Introduction

The Umpqua River drains 12,103 km² of western Oregon before entering the Pacific Ocean at Winchester Bay near the town of Reedsport (fig. 1). For much of its length, the Umpqua River and its two main tributaries, the North Umpqua and South Umpqua Rivers, flow on a bed alternating between bedrock and coarse alluvium, locally flanked by gravel bars and sandy flood-plain deposits (fig. 2). The lowermost 40 km of the Umpqua River is tidally affected, where the low gradient river flows over a sand and gravel bottom flanked by muddy tidal flats and flood-plain deposits.

For the last several decades, some of these gravel bars and in-stream alluvial deposits, particularly along the South Umpqua River and main stem Umpqua River, have been mined for aggregate. Ongoing permitting actions have instigated questions of possible effects from such mining and other land-use activities on physical channel conditions (for example, Kondolf, 1994, 1997), prompting the U.S. Army Corps of Engineers (USACE), in conjunction with regulatory agencies and stakeholder groups, to request from the U.S. Geological Survey (USGS) an assessment of bed-material transport and changes in channel and gravel-bar conditions for the Umpqua River and alluvial reaches of the North and South Umpqua Rivers. This study incorporates and supersedes a 2009 reconnaissance study (O’Connor and others, 2009).

Purpose and Scope

This report summarizes temporal trends of channel and gravel-bar area and provides estimates of sediment flux and sediment yield, with the goal of estimating temporal and spatial trends in bedload transport, deposition, and erosion in the main stem Umpqua River, as well as the semi-alluvial portions of the North Umpqua and South Umpqua Rivers. These analyses were based on mapping of the channel and flood plains from historical and current aerial photographs, sampling of bed-material size distributions, survey records from aggregate mining operations, sediment yield estimates derived from regional analyses, and site-specific sediment transport modeling. The detailed channel maps developed in this study can also be used in future analyses to detect changes in planform and bar morphology that may arise due to changes in sediment balances and transport. The scope of the study follows a process established in the State of Oregon to address permitting issues for in-channel gravel extraction.

Background

The natural resources of the Umpqua River basin are numerous, ranging from highly productive Douglas-fir forests in the upper basin, to ranches along the low-lying valleys, to coastal fisheries at its mouth. The basin also provides diverse habitats for aquatic and riparian species and supports populations of steelhead, coho salmon, and Chinook salmon, as well as Pacific lamprey and cutthroat trout (Geyer, 2003 a–d). Issues of fish habitat, water quality, and changing land‑use laws, similar to other basins in the Western United States, have motivated new efforts to manage the Umpqua River and its tributaries for multiple resources.

In Oregon, rivers potentially subject to in-channel gravel extraction undergo a two-phase process of review and assessment by an interagency team co-chaired by the USACE and the Oregon Department of State Lands. The first phase is a preliminary assessment of “vertical stability” primarily based on available information. If 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 Umpqua River, the Phase I assessment was completed by the USGS in 2009 (O’Connor and others, 2009). Among the findings from this preliminary assessment of gravel transport and historical changes to channel conditions was that the Umpqua River was in a “long-term (over time scales of thousands of years) state of incision” and that the extensive presence of in-channel bedrock indicated that the main stem Umpqua River was historically, and presently is, sediment supply limited—meaning that the transport capacity of the channel (the amount of sediment the channel could, theoretically, transport given its geomorphic and hydrologic characteristics) probably exceeds the volume of sediment entering the river system.

These findings prompted the interagency team to consider permitting of future in-stream gravel extraction subject to the completion of a more extensive Phase II analysis consisting of data acquisition and analysis aimed at:

1. Assessing planform changes to the Umpqua River, as well as the semi-alluvial portions of the North Umpqua and South Umpqua Rivers;
2. Determining spatial and temporal trends in bed-material flux; and
3. Evaluating linkages between sediment source areas in the upper basin and channel conditions along lower reaches of the main stem Umpqua River.

Locations and Reporting Units

Analyses and results are presented in SI (metric) units, except for bed-material flux values, which are presented in terms of mass in metric tons, which is equivalent to the SI unit megagram. Conversions to English units are provided in the report front matter. To convert between sediment mass and volume, we used an in situ bulk density value of 2.1 metric tons/m³ on the basis of measurements conducted by Milhous (2001) and reported in Bunte and Abt (2001).

Locations along the channel are referenced to river kilometers (RKM) measured along the channel centerline from the mouth of the Umpqua River and continuing upstream along the South Umpqua River, as mapped from orthoimagery acquired in summer 2005 by the National Agriculture Inventory Program (NAIP). These distances do not correspond exactly with river miles (RM) shown on current USGS quadrangle maps. Measured from the same orthoimagery, river kilometers for the North Umpqua River (NURKM) begin at the confluence of the North Umpqua and South Umpqua Rivers, and continue upstream along the centerline of the North Umpqua River.

To avoid ambiguity due to channel shifting, locations and analyses for the study area are referenced to a flood-plain kilometer (FPKM) centerline, measured from the river mouth along the centerline of the Holocene flood plain upstream along the main stem Umpqua and South Umpqua Rivers (fig. 1). This flood-plain reference frame provides a static template from which to consider temporal changes in channel morphology and is not intended for use as a regulatory or flood-hazard tool. In 2005, approximately 179.5 km of river channel lay along 169 km of the main stem Umpqua River flood plain and 123.4 km of river channel were within 106 km of flood plain flanking the South Umpqua River. The North Umpqua flood-plain kilometer (NUFPKM) centerline begins at the mouth of the North Umpqua and extends 45 km along the North Umpqua River valley bottom, containing 47 km of river channel in 2005.

Prominent landmarks and locations along the main stem Umpqua River include the mouth of Umpqua River near Winchester Bay FPKM 0 (RKM 0), the head of tide near Scottsburg at FPKM 40 (RKM 44), and the confluence of the North Umpqua and South Umpqua Rivers FPKM 169 (RKM 179.4). Major landmarks on the South Umpqua River include the city of Roseburg at FPKM 182 (RKM 197), and the mouth of Cow Creek at FPKM 230.9 (RKM 256). On the North Umpqua River, Winchester Dam is located at NUFPKM 10.2 (NURKM 11.3). Numerous gravel bars are referenced in this report, some of which have place names derived from USGS topographic maps and gravel mining permits, whereas others were assigned informal names during this study using nearby place names.

First posted September 29, 2011