Chetco River Gravel Transport Study

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In 2009, the USGS completed a comprehensive study of gravel transport and storage along the lower Chetco River. 

PROJECT DESCRIPTION

The Chetco River is a steep, gravel-bed river in southwestern Oregon that drains 914 km2 (354 mi2) of the rugged Klamath Mountains before emptying into the Pacific Ocean 5 km (3.1 mi) north of the California-Oregon border. The lower 18 km (11.2 mi) of the channel is a wandering gravel bed river flanked by abundant, large gravel bars.

Location Map for Checto River Study

Location Map for Checto River Study(Public domain.)

The U.S. Army Corps of Engineers and other agencies charged with permitting instream gravel extraction in Oregon requested that the USGS conduct studies to characterize channel processes and gravel transport on the Chetco River and to also evaluate possible effects of gravel extraction on physical channel and aquatic habitat conditions. The study area for this project spans the lower 18 km (11.25 mi) of the Chetco River corridor, including the USGS streamflow gage at river kilometer 16.9 (river mile 10.7) and the extent of commercial gravel extraction.

In 2009, the USGS completed a comprehensive study of gravel transport and storage along the lower Chetco River. The objectives of the study were to:

  1. Determine the annual influx of bed-material sediment.
  2. Evaluate spatial and temporal variation in bed-material transport rates.
  3. Assess planform and vertical changes to the river channel.
  4. Provide digital maps of floodplain geomorphology and general vegetation conditions for different time periods.

 

DATA AVAILABILITY

Download geodatabase

 

Computed bed-material influx for Chetco River

As part of the comprehensive study of gravel transport on the Chetco River, USGS staff computed sediment influx to the lower Chetco River for water years 1970–2008 using the Parker (1990 a,b) and Wilcock-Crowe (2003) equations of bedload transport (Wallick and others, 2010). The basic premise of this approach is that for a site where hydraulic geometry and bed-material characteristics are known, transport capacity can be estimated as a function of discharge. Sediment transport capacity is defined as the “maximum load a river can carry” (Gilbert and Murphy, 1914, pg. 35). For gravel-rich rivers with an ample supply of bed material sediment, transport capacity can be used to approximate bedload flux, so long as the correct transport relation is applied and an accurate description of channel hydraulics and bed material are used as inputs to the formula.

For the Chetco River, the bedload-transport equations were applied for the location of the USGS streamflow gaging station at river kilometer 16.9 (river mile 10.7). The formulas were implemented by the software package Bedload Assessment in Gravel-bedded Streams (BAGS), a program operating within a Microsoft Excel workbook (Pitlick and others, 2009). Bed-material size data were collected at the site in 2008, and channel hydraulics were characterized with a validated HEC-RAS model constructed using 2008 LiDAR topography and 2008 bathymetric survey data (see Wallick and others, 2010 for complete model description). Two bedload transport measurements obtained at the USGS gaging station during water year (WY) 2008 are consistent with calculated values of bed-material transport. Because the Chetco River has abundant gravel deposits, and because measured bedload transport rates are similar to calculated transport capacity, the volume of bed-material sediment entering the lower 18 km of the Chetco River is presumably limited by the availability of sediment-transporting flow events, and not limited by bed-material supply. Hence, the transport capacity estimates, as computed using the Parker and Wilcock-Crowe equations, probably provide a reasonable approximation of the influx of bed material sediment.

The same methodology described by Wallick and others (2010) was used to calculate sediment influx using mean daily flows for the period April 1, 2008 to March 31, 2011. Daily transport values were then summed to determine annual sediment inflow for the period beginning April 1 and extending through March 31 of the following year. Also calculated was annual bedload transport in terms of the more commonly used ‘water year’ beginning October 1 and extending through September 30 (Table 1, Figure 1).

Although daily mean flow for the Chetco River gaging station is automatically computed on a daily basis, these data are considered provisional (and subject to revision) until the data is formally reviewed and approved. The Annual Data Report, typically published in the spring of each year, contains the final approved data from the previous water year. However, data are approved on an ongoing basis, so the Chetco River USGS streamflow website may contain approved data that have not yet been published in the Annual Data Report. Estimated annual recruitment values, for April 1-March 31 are:

April 1, 2008 to March 31, 2009:

  • Bed-material flux calculated by Parker equation: 84,700 metric tons
  • Bed-material flux calculated by Wilcock-Crowe equation: 111,700 metric tons

April 1, 2009 to March 31, 2010:

  • Bed-material flux calculated by Parker equation: 45,500 metric tons
  • Bed-material flux calculated by Wilcock-Crowe equation: 79,100 metric tons
  • These estimates, originally calculated in April 2010 using several months of provisional flow data, were updated in April 2011 using approved flow data. The updated daily mean flows resulted in a slight change to the estimated bedload flux values

April 1, 2010 to March 31, 2011:

  • Bed-material flux calculated by Parker equation: 67,100 metric tons
  • Bed-material flux calculated by Wilcock-Crowe equation: 134,300 metric tons
  • Calculations made in April 2011 rely upon provisional flow data for the period December 29, 2010 to March 31, 2011