Role of storms and forestry practices in sedimentation in an Oregon Coast Range Lake

Abstract:

Contemporary harvesting practices have been shown to reduce sediment transport rates relative to historic practices; however, it is not well understood how these practices interact with climate to influence sediment flux and source. We are currently studying sedimentation rates in Loon Lake, in the Oregon Coast Range. The watershed of Loon Lake is 80% privately owned, and is therefore an ideal system for examining the effect of policy affected management shifts on water quality and sediment transport. We analyzed one 690cm core that represents about 1500yrs of sedimentation in the lake. We measured accumulation by layer thickness and character by proxies of elemental analysis, magnetic susceptibility, particle size, and bulk density. Radionuclides and radiocarbon dates provide an age model with depth downcore. Our preliminary results show that there is some evidence that the sedimentation rate in the lake decreased around the time of passing of the Oregon Forest Practices Act when climate is taken into account; pre-OFPA, the estimated mean sedimentation rate was 1.0 cm/year, and post-OFPA, the estimated mean sedimentation rate was 0.76 cm/year. Both of these sedimentation rates appear to be different than pre-European settlement period which averaged about 0.40 cm/year; however, this conclusion is preliminary. In general, it seems there is some evidence that the Best Management Practices instituted with OFPA are associated with lower sedimentation rates. In our presentation we will discuss the role of forestry and climate in changing these sedimentation rates and compare these modern sedimentation rates with sedimentation in the pre-settlement period.