Managing Flows to Promote Natural River Functionality and Steelhead Habitat Development

The loss of habitat connectivity between mainstem river channels and upstream tributaries, due to the presence of impassable dams, has been widely documented throughout the world.  However, few studies have assessed the degree to which flows can be managed to restore geomorphic processes and habitat development in mainstem rivers, where access to upstream tributaries no longer exists.  Here we investigate the potential for using environmental flows to enhance endangered steelhead (Oncorhynchus mykiss) habitat below a large, impassable dam on the Santa Ynez River, CA near the southern extent of the species’ range.  We report on the downstream geomorphic effects of the dam and evaluate a range of flow conditions in terms of their ability to initiate geomorphic processes that form and maintain habitat within the mainstem channel.  Using a time-series of aerial photographs we documented that following dam construction, the active river channel width has reduced and the channel has become disconnected from the floodplain due to vegetation encroachment along the low-flow channel.  The presence of persistent vegetation, in conjunction with diminished peak flows and a ~50% reduction in the coarse sediment supply has resulted in a simplified, narrow channel with relict gravel bars that are rarely inundated under the managed flow regime.  To explore the potential for reservoir releases to form and maintain more complex spawning and rearing habitat we constructed a two-dimensional flow model capable of predicting flow hydraulics at discharges ranging from base flow to a 5-yr flood.  Ongoing model simulations are focused on evaluating the effectiveness of managed flows in mobilizing the channel bed, building gravel bars and scouring pool habitat, in a manner that more closely approximates the pre-dam flow regime.  Results from this work will help to assess both the feasibility and potential limitations for using environmental flows to promote natural river functionality and habitat development in mainstem channels that have been simplified due to the presence of large dams.