Investigating the Influence of Timber Harvests on Bedload Transport

The bedload transport response to timber harvest depends on a variety of harvest-related factors that include post-harvest hydrologic changes, fining or coarsening of the stream bed, modified channel morphology due to direct disturbance, or changes to instream wood quantity or structure. Until the 1970s in the United States, limited effort was made to protect the channel and riparian area from direct disturbance during timber harvests. These disturbances included building stream crossings, using streams as tractor corridors, constructing logging roads near streams, felling and yarding trees in the riparian corridor, removing large wood from the channel, and releasing splash dams that flooded the channel and floated cut logs downstream. Modern best management practices either prohibit or greatly diminish most direct channel disturbances, but legacy effects still exist. We investigate post-harvest bedload response by compiling bedload transport data collected during timber harvests for experimental sites around the world. We also examine how bedload transport responded to experimental timber harvests at the Caspar Creek Experimental Watersheds in northern California and how increased post-harvest discharge may have influenced bedload transport. Specifically, we consider how direct channel disturbance during an experimental timber harvest of the South Fork watershed in the early 1970s may have influenced bedload transport. This harvest occurred prior to the implementation of the Forest Practice Act in 1973 that mandated stream corridor protection. We also assess how bedload transport responded to a harvest in the North Fork watershed from 1985 to 1992, which included stream corridor protection following California Forest Practice Rules.