Decreased bed load transport following timber harvesting and wood input to the North Fork of Caspar Creek, CA

Two timber harvesting experiments have been completed in the mixed conifer-coast redwood forest at the Caspar Creek Experimental Watersheds in the northern part of the California Coast Ranges. During the second harvesting experiment, which primarily occurred between 1989 and 1992, buffer strips were left along much of the main stem of the North Fork following harvesting. Large wind storms, which occurred in 1990, 1994, and 1995, blew down many of the recently exposed trees. This resulted in a dramatic increase in the quantity of wood in the North Fork main stem channel. These blow downs introduced an opportunity to investigate how wood organizes and stores sediment in the channel. Birkbeck-style bed load pits along the main stem of the North Fork were operated during 13 storms between 1988 and 1995. Measured transport rates reveal a decrease in transport efficiency during this period. Here, we extend the investigation of the bed load transport response to the wood input through 2017 by reconstructing a record of annual gravel yields from weir pond surveys and estimates of settled suspended sediment and organic matter. We also compare our annual reconstructed gravel yields against gravel yields predicted with a gravel transport model and find that the most significant decrease in gravel transport occurred after harvesting ended and the majority of wood entered the channel. We explore the timing of this departure and complete spectral analysis of mapped downed wood, which has been surveyed along the main stem of the North Fork every ~2 years since 1984, to determine if wood jam organization, as opposed to wood quantity only, is responsible for the decrease in bed load transport. Ultimately, the blow downs increased channel storage space for sediment, contributed to aggradation, and improved channel complexity along the North Fork of Caspar Creek. Although this sequence of events following timber harvesting is most likely specific to Caspar Creek, it is strong evidence that increased wood input can lead to positive changes in the channel environment.