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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5126

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Introduction

Between 2005 and 2040, the population of Oregon is expected to increase by about 1.8 million (http://www.oregon.gov/DAS/OEA/popsurvey.shtml, accessed July 16, 2008). As a consequence of projected population growth in urban and rural locations, competition over water resources in the State could become more profound. In many locations within Oregon, water supplies are already insufficient to meet the needs of aquatic habitat, agricultural irrigation, industry, and urban drinking-water consumption. To meet future challenges, improved information-based tools are needed to better characterize and manage water resources.

Flow statistics can be used to characterize flow of a certain magnitude at a location of interest on a stream. Flow statistics are crucial to Federal, State, and local agencies for water-quality regulatory activities and water-supply planning and management. These statistics are used as benchmarks when setting wastewater-treatment plant effluent limits and allowable pollutant loads to meet water-quality standards. Hundreds of river reaches in Oregon have been designated as impaired (exceeding water-quality and/or biological criteria) by Total Maximum Daily Load (TMDL) assessments. Reliable estimates of expected streamflow are needed at specific periods of the year when determining the maximum allowable load of a pollutant. Aside from water-quality regulatory activities, flow statistics are used in design and management decisions for hydroelectric facilities, reservoir storage, fish passage, stream restoration, temporary control of water during construction, culverts, bridges, and agricultural irrigation systems. Low-flow statistics, in particular, are used in water-use permit decisions and the adjudication of water conflicts between competing users. Low-flow statistics also are increasingly being used in ecological research. Low-flow conditions can create biological responses and changes in habitat such as reduced population size of aquatic species and shifts in the quantity of species type.

An accurate calculation of flow statistics is dependent on the availability and quantity of measured flow records on a stream. The U.S. Geological Survey (USGS), Oregon Water Resources Department (OWRD), and other public agencies operate continuous streamflow-gaging stations in Oregon and surrounding States that provide flow data needed for various purposes. Although flow statistics can be calculated at these locations, techniques can be used to make estimates of flow statistics at locations where streamflow-gaging stations do not exist. If the stream location where a flow statistic is needed is close to a gaging station then streamflow information can be extrapolated from the gaging-station record. For locations farther away from gaging stations, regression equations that relate flow statistics with physical and climatic characteristics of drainage basins can be used.

Regression equations for estimating flow statistics were developed for use in Oregon and are described in this report. In addition, the regressions equations developed from this study also are included in the USGS StreamStats Web-based tool (http://water.usgs.gov/osw/streamstats/index.html, accessed July 16, 2008). StreamStats allows users to obtain flow statistics, drainage-basin characteristics, and other information for user-selected sites on a stream. Using a GIS-based interactive map of Oregon, the user can ‘point and click’ on a location and StreamStats will rapidly delineate the basin upstream of the selected location. The user also can ‘point and click’ on USGS streamflow-gaging stations and receive flow statistics and information about those stations.

Purpose and Scope

This report presents the results of statistical analyses used to compute period of record annual and monthly flow duration and low-flow frequency statistics at unregulated sites throughout Oregon. These statistics include flow durations (5th, 10th, 25th, 50th, and 95th percent exceedances) and the 7-day, 10-year (7Q10) and 7-day, 2-year (7Q2) low flows. In addition to providing methods for calculating flow duration and low-flow frequency statistics from streamflow records, the report also describes the development of regression equations that relate basin physical and climatic characteristics to flow statistics. These equations provide estimates of unregulated flow conditions at locations where streamflow data are unavailable (ungaged sites). The report also provides a discussion of the accuracy and limitations of the flow statistics and regression equations.

Description of Study Area

The equations for estimating flow statistics were developed for use only in Oregon. The equations were developed from flow statistics and basin characteristics at streamflow-gaging stations in Oregon and adjacent areas of the neighboring States of Washington, Idaho, Nevada, and California. The study area includes a wide range of geologic, physiographic, biological, and climatic characteristics. The area contains all or portions of nine U.S. Environmental Protection Agency (EPA) Level III ecoregions: Coast Range, Klamath Mountains, Willamette Valley, Cascades, Eastern Cascades Slopes and Foothills, Columbia Plateau, Blue Mountains, Snake River Plain, and Northern Basin and Range (U.S. Environmental Protection Agency, 1996) (fig. 1). These ecoregions were initially used as the basis for grouping streamflow-gaging stations for the development of the flow statistics regression equations. Ecoregion boundaries were adjusted to provide hydrologic regions within which various sets of regression equations were applicable as discussed in section, “Modeling Regions.”

In western Oregon, the Coast Range ecoregion separates the Pacific Ocean and the Willamette Valley ecoregion. Altitudes in the Coast Range are relatively low compared to other mountainous regions of Oregon. Major river basins in the Coast Range include the Nehalem, Siletz, Siuslaw, and parts of the Umpqua, which all drain to the Pacific Ocean. The ecoregion is dominated by lush conifer rain forests composed of Sitka Spruce, Western Hemlock, and Douglas Fir. Mean annual precipitation typically ranges from 80 to 100 in. High-altitude areas can get more than 200 in/yr of precipitation (http://www.ocs.oregonstate.edu/index.html, accessed July 16, 2008). Although winters are wetter than summers, air temperatures are mild and nearly constant year round at many locations.

