MECHANISMS AND CONTROLS ON POST-HIGH PARK FIRE SEDIMENT DELIVERY IN THE SOUTH FORK CACHE LA POUDRE BASIN: PRELIMINARY RESULTS

The 2012 High Park fire burned over 35,000 ha within the Cache la Poudre basin. Pre-fire vegetation was sub-alpine dominated by mature lodgepole pine characterized by infrequent, high intensity fires with estimated recurrence intervals of over 100 years. Previous studies suggest that post-fire erosion in the Colorado Front Range is driven by large convective storms rather than annual snow melt processes. Qualitative observations of post-fire channel morphology and turbidity suggest that small tributaries of the South Fork Cache la Poudre River are delivering substantial amounts of sediment that reaches the main stem Cache la Poudre River. The objective of this research is to evaluate the controls and mechanisms of sediment delivery, and to identify thresholds that cause a change in the dominant sediment delivery process in one mulched and one reference basin.

As part of the US Forest Service’s Burned Area Emergency Response, aerial mulching on severely burned hillslopes was implemented within the National Forest Systems lands during late summer 2012 and completed in June 2013 to minimize flood runoff and soil erosion. Tipping bucket rain gauges were installed at 1st and 3rd order cross sections, and sonic sensors were installed at 3rd order channel cross sections in July 2013 to relate rainfall events of differing depths and intensities to the timing and magnitude of runoff in each basin.

Three events were recorded between July 14-July 31, 2013 with precipitation measuring 6-12mm per event. Cross section surveys were performed at 1st, 2^nd, and 3rd order channels in each basin following each event. Preliminary results suggest that all reaches with surveyed cross sections are acting as transport zones, with no significant changes in cross sectional areas observed. This may be attributed to events that do not meet depth or intensity thresholds required to initiate channel incision, or by other controls such as contributing area. Qualitative observations from reaches downstream of the study sites suggest that precipitation depth and intensity have been sufficient to cause significant channel incision and bank erosion. Additional cross sections survey sites will be selected in summer 2014 to identify the controls causing a change in the dominant sediment delivery process as contributing area to a reach increases.