Rates and Form of Fluvial-System Response to Climate Variability Over Decadal to Holocene Timescales - Cuyama Valley, California

Fluvial-system response to climatic perturbation over multiple timescales is well-preserved in Cuyama Valley, southern California. Here we present analysis of rates and forms of 1) aggradation, 2) incision, and 3) lateral erosion over decadal to Holocene timescales. After deposition of a >12m thick alluvial unit (P-H) (dated at ~10,800 ¹⁴C yrs BP) at the Pleistocene-Holocene climatic transition, the Cuyama River incised and eroded laterally, removing the P-H fill from all but the widest reach of the Cuyama Valley. Alluvium from between ~10,000 ¹⁴C yrs BP and ~2,000 ¹⁴C yrs BP has not been observed in the main valley. If aggradation did occur during this time it was subsequently removed by incision and lateral erosion. ~9-12m of aggradation has occurred since ~2,000 ¹⁴C yrs BP along a 35km reach. Aggradation initiated before 1691 ¹⁴C yrs BP, which corresponds to a wet-dry climatic transition recorded at nearby Soda Lake after ~2100 cal yrs BP Subsequently, at least one episode of floodplain degradation punctuated an overall aggradational regime that continued until recent arroyo cutting initiated in the downstream reaches. A minimum age estimate for the initiation of arroyo cutting is 469 ¹⁴C yrs BP Incision has propagated upstream at an average rate of ~100m/yr. Anecdotal evidence indicates that the Cuyama River reached its base level of erosion (indicated by exposed bedrock channel-bed) only within the last century. The dominant mode of the fluvial system at present is lateral erosion. GIS analysis of remote sensing data capture patterns of lateral erosion that have occurred between 1989 and 2002 removing up to 100m of bank material in localized areas. Analysis of streamflow data indicate that this erosion was dominantly a result of El Niño flooding in February of 1998, a 25-yr flood event that resulted in loss of human life and agricultural land in Cuyama Valley. Response of the fluvial system to well-constrained triggers appears generally well-preserved in the stratigraphy and morphology of Cuyama Valley. However, these results also indicate that the timescale for preservation of sedimentary fill in a topographically confined axial system may be on the order of only a few millennia, and gaps in a preserved alluvial sequence do not necessarily indicate lack of response to perturbation.