Channel Change and Bed-Material Transport in the Lower Chetco River, Oregon

摘要

The lower Chetco River is a wandering gravel-bed river flanked by abundant and large gravel bars formed of coarse bed-material sediment. Since the early twentieth century, the large gravel bars have been a source of commercial aggregate for which ongoing permitting and aquatic habitat concerns have motivated this assessment of historical channel change and sediment transport rates. Analysis of historical channel change and bed-material transport rates for the lower 18 kilometers shows that the upper reaches of the study area are primarily transport zones, with bar positions fixed by valley geometry and active bars mainly providing transient storage of bed material. Downstream reaches, especially near the confluence of the North Fork Chetco River, are zones of active sedimentation and channel migration. This report summarizes analyses of temporal trends in channel width and gravel bar area, and bed-material transport measurements and calculations, with the goal to estimate temporal and spatial trends in bedload transport, deposition, and erosion within the lowermost 18 km of the Chetco River, and further to assess historical changes to the channel and flood plain. These analyses were supported by systematic mapping of channels and flood plains from historical and current aerial photographs, sampling of bed-material size distributions, bedload transport measurements, and hydraulic modeling. Additionally, the channel mapping in conjunction with new surveys allows for assessment of planform and vertical changes in channels, possibly attributable to changes in sediment balances and transport. The datasets developed in this study also will form a basis for ongoing monitoring programs to evaluate future changes in channel and bar morphology along the Chetco River. The scope of the study follows a process established in the State of Oregon to address permitting issues for inchannel gravel extraction.