POST-FIRE DEBRIS FLOW IMPACTS ON STEELHEAD SPAWNING GRAVEL IN THE CARMEL RIVER, CALIFORNIA

Spawning gravel scarcity is a limiting factor for successful recovery of federally-threatened anadromous fish like steelhead of central California, especially where dams cut off gravel that was once transported to downstream reaches.

San Clemente Dam eliminated gravel transport to the lower Carmel River from 1921 until removal in 2015. The loss of bedload supply produced boulder and large cobble armoring from just below the dam to at least 2.5 km downstream of the dam. Despite the increased supply of sand and gravel when the dam was removed, the bed was still too coarse for spawning in 2020.

The Carmel Fire of August 2020 intensely burned the steep chaparral-covered slopes within small sub-watersheds adjacent to the coarse-bed reach. US Geological Survey debris flow probability models indicated that several of the watersheds would likely generate debris flows if rain intensity exceeded 24 mm/hr for a 15-minute duration. On January 27^th 2021, a rain event surpassed that intensity threshold, and debris flows carrying gravelly, granitic colluvium were triggered in almost all the study area basins. During the storm,10 of the 13 debris flows we studied entered the flood water of the Carmel River. Particle counts of the eroded debris flow snouts indicated that 45% of the material was spawning-sized gravel, and that virtually no mud was present. Although the debris flows were rich in spawning-sized gravel, and several debris flows reached the river, both reconnaissance observations and a BACI-sampling design of pebble counts in the Carmel River channel show that no significant patches of new gravel were present in the study area when the flood waters receded. Spawning gravel in the Carmel River channel before and after the debris flow event was approximately 12% of the bed, and it was too dispersed to be useful for redds.

Fire-mediated debris flows are a potential source of spawning gravel in the Carmel River. The volume of gravel delivered to the river was too low to produce bars or patches that could be used for spawning. The debris flows were less effective at delivering gravel to the river in the study area because some flows had to cross a rural road and water supply pipe infrastructure before reaching the river, and some did not have enough energy to traverse a broad floodplain at the base of slope.