FLOODPLAIN AND CHANNEL EVOLUTION FOLLOWING CATASTROPHIC SEDIMENTATION: EARLY AND ON-GOING RIVER-BASIN MANAGEMENT IN THE WEST

Hydraulic gold-mining sediment produced in the late 19th century in the Sierra Nevada foothills of California, caused severe channel aggradation in the lower Feather, Yuba and Bear Rivers. Topographic and planimetric data from historical maps, surveys, aerial photographs, and LiDAR data reveal various styles of channel change and floodplain evolution in these rivers. For example, early engineered levees in the lower Yuba River were set back as much as four km to encourage sequestration of mining sediment and reduce its delivery to navigable waters downstream. By 1906, the lower Yuba floodplain was a multi-thread channel system with braiding indices >12 in some reaches, and many of these channels remain clearly visible on aerial photographs and LiDAR imagery. Comparisons between detailed 1906 topographic maps and 1999 LiDAR data indicate substantial channel morphologic changes including abandonment and filling of former channels and the evolution towards a single-thread channel system. Implications to sediment budgets include a progressive shifting of stored historical sediment from the Sierra piedmont to lower in the Sacramento Valley.

Early river engineering and management of these channels represent the first efforts at integrated river basin management west of the Mississippi. The California Debris Commission (CDC) was formed by Congress in 1893 to assess and mitigate the damage of hydraulic mining sediment to navigation and flooding in the region. On-going engineering changes to these rivers, such as major levee setback projects on the Bear and Feather Rivers, represent modern river management efforts to reduce flood risks as residential developments encroach on flood-prone lands. Modern river management should consider how the unique disturbed character of these channels, past successes and failures of local river management, and the imprint of past channel and floodplain evolution affect morphologic stability and sediment production potential.