THE INFLUENCE OF SEDIMENT SUPPLY ON THE STRATH TERRACE CYCLE AS DETERMINED BY COSMOGENIC RADIONUCLIDES

Paleoerosion rates derived from cosmogenic 10Be concentrations in strath terrace sediments along the South Fork Eel River in northern California are as much as a factor of two greater than erosion rates derived from modern stream sediment collected at the same location along the river profile. We use these results to interpret sediment supply conditions during different phases of the strath terrace cycle such that paleoerosion rates from strath terrace samples represent conditions during lateral planation and erosion rates from modern stream samples represent conditions associated with vertical incision and abandonment of the strath surface. In addition, we have used optically stimulated luminescence to date strath terrace sediment and find that the highest paleoerosion rates in our study come from surfaces that are late Pleistocene in age. Our results provide quantitative, field-based evidence in support of the hypothesis that lateral planation of a strath surface can be caused by increased sediment supply. Field evidence within the South Fork Eel River basin suggests that deep-seated landslides were a prominent erosional mechanism at some time in the past. We suggest that a wetter climate during the late Pleistocene increased the frequency of deep-seated landslides, thus causing a change in the dominant erosional mechanism of the system leading to extensive lateral planation. We believe a change in the dominant erosional mechanism could be a key factor in causing fluvial systems to switch between phases of the strath terrace cycle.