The Power of Long-term Observation: New Insights from 50+ Years of Monitoring Sediment Fluxes at the Caspar Creek Experimental Watershed, CA

We investigate fluvial sediment transport at the Caspar Creek Experimental Watershed (CCEW), a long-term monitoring site in a mixed coast redwood-Douglas-fir forest in northern California. CCEW includes two main catchments—the North Fork (4.73 km²) and the South Fork (4.24 km²) —which have been continuously monitored for discharge and suspended sediment since 1962. Sediment deposition in the weir ponds for both forks has also been measured each summer. Two logging experiments have been completed: the South Fork was selectively logged in 1971-1973 while the North Fork was partially clear cut in 1989-1992. A third experimental logging campaign is underway. To investigate the effect of logging and natural disturbances on suspended sediment transport, we develop a sediment rating curve and identify departures from predicted suspended sediment yields for each catchment using the long-term suspended sediment record. To investigate the same effects on bedload transport, we implement the Wilcock two-fraction bedload transport model using the long-term discharge record for each catchment and calibrate the model from an 8-year sediment transport study on the North Fork. We compare the bedload predictions to a recently constructed 55-year record of bedload yields created by explicitly accounting for bedload trapped in the weir ponds. Results from our single-catchment approach compare well to the results using a paired-catchment approach for years in which a comparison is possible. However, the single-catchment approach enables us to investigate sediment departures for years in which experiments or atypical transport occurred in the other catchment, freeing us from the usual constraints imposed by paired-catchment analysis. We identify a previously unrecognized negative bedload departure on the North Fork. This departure occurred 5-6 years after logging concluded and was likely due to a cascading series of events initiated by logging that culminated with substantial input of coarse wood and channel aggradation. Previous work suggests that CCEW sediment flux recovers 5 years after logging concludes. The significant response lag reported here demonstrates the value of continued monitoring beyond what was previously identified as a typical response period.