Evolution of Sediment Accommodation Space in Steady-State Bedrock-Incising Valleys Subject to Episodic Aggradation

Steepland valleys subject to debris flows incise bedrock even as episodic deposition typically covers valley bottoms. Valley bottom morphometry and valley cross-section modeling address the hypothesis that, while continual fluvial processes evacuate deposits, storage of episodic deposition drives valley widening and, thereby, creation of accommodation space for sediment storage on the valley floor. In three headwater valleys in the Oregon Coast Range (OCR), valley-to-channel width ratios and valley bottom deposit depths are variable, have little systematic variation with respect to contributing area, and are similar on average among sites. A model of valley cross-section evolution couples soil production, nonlinear diffusion, contrasting rates of channel incision into deposits and bedrock, and stochastic valley bottom deposition (mean recurrence interval 2-20 a). The model reproduces observed flat, deposit-covered valley bottoms and abrupt transitions to valley sides with oversteepened toe slopes (i.e., inner gorges). Simulations address sensitivity of valley morphologies and incision rates to dimensionless numbers, the ratios of instantaneous bedrock and deposit erosion rates (incision number) and of deposition and evacuation rates (deposition number). For steady-state simulations, increasing deposition number by <101 leads to deposit depth and valley bottom width increasing by 101 and 101.5, respectively, and valley bottom incision relative to the instantaneous rate decreasing by 10-3. For incision number increasing by 103, valley capacity (width times toe slope height) relative to mean deposit volume increases by 101.5. Simulations, consistent with field data, imply that steady-state valley widths are adjusted to episodic deposition rates and respond more quickly to changes than profile gradients because of contrasting limitations by instantaneous vs. long-term lowering rates. Moreover, stochastic deposition introduces substantial fluctuations (>20%) in simulated valley-bottom bedrock lowering rates, even over periods (>100 ka) much greater than the averaging time for cosmogenic radionuclides in the OCR.