Research about Chetco River

USGS2011

Estimation of bed-material transport in the lower Chetco River, Oregon, water years 2009-2010

This assessment of bed-material transport uses methods developed in a previous study (Wallick and others, 2010) to estimate bed-material flux at the USGS Chetco River streamflow gaging station located at flood-plain kilometer 15 (14400000). On the basis of regressions between daily mean flow and transport capacity, daily bed-material flux was calculated for the period October 1, 2008 to March 30, 2011. The daily flux estimates were then aggregated by water year (WY) for WY 2009 and WY 2010 and the period April 1-March 31 during 2008-09, 2009-10 and 2010-11. The main findings were: *Estimated bed-material flux for WY 2009 (October 1, 2008 to September 30, 2009) was 87,300 metric tons as calculated by the Parker (1990a, b) equation (hereinafter \'the Parker equation\') and 116,900 metric tons as calculated by the Wilcock and Crowe (2003) equation (hereinafter \'the Wilcock-Crowe equation\'). *Estimated bed-material flux for water year 2010 (October 1, 2008 to September 30, 2009) was 56,800 metric tons as calculated by the Parker equation and 96,700 metric tons as calculated by the Wilcock-Crowe equation. *Estimated bed-material flux for April 1, 2008, to March 31, 2009, was 84,700 metric tons as calculated by the Parker equation and 111,700 metric tons as calculated by the Wilcock-Crowe equation. Flux values from April 1 to September 30, 2008, are from Wallick and others (2010). *Estimated bed-material flux for April 1, 2009, to March 31, 2010, was 45,500 metric tons as calculated by the Parker equation and 79,100 metric tons as calculated by the Wilcock-Crowe equation. *Estimated bed-material flux for April 1, 2010, to March 31, 2011, was 67,100 metric tons as calculated by the Parker equation and 134,300 metric tons as calculated by the Wilcock-Crowe equation. These calculations used provisional flow data for December 29, 2010, to March 31, 2011, and may be subject to revision. *Water years 2009 and 2010 both had less bed-material transport than the average annual transport values of 105,300 and 152,300 metric tons for the period 1970-2010 as calculated by the Parker and Wilcock-Crowe equations, respectively.

Oregon↗

USGS2010

Channel change and bed-material transport in the Lower Chetco River, Oregon

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. Multiple analyses, supported by direct measurements of bedload during winter 2008–09, indicate that since 1970 the mean annual flux of bed material into the study reach has been about 40,000–100,000 cubic meters per year. Downstream tributary input of bed-material sediment, probably averaging 5–30 percent of the influx coming into the study reach from upstream, is approximately balanced by bed-material attrition by abrasion. Probably little bed material leaves the lower river under natural conditions, with most net influx historically accumulating in wider and more dynamic reaches, especially near the North Fork Chetco River confluence, 8 kilometers upstream from the Pacific Ocean. The year-to-year flux, however, varies tremendously. Some years may have less than 3,000 cubic meters of bed material entering the study area; by contrast, some high-flow years, such as 1982 and 1997, likely have more than 150,000 cubic meters entering the reach. For comparison, the estimated annual volume of gravel extracted from the lower Chetco River for commercial aggregate during 2000–2008 has ranged from 32,000 to 90,000 cubic meters and averaged about 59,000 cubic meters per year. Mined volumes probably exceeded 140,000 cubic meters per year for several years in the late 1970s. Repeat surveys and map analyses indicate a reduction in bar area and sinuosity between 1939 and 2008, chiefly in the period 1965–95. Repeat topographic and bathymetric surveys show channel incision for substantial portions of the study reach, with local areas of bed lowering by as much as 2 meters. A specific gage analysis at the upstream end of the study reach indicates that incision and narrowing followed aggradation culminating in the late 1970s. These observations are all consistent with a reduction of sediment supply relative to transport capacity since channel surveys in the late 1970s, probably owing to a combination of (1) bed sediment removal and (2) transient river adjustments to large sediment volumes brought by floods such as those in 1964 and, to a lesser extent, 1996.

Oregon↗