Fluvial geomorphology of a boulder-bed, debris-flow — Dominated channel in an active tectonic environment

Numerous streams, with a drainage area of a few square kilometers, draining the south flank of the Santa Ynez Range in Southern California are characterized by steep boulder-bed, debris-flow-dominated channels. Rattlesnake Creek is a good example. Approximately 90% of the stored sediment in the stream valley is stable debris flow deposits found in debris flow terraces and levees adjacent to the channel. The remaining 10% of stored sediment resides in the active channel, where the channel morphology is dominated by large boulders (1 to 4 + m in diameter) derived from debris flows or rock fall that, along with bedrock outcrops, form rock or boulder transverse ribs with frequent step-pools.The dominant channel pool morphology is the step-pool, which accounts for most of the pools and approximately 30% of relief (drop in elevation) through the study reaches. Less common pool types identified in the study reaches include forced pools that result from convergence of flow with scour between LBREs (large boulder roughness elements) and bedrock outcrop along the channel at near bankfull discharge.The LBREs and boulders in step-pools armor the channel and are primary controls on channel morphology and gradient. Rock strength is a secondary control on channel morphology, location, and abundance of pools.We hypothesize that step-pools, when inundated with sand and fine gravel from a disturbance (such as wildfire) during a flow event, fill near bankfull stage and scour on falling stage below bankfull when unit stream power is a maximum over the step. Thus, as with forced pools, step-pool maintenance depends on a specific pattern of discharge and process. The process of scour during falling stage reestablishes step-pools during moderate flows, following disturbance.