CHANNEL MORPHODYNAMIC RESPONSE TO HUMAN ACTIVITIES AND NATURAL INFLUENCES IN THE LOWER SANTA CLARA RIVER, VENTURA COUNTY, CALIFORNIA

The Santa Clara River is the largest river in southern California without significant mainstem flow regulation. Flow is extremely variable resulting from a semi-arid climate and has a very high sediment supply. Anthropogenic influences on channel morphodynamics include local-scale impacts (e.g., levee construction and in-channel gravel mining) and regional-scale impacts (e.g., legacy of ranching following European colonization, changes caused by groundwater extraction, flow and sediment regulation by large dams on tributaries, and possible impacts stemming from the 1928 St. Francis Dam disaster). Rapid urbanization within the watershed has the potential to cause future channel impacts.

Risk analysis for river management and restoration is concerned with discerning hazards and assets associated with changes in reach-scale channel morphodynamics and requires gathering and interpreting data from a combination of historical sources. To assist in managing the lower Santa Clara River, the morphodynamics of the 60-km lower mainstem river channel from 1927-2005 were investigated using flow and sediment records, aerial photographs, LiDAR data, repeat thalweg elevation data, in-channel vegetation data,, field observations, and numerical modeling of high flow events. The lowest 25 km of river demonstrates on-going incision (up to 8 m), without appreciable recovery except where grade-controlled, and a mainstem channel that can now convey the 10-year flood. Planform overlays show a progressive narrowing of the active channel until recent years due to mainstem levee construction and continuing incision. Conversely, further upstream, active channel width responds in tandem with the magnitude of the last large flood. A sediment budget of the lower river provides corroboratory evidence both for the observed changes and for the importance of the largest flood events.

Our analysis suggests that morphodynamic response to human influences in the Santa Clara River was underway by 1927 and that the incised lower Santa Clara River is now more morphodynamically sensitive than it was historically. This reinforces the need to manage and restore the river and floodplain to accommodate large flood events and underpins the importance of understanding the ‘catchment historical' context to river management.