The 2007 Zaca Fire burned 248,000 acres of northern Santa Barbara County; including portions of the watersheds supply most of the County’s water supply. Over 60 percent of area contributing runoff to the City of Santa Barbara’s Gibraltar Reservoir was burned, as well as a large part of the watershed of Lake Cachuma.
Balance was retained by the reservoir managers to plan sediment and debris management in the two lakes, beginning with assessment of hazards to their facility and road network, as well as possible ash and sediment deposition in the tunnels which convey water through the Santa Ynez Mountains to the coastal communities. Next, our geomorphologists and engineers addressed managing the initial pulse of organic fines (wet ash and burnt limbs) flowing into the reservoir during the first storms. We then estimated the volume of sediment likely to enter the reservoir, comparing it with estimates by the U.S. Forest Service’s BAER team, and modeled the likely prism of deltaic deposition, as well as the related long-term loss of storage and access to the lakes. Hazards of subaqueous landsliding caused by collapse of the rapidly-accumulating delta front were simulated, and measures to limit this hazard were recommended. Finally, Balance hydrologists evaluated likely turbidity persistence resulting from this fire, given the widespread occurrence of friable diatomaceous earth and black shales in both watersheds, and helped client staff project likely effects on water treatment, as well as on the quality of water released from the reservoirs to sustain steelhead and riparian systems.