CONCENTRATIONS IN THE SOUTH FORK CACHE LA POUDRE FOLLOWING THE HIGH PARK FIRE

The High Park fire burned over 35,000 ha within the Cache la Poudre basin in early summer 2012, including an eastern portion of the South Fork Cache la Poudre (SFCLP) watershed. Given the proximity of the burn and the implications for water quality supplied to local municipalities, there was an expressed interest in improved understanding of post-fire sediment loads from the contributing burned watersheds. We present data on suspended sediment concentrations (SSC) collected over two years post-fire (2013 and 2014) at a site on the SFCLP. Prior to burning, data on SSC were collected at this site by researchers from the US Forest Service, providing baseline information on pre-fire sediment concentrations. Post-fire, turbidity sensors were deployed as a continuous, surrogate measure of SSC. This signal was calibrated using samples obtained from an automated water sampler triggered when values exceeded a turbidity threshold. Peak SSC during snowmelt runoff in 2013 was about 70 mg L^-1, which is about 5 times higher than pre-fire values for comparable levels of flow. Values of SSC ranged from 100 to 1000 mg L^-1 during individual moderate-to-high intensity summer storms, with instantaneous concentrations up to 7000 mg L^-1. In 2014, SSC varied between 2 and 10 times greater than pre-fire snowmelt values, but primarily on the rising limb of the seasonal hydrograph. This pattern is likely due to re-entrainment of sediment deposited following a large flood in September 2013. During summer 2014, SSC exceeded 100 mg L^-1 only during 2 storms: July 12 (60 to 7000 mg L^-1) and July 29-30 (50 to 3000 mg L^-1).

Additional sampling stations were established downstream of the SFCLP site in conjunction with assessments of channel extension and sedimentation from severely burned gulches, one of which was mulched for erosion control in spring 2013. SSC values measured during summer storms below both tributaries show progressive increases in SSC relative to the upstream site in 2013. Preliminary data analysis for 2014 (and 2015) is in progress, but suggests that increases in suspended sediment from the 2 tributaries were lower than those generated in 2013. While there are apparent differences in the contributions from the two sub-watersheds, the overall effectiveness of mulching on sediment reduction is unclear at this stage of analysis.