Local- and Segment-Scale Considerations for Managing Gravel Regimes in Regulated Rivers

Abstract:

Gravel augmentation programs intended to improve riverine habitats are currently underway in a number of stream where the natural delivery of gravel from upstream is impeded by dams. Augmentations are sometimes intended to improve local habitat conditions at a specific stream location. However, gravel management objectives may also include establishing a more natural bedload regime in which gravel fluxes are sufficient to support the geomorphic processes that maintain channel complexity, substrate quality, and physical habitat integrity at the river segment scale. Determining the quantity of gravel to add to a stream and where to add it involves consideration of both spatial scales and requires a variety of analysis methods. Four general methods used to inform multi-scale gravel augmentation planning for the Trinity River of California are described. Recent analyses have produced recommendations for long-term average augmentation rates for the near-dam reaches based on measured bedload fluxes downstream from several unregulated sediment-producing tributaries. Monitoring of past augmentation projects is used to empirically evaluate the fate of added gravel at the augmentation locations, as well as the effects of those augmentations on reaches immediately downstream. Expectations for longer-term routing characteristics, including the identification of reaches likely to respond to upstream gravel additions, are assessed through morphodynamic modeling. A fourth approach to guiding augmentation activities involves integrating physical and biological information to identify specific locations where increasing the gravel supply are expected to produce specific habitat benefits. This is arguably most relevant way to approach habitat restoration, but it is also the most complex and least developed. It requires understanding the relative values of various habitat characteristics, linking ecological function to assemblages of channel features, and anticipating the geomorphic evolution of those features in response to gravel additions.