Abstract: Opening Plenary Session 1:15 pm (BACK)

__________________________________________

Effects of Sediment on River Channel Form and Behavior

Thomas Dunne
Bren School of Environmental Science & Management
University of California Santa Barbara
Santa Barbara, CA

The effects of sediment supply on river channel form and behavior are difficult to anticipate and especially to predict because of: (a) the intense stochasticity of sediment supplies in both frequency and magnitude; (b) diversity of their grain-size composition; and (c) the fact that channels do not always respond to sediment influxes by conveyance alone but also by storage. Identifying where and how these aspects of sediment supply might become important to channel design or resource management and where more benign conditions exist can indicate whether or not extra investments of fieldwork and analysis are required. Storage changes cause a range of morphological responses such as bar growth, pool filling, bank erosion, bend migration, and avulsion. These are the channel changes that promote the complexity considered to be welcome or unwelcome by various interested parties. Identifying and then quantifying these features of channel response to sediment supply helps to anticipate likely changes of channel properties, and to address whether any such changes have hazardous consequences or ecological and esthetic value. Analyzing the geomorphological role of sediment in this way also facilitates a first approach to estimating response to environmental changes of either sediment sources or their hydrologic drivers (rainfall, groundwater pressures, streamflow). Strong, field-based conceptual models of these sources and drivers then provide a basis for employing empirically supported and calibrated prediction models, which — it must be emphasized — can provide only inexact (but still potentially useful) guidance in situations that lack records of transport and morphological change.

It is difficult to design complexity into a restored river channel, but there might be opportunities for designing into a project resilience to the stochasticity of sediment supplies and their morphological consequences. Doing so requires field activities to define any potential for strong fluctuation in sediment supply, poorly sorted sediment influxes, and morphologically disruptive sediment storage.