Bagley Fire Sediment Study: Shasta-Trinity National Forest, Eastern Klamath Mountains, Northern California

The US Forest Service is conducting a study of sediment mobilization, transport, and deposition on the Bagley Fire, which burned about 18,000 hectares in late summer, 2012, on the Shasta-Trinity National Forest, south of McCloud, CA. The fire area is in steep terrain of the Eastern Klamath Mountains that are underlain primarily by metasedimentary rock. The watersheds affected drain into the headwaters of Squaw Creek, along with small streams tributary to the McCloud and Pit Rivers, all of which flow into Shasta Lake Reservoir. In November and December of 2012, intense storms occurred over the fire area with estimated return intervals of 25-50 years, based on 4-day storm totals in ranging from 38 to 56 cm. The Squaw Creek storm response was unique for this area, in that it remained turbid for about 2 months following the storms. Subsequent small storms through June, 2013 have also generated prolonged turbidity. This may be attributable to the remobilization of fine particles temporarily stored in the channel network. Preliminary observations from field reconnaissance include the following: a) Erosional processes were dominated by sheet, rill, and gully erosion, and the resulting sediment delivered to channels was rich in fine particles and gravels; b) Landslides were infrequent, and as a result, a limited amount of large rock and logs were delivered to channels; c) Sediment laden flows occurred in most burned low order channels, but classic debris flows, those scouring all vegetation from channel bottoms, were very uncommon; d) Most road stream crossing culverts failed in high severity burn areas; e) Low gradient stream reaches in Squaw Creek were aggraded with fine sediment; f) Sustained high levels of turbidity occurred in the main stem of Squaw Creek. The goals of this study are to characterize relative roles of surface erosion, landslides, and debris flows in delivering sediment to streams after the fire, and if possible, to develop a rough sediment budget, comparing the amount of sediment delivered to the reservoir to that mobilized on hillslopes and in channels. A combination of remote sensing and field methods are being used. Remote sensing methods include post-fire air photo interpretation and mapping, LiDAR data analysis, and reservoir bathymetry. Field methods include reconnaissance traverses, and transects for direct estimates of sediment volume from surface erosion, gullies, and landslides. Results of this study will improve our understanding of erosional and sedimentation processes in this specific post-wildfire response domain, including reservoir sedimentation rates. They will also provide land managers with sound information upon which to base future decisions on the management of the local natural resources. Lastly, they will facilitate the work of Burned Area Emergency Response teams (BAER) which respond to future wildfires in this domain, and promote development of better designs for road/stream crossings.