Erosion rates over millennial and decadal timescales at Caspar Creek and Redwood Creek, Northern California Coast Ranges

Comparing millennial‐scale denudation rates from cosmogenic nuclides with decadal‐scale sediment yields can shed light on erosional processes and on the effects of land use on sediment delivery to streams. Detailed measurements of sediment fluxes in the Northern California Coast Ranges at Caspar Creek and Redwood Creek have provided estimates of physical erosion rates since 1963 and 1971, respectively. We used cosmogenic 10 Be to measure millennial‐scale denudation rates averaged over 1400–8700 years at six catchments in Caspar Creek and four catchments in Redwood Creek. Our 10 Be measurements at Caspar Creek imply denudation rates that are nearly spatially uniform across the entire catchment and average 0·09 ± 0·02 mm a −1 . These millennial‐scale rates implied by cosmogenic 10 Be are faster than physical erosion rates of 0·005 ± 0·001 mm a −1 to 0·046 ± 0·007 mm a −1 inferred from sediment flux measurements over the past few decades in the same catchments. At Redwood Creek, our cosmogenic 10 Be measurements imply millennial‐scale denudation rates that vary across the catchment from 0·14 ± 0·03 mm a −1 to 0·44 ± 0·09 mm a −1 , in contrast to physical erosion rates ranging from 0·038 ± 0·011 mm a −1 to 0·48 ± 0·09 mm a −1 derived from sediment flux measurements made over the past few decades at the same catchments. The decadal‐scale and millennial‐scale measurements tend to differ most at the smallest tributaries, but differ by less than a factor of three for the Caspar Creek and Redwood Creek catchments as a whole. These measurements suggest that denudation rates at Caspar Creek are slower than rock uplift rates of 0·3–0·4 mm a −1 , implying that Caspar Creek is not in topographic steady state. Copyright © 2005 John Wiley & Sons, Ltd.