HISTORICAL AND ANTHROPOGENIC TRAJECTORY ANALYSIS FOR RIVER MANAGEMENT

Analysis of evolutionary trajectories is increasingly being practiced in Australia and Europe as an alternative river management strategy to conventional river restoration. This perspective and associated methods are described, contrasted with conventional restoration, and applied to the lower Yuba and Bear Rivers in northern California. River restoration is generally based on geodynamic process analysis assuming equilibrium conditions and aimed at a reference channel system that is either ‘natural’ or ‘historical.’ Basing target design on a past morphology presumes—often erroneously—that current water and sediment loadings are best accommodated by a fluvial system that was adjusted to past watershed loadings.

From a watershed perspective, channel restoration or rehabilitation are often cost prohibitive resulting in spotty treatment of extreme sites and neglect of most of the rest of the system. Instead, an integrated river-management perspective is needed that recognizes historical and anthropogenic changes at the watershed scale. Most river systems have been substantially altered by human activities that introduce complex, non-linear dynamics and may not be equilibrium systems when considered over decadal time scales. The evolutionary trajectory approach recognizes how historical and human-induced changes influence channel dynamics, associated uncertainties, and seeks to project likely scenarios of future change.

River trajectories for various locations in the lower Yuba and Bear Rivers are described for pre-settlement (early 19^th century), rapid aggradation during hydraulic mining sediment production (1854-1884), post-mining channel degradation, engineering structures (dams, levees, bank protection), and modern adjustments. In addition, arrested trajectories are identified that have the potential for rapid, non-linear responses.