Textural Adjustments of a Riffle-pool Stream to Multiple Sediment Mobilizing Floods

Mountain streams exhibit a diversity of bed sediment textures (patchiness) that manifest at a range of scales, bearing significance, for example, to localized rates of sediment transport. The coupling of bed texture and sediment transport at relevant scales is a particularly difficult process to represent, as some aspects are driven by grain to grain interactions. Field studies can somewhat address this challenge by mapping event-driven textural conditions to characterize the range of responses under prevailing conditions, thereby capturing the statistical tendencies of the system. We have applied this approach within a small, supply-rich riffle-pool stream over three winter seasons in the Santa Cruz Mountains, California. During the investigation discharge was continuously monitored at a semi-permanent gaging station, and bedload and suspended load sediment discharge were measured during every mobilizing storm. Immediately following each sediment mobilizing event we measured the fractional volume of pools filled with fines (V*), mapped riffle texture (sampling frame and template procedure), and measured cross-sectional and longitudinal topography. Thus far we have collected relevant data for eleven different sediment mobilizing events representing a wide range of hydrologic conditions. Preliminary results indicate that bed texture is consistently finer than the subsurface material, yet surprisingly quite varied from storm to storm. Using descriptive statistics we will demonstrate that V* and riffle texture changed with each sediment mobilizing event, and with little apparent similarity in trend for riffle and pool pairs. To explore possible explanations for observed conditions we will compare event-driven riffle and pool texture to the associated flow and sediment regimes, including the fractional volumes of transported bedload. Spatially-oriented comparisons will also be made for riffles using texture maps to estimate apparent levels of bed mobility from event to event. Use of this later approach in conjunction with fractional bedload data may prove illustrative.