Introduction


The U.S. Geological Survey (USGS) in cooperation with the cities of Damascus, Gresham, Happy Valley, Milwaukie, and Portland; Clackamas County Water Environment Services; Multnomah County; and the East Multnomah Soil and Water Conservation District monitors streamflow and water quality at stations in the Johnson Creek basin, Oregon (fig. 1, table 1). Data are recorded, transmitted or collected, stored, and made available on the USGS National Water Information System (NWIS) web site (U.S. Geological Survey, 2011b). These resulting data can be used in evaluating stream water-quality effects of land-use practices and restoration projects, conversion of agricultural or rural land to more urban uses, and modification of urban storm-drain networks.


Study Background


For water years 2007 through 2010 (a water year is the 12-month period between October 1 and September 30), the USGS conducted a study of suspended sediment occurrence and transport in the Johnson Creek basin. Suspended sediment is often an important aspect of water quality in a watershed because it can have adverse effects on fish and other aquatic life (Angino and O’Brien, 1968). The U.S. Environmental Protection Agency (2011) lists sediment as one of the most prevalent sources of impairment in surface waters of the United States.


Past water-quality studies in Johnson Creek included analyses and interpretation of chemical and physical characteristics (alkalinity, dissolved oxygen, pH, specific conductance, and temperature), nutrients, trace elements, anthropogenic organic compounds, and sediment during high- and low-flow periods (Edwards, 1992, 1994; Edwards and Curtiss, 1993). As part of a larger Willamette River basin study, Anderson and others (1996) sampled Johnson Creek for organic compounds, suspended sediment, and trace elements. Similarly, as part of a larger Columbia River basin study, McCarthy and Gale (1999) assessed the distribution of organic compounds using semipermeable membrane devices. Tanner and Lee (2004) evaluated the presence of organochlorine pesticides in whole-water samples from several locations in the Johnson Creek basin, including the temporal variations in the relation between suspended sediment and pesticides.


Suspended-sediment concentration (SSC) time-series data traditionally have been derived from laboratory analyses of water samples (Edwards and Glysson, 1999), and suspended-sediment load (SSL) has been computed using SSC and streamflow time series (Porterfield, 1972). This approach relies, in part, on interpolation of SSCs and streamflows between measurements using hydrologic judgment. Consequently, results are somewhat subjective and not easily reproduced by others.


Turbidity is an expression of the optical properties of a sample of water (Anderson, 2005). The presence of dissolved and suspended material (clays, silt, fine organic matter, and other material) in streamwater typically results in more turbid (less clear) water (ASTM International, 2007). Numerous studies have shown that turbidity values often are highly correlated with SSC values (Walling, 1977; Gilvear and Petts, 1985; Uhrich and Bragg, 2003; Rasmussen and others, 2005; Bragg and others, 2007; Bragg and Uhrich, 2010). By developing a turbidity-SSC regression model at a sampling station, time-series SSCs can be computed from continuous turbidity data. If the turbidity-SSC regression model is deemed adequate, the SSC and streamflow time series can be used to compute SSL (Rasmussen and others, 2009). Using this approach, SSL has been computed successfully in recent USGS studies in Florida (Lietz and Debiak, 2005), Oregon (Bragg and others, 2007; Bragg and Uhrich, 2010), and Virginia (Jastram and others, 2009). This study was an attempt to apply the technique in the Johnson Creek basin to determine whether the turbidity-SSC correlation exists there, and, if so, to use the technique to help calculate SSL and locate sediment sources in the watershed. The purpose of the study also was to calculate a suspended-sediment budget for the study period 2007–10.


Description of Study Area


The Johnson Creek basin (fig. 1) is in northwestern Oregon, on the eastern side of the Portland metropolitan area. The creek forms a wildlife and recreational corridor through densely populated areas in the cities of Gresham, Milwaukie, and Portland, and through agricultural and rural areas of Clackamas and Multnomah Counties.


Johnson Creek is about 24 mi long and has a topographic, surface-water drainage area of about 54 mi². The headwaters are in northern Clackamas County, northeast of the city of Boring. The watershed elevation ranges from a low of about 8 ft, at the confluence with the Willamette River, to near 1,130 ft, at one of the volcanic buttes in the southern part of the watershed (U.S. Geological Survey, 2011a).


The Johnson Creek basin has a temperate marine climate, characterized by wet winters with relatively mild temperatures and a dry season from summer to early autumn. Mean (average) annual precipitation in the watershed is about 53 in. and ranges from about 40 in. at the mouth of Johnson Creek to more than 60 in. at higher elevations to the east (U.S. Geological Survey, 2011a). Historically, about two-thirds of the annual precipitation falls during November–March (U.S. Geological Survey, 2011d). The summer months of June–September typically account for about 12 percent of annual precipitation. Most precipitation falls as rain, although periodic snow accumulation is not uncommon at higher elevations. Snowpack in most of the watershed is short-lived, as it usually melts rapidly in subsequent rain events.


The watershed is 65 percent urban and 22 percent forested. The remainder of the watershed consists of a mixture of agricultural and rural uses (U.S. Geological Survey, 2011a). About 30 percent of the watershed is covered by impervious surfaces (U.S. Geological Survey, 2011a). More information about the hydrology, hydrogeology, and land use of the Johnson Creek basin, including a list of previous investigations in and around the watershed, can be found in Snyder (2008) and Lee and Snyder (2009).


Purpose and Scope


This report presents the results of an investigation into the occurrence and transport of suspended sediment in the Johnson Creek basin for the water years 2007–10. Objectives of this investigation were to:


1. Evaluate the use of turbidity and (or) streamflow as a surrogate method for quantifying SSL in Johnson Creek, which, if valid, could significantly reduce the cost of future SSC monitoring; 

2. Compute the mean annual suspended-sediment budget for the watershed, which could be used for evaluating the efficacy of future restoration efforts, land-use practices, and policies aimed at reducing sediment load; and

3. Investigate the timing and spatial distribution of SSL within the watershed, so that future remediation efforts can focus on when and where sediment transport is most problematic.


The annual and monthly suspended-sediment loads were computed using data from two gaging stations, Johnson Creek at Regner Road, at Gresham (14211400) and Johnson Creek at Milwaukie (14211550). Historical streamflow data from a third long-term station, Johnson Creek at Sycamore (14211500), also were examined. The analysis method included developing regression models relating SSC to turbidity and streamflow, and calculating continuous SSC and SSL for each station.


First posted October 3, 2012