Introduction

The Chetco River is a steep gravel-bed river in southwestern Oregon draining 914 km² of the rugged Klamath Mountains before entering the Pacific Ocean 5 km north of the California–Oregon State line (fig 1). Downstream of the confluence of the South Fork Chetco River at river kilometer (Rkm) 29, the Chetco River is flanked by varying widths and areas of gravel bars and flood plains. Downstream of Rkm 18, several of these gravel bars have been mined as a source of aggregate during the last century. Ongoing permitting actions have instigated questions of possible effects from such mining on physical channel conditions (for example, Kondolf, 1994, 1997), prompting the U.S. Army Corps of Engineers, in conjunction with regulatory agencies and stakeholder groups, to request from the U.S. Geological Survey (USGS) a measurement and analysis program to evaluate transport rates of bed material and to assess changes in channels and flood plains for the lower 18 km of the Chetco River.

Purpose and Scope

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.

Background

The Chetco River is like many rivers in the western United States for which issues of fish habitat, water quality, climate change, and changing land use have motivated new efforts to manage rivers and flood plains for multiple resources. In Oregon, rivers potentially subject to inchannel gravel extraction undergo a two-phase process of review and assessment by an interagency team co-chaired by the U.S. Army Corps of Engineers and the Oregon Department of State Lands. This team is subdivided into an executive team of policy managers and a technical team of supporting resource experts. The first phase is a preliminary assessment of “vertical stability” based primarily on available information. If the Phase I analysis shows no clear evidence of adverse channel or flood-plain conditions, a Phase II analysis may be initiated to provide more information relevant to permitting decisions. For the Chetco River, the Phase I assessment was completed in May 2007 (Janine Castro, U.S. Fish and Wildlife Service, written commun., 2007). This assessment of maps and surveys concluded that although the lowermost 2 km of the river “appears to have deepened slightly over the past 20 years,” there was no evidence of systematic channel incision for the balance of the lower 18 km of the Chetco River. These findings prompted the executive team to consider permitting of future instream gravel extraction upon completion of a more extensive Phase II analysis consisting of data acquisition and analysis aimed at:

1. Determining spatial and temporal rates of bed-material transport.
2. Assessing planform and vertical changes to the river channel.

In addition to these specific Phase II analysis components, the USGS was requested to provide broadscale maps of flood-plain geomorphology and general vegetation along the flood plain flanking the lower 18 km of the river corridor. The lower 18 km forms a convenient analysis segment because the upstream end approximately corresponds to the location of the USGS streamflow-gaging station for the Chetco River, which is 16.9 km upstream from the mouth and encompasses the extent of commercial gravel extraction. These findings and maps will be used by the regulatory agencies as supporting information for future permitting decisions for instream gravel extraction along the Chetco River.

Our approaches for assessing channel changes, as well as mapping current and historical channels and vegetation, followed established procedures of aerial photograph analysis, and channel and flood-plain surveys. Our analysis period extends back to include aerial photographs and bathymetric surveys from 1939. Assessing sediment transport rates and budgets is more difficult (Reid and Dunne, 1996, 2003), particularly for bed material (Edwards and Glysson, 1999; Hicks and Gomez, 2003; Reid and Dunne, 2003). Because of the challenges in assessing bed-material transport rates, we have adopted several measurement, modeling, and analysis approaches to ensure the greatest likelihood of meaningful results and to provide qualitative assessment of their accuracy.

Locations

All analyses and results are presented in metric units. Conversions to English units are provided in the report front matter. Locations along the channel alignment in summer 2008 are referenced to river kilometers (Rkm) measured from the mouth of the Chetco River (fig. 1) along the channel centerline mapped from Light Detection and Ranging (LIDAR) topography acquired in 2008. Ambiguity because of channel shifting was avoided by referencing locations and analyses for the lowermost 18 km to a flood-plain kilometer (FPkm) centerline, measured from the river mouth along the centerline of the Holocene flood plain. In 2008, approximately 18 km of channel were within the 16-km-long length of flood plain composing the study reach. Prominent landmarks and locations include the Highway 101 bridge at FPkm 0.9 (Rkm 1.4), Tide Rock at FPkm 4.2 (Rkm 4.9), North Fork Chetco River confluence at FPkm 7.6 (Rkm 8.3), and the USGS streamflow-gaging station (at Second Bridge) at FPkm 15.2 (Rkm 16.9).

Revised July 2012

First posted May 26, 2010