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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5101–D

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Introduction

Research has shown that stream ecosystems are increasingly degraded by urban development and human population growth (Booth and Jackson, 1997, Paul and Meyer, 2001; Walsh and others, 2005; Tate and others, 2005). The growth of urban areas changes landscapes and increases stresses to freshwater systems by adversely altering water-quality, habitat, biodiversity, and ecosystem processes (McDonnell and Pickett, 1990; Sala and others, 2000; Paul and Meyer, 2001; Brown and others, 2005; Sprague and others, 2006). Urban growth, or urbanization, is defined as the development of rural, agricultural, or forested land into urban land, such as buildings and roads. Urbanization may be further defined by incorporating population density estimates, percentages of urban land use classification from remote sensing data (Brown and others, 2005; Tate and others, 2005), or percentage of impervious surface cover (Arnold and Gibbons, 1996). Regardless of how urbanization is characterized, it directly changes the physical habitat and stream hydrology of a river system (Sinokrot and Stefan, 1993; LeBlanc and others, 1997). For example, encroachment of urban land into riparian areas decreases canopy cover, allowing more solar radiation to heat streams (Waite and Carpenter, 2000; Jacobson and others, 2001; Sprague and others, 2006). The expansion of urban land also introduces more industrial and human waste to rivers, which combines with more urban and agricultural pesticide applications that deteriorate water quality in streams. Additionally, urbanization brings increased development and more impervious surfaces. Impervious surfaces, such as roads, parking lots, and rooftops, increase surface runoff volumes and reduce the amount of water that infiltrates into the soil and ground water. As a result, the excess runoff modifies stream hydrology and channel morphology causing the degradation of aquatic habitats (Winterbourne and Townsend, 1991), the increase in sedimentation rates (Waite and Carpenter, 2000), and a greater fluctuation in frequency and magnitude of stormflows.

To investigate the effect of multiple urban stressors on stream ecosystems, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Effects of Urbanization on Stream Ecosystems (EUSE) study examined the effects of varying degrees of urbanization among various watersheds in the Willamette River basin and surrounding area. The approach integrated multiple parameters, such as socioeconomic variables, population statistics, and land use metrics, into a single index measurement of urbanization intensity (Cuffney and others, 2000; Tate and others, 2005), and was based on a common design and sample collection technique (McMahon and Cuffney, 2000). Using this multimetric indicator of urban intensity, 28 watersheds in the study area were selected with increasing degrees of urbanization (table 1). The urban land use gradient ranged from minimal urban development to highly developed land, while limiting differences in natural features and local disturbances. The gradient was used to assess the effects of urbanization on stream water chemistry, habitat, and biological conditions (Walsh and others, 2001; Fitzpatrick and others, 2004; Sprague and others, 2006).

Purpose and Scope

This report describes the physical (stream hydrology, water temperature, and stream habitat), chemical (nutrients and pesticides), and biological (algae, macroinvertebrate, and fish assemblages) characteristics of stream ecosystems in 28 watersheds along a gradient of urbanization in the Willamette River basin and surrounding area of Oregon and southwestern Washington from 2003 through 2005. Watersheds were selected to minimize natural variability between sites due to watershed size, elevation, and climate, and to maximize coverage of different degrees of urban development. The objectives of the study were to (1) examine physical, chemical, and biological responses along a gradient of urbanization and (2) determine the major physical, chemical, and landscape variables associated with aquatic communities.

Study Area

The Willamette River basin and surrounding area includes 35,000 km² in northwestern Oregon and southwestern Washington (fig. 1). Although the Willamette River basin is in Oregon, the study area was extended into Washington because of similar socioeconomic, climatic, ecologic, and topographic settings. For example, 1,000 km² of the Willamette Valley-Level III ecoregion, as defined by Omernik (1987) extends across the Columbia River into Washington (fig. 2). An ecoregion—unlike a watershed, which delineates an area of convergent drainage—denotes an area of shared natural characteristics, such as soil types, elevation, and climate. The Willamette Valley ecoregion contains a mixture of rolling prairies, mixed forests, and extensive lowland valley wetlands.

Land cover in the basin (fig. 3) is predominately forest (66 percent), with moderate agriculture (29 percent) and minimal urban (3.5 percent) and surface water (1.5 percent) (U.S. Geological Survey, 2005). The valley plains and foothills primarily are used for cultivated crops, pasture, and grasslands, although minimally developed areas, such as Dundee, Oregon, to highly developed urban areas, such as Portland, Oregon, also are in the valley. Fertile soils and a temperate climate make the Willamette Valley a thriving agricultural region (Thorson and others, 2003). Land use in the forested Coastal Range and Cascades is a combination of timber harvesting, recreation, and development. Centered on the confluence of the Columbia and Willamette Rivers, Portland is the most populous city in Oregon, with 539,000 people in city limits and nearly 3 million people in the Portland/Salem/Vancouver metropolitan area (U.S. Census Bureau, 2000). The population in the metropolitan area increased almost 30 percent from 1990 to 2000, with some suburban populations increasing more than 80 percent during the same period (U.S. Census Bureau, 2000).

With temperate, dry summers and cool, wet winters, the Willamette River basin and surrounding area is characteristic of Pacific Northwest climate. About 90 percent of the annual precipitation occurs during October through May (Uhrich and Wentz, 1999), falling as rain in the valley and snow in the mountains. The drainage network in the Willamette Valley combines natural dendritic tributaries, complex networks of canals in agricultural areas, and sewer piping in cities. Dams and reservoirs regulate most large rivers, such as the McKenzie, Santiam, and Willamette Rivers, which supply drinking water, power generation, and irrigation to different parts of the region.

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