The Klamath Mountains ecoregion is in southwestern Oregon. In the Oregon portion of this ecoregion, most runoff drains from the Rogue River basin into the Pacific Ocean. Mean annual precipitation in the western side of the Klamath Mountains, close to the Pacific Ocean, typically ranges from 80 to 100 in. However, areas to the east, around Medford and Ashland, typically receive only 20 in/yr of precipitation. Natural vegetation includes Oregon White Oak, Douglas Fir, and Ponderosa Pine.

Most of the Willamette Valley ecoregion is contained within the Willamette River basin. Runoff flows from the Willamette River into the Columbia River before it reaches the Pacific Ocean. Composed of flood alluvial material, the Willamette Valley is fairly flat and gently slopes from south to north. Altitudes near Eugene are around 500 ft and at sea level near Portland. Mean annual precipitation ranges from 40 to 50 in. About 80 percent of annual precipitation falls between October and May. Summers in the Willamette Valley can be hot and dry. Natural vegetation at high altitudes is dominated by Douglas Firs and other conifers. Oregon White Oak, Douglas Fir, ashes, alder, and maples are more common in low altitudes.

The Cascades ecoregion is immediately to the east of the Willamette Valley. As part of a larger mountain range extending from British Columbia to California, the Cascades are a natural north-south dividing line between western and eastern Oregon. Mean annual precipitation can be more than 140 in. at high altitudes. Several peaks in the region are glaciated and at altitudes of more than 10,000 ft. Natural vegetation in the Cascades is dominated by conifers such as Douglas Fir, Western Hemlock, and Western redcedar. The Cascades are composed mostly of highly permeable volcanic materials. As a consequence, many streams are spring fed and have a near constant discharge throughout most of the year (Manga, 1996). The Cascades contain the headwaters of the Clackamas, Santiam, and McKenzie Rivers, all of which flow into the Willamette River. The Cascades also include the headwaters of the Umpqua and Rogue Rivers, tributaries of the Pacific Ocean.

The Eastern Cascades Slopes and Foothills ecoregion extending from Washington to California contains snowmelt fed streams flowing eastward off of the Cascades into the Deschutes or Klamath River basins. This ecoregion is noteworthy for its numerous and highly productive spring fed streams, which includes the Metolius River, Fall River, Wood River, Annie Creek, Spring Creek, and Sheep Creek. The Eastern Cascades Slopes and Foothills ecoregion is in the rainshadow of the Cascades and receives significantly less precipitation ranging from about 20 to 50 in/yr. Natural vegetation in this ecoregion contains open stands of Ponderosa and Lodgepole Pine.

The Columbia Plateau ecoregion in north-central Oregon contains the Umatilla River basin in addition to the lower portions of the Deschutes and John Day River basins. This region drains the north side of the Blue Mountains and slopes from the south from an altitude of about 3,000 ft to a few hundred feet above sea level along the Columbia River in the north. Like much of eastern Oregon, annual precipitation in this region is less than 20 in/yr. The natural landscape is dominated mostly by grasslands and sagebrush.

The Blue Mountains ecoregion dominates northeastern Oregon. As the wettest ecoregion in eastern Oregon, conifer forests and alpine vegetation are present at high altitudes. The Wallowa Mountains on the eastern side of the Blue Mountains have several peaks more than 9,000 ft in altitude with a mean annual precipitation greater than 70 in. Runoff in the western side of the Blue Mountains flows into the Deschutes or the John Day Rivers and then into the Columbia River. Runoff in the eastern side drains into the Grand Ronde or the Powder Rivers and then into the Snake River.

A small portion of eastern Oregon is in the Snake River Plain ecoregion. Mean annual precipitation generally is less than 12 in/yr. Natural vegetation is dominated by grasslands and sagebrush. The limited runoff generated in this region flows into the Malheur and Snake Rivers.

Southeastern Oregon is a part of the Northern Basin and Range ecoregion. This ecoregion has few perennial streams and contains some of the driest areas of the State. The natural landscape is characterized by grasslands, creosote, and sagebrush. Mean annual precipitation in the Alvord Desert is less than 4 in. Most basins in this part of Oregon have no outlet to the sea and are within the Great Basin of the Western United States. Runoff in the closed basins terminates at existing or dried-up lakes. The largest of these lakes include Malheur, Abert, Harney, and Summer.

Previous Studies

Lystrom (1970) published a statewide [Oregon] evaluation of low-flow characteristics that included low-flow equations. Equations for determining water availability in Oregon are provided in Cooper (2002). Harris and others (1979) developed regression equations for predicting peak discharges in rural unregulated streams in western Oregon. Harris and Hubbard (1983) developed peak-discharge regression equations for eastern Oregon. Using additional years of data and streamflow sites, Cooper (2005; 2006) developed peak-discharge regression equations for streams in western and eastern Oregon, respectively. In the 1960s, the USGS published a series of Water-Supply Papers analyzing the magnitude and frequency of floods throughout the continental United States. Publications from the series that included portions of Oregon are Thomas and others (1963), Hulsing and Kallio (1964), Butler and others (1966), and Young and Cruff (1967). Portions of southern Oregon also were included in a flood regionalization study by Thomas and others (1993).

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