Methods

Hydrologic analyses in the Johnson Creek basin were based on data collected at numerous sites. Each groundwater and streamflow site has a unique 8 or 15 digit number and is associated with a map number that is used throughout the report (table 1; pl. 1). Map numbers 1 through 24 are groundwater sites and map numbers 25 through 45 are surface-water sites. Climate site numbers are assigned by the National Weather Service (NWS).

Groundwater

Groundwater level data were collected and evaluated using a monitoring well network consisting of wells measured quarterly or more frequently and supplemented by data from wells that were equipped with continuous groundwater level recorders for this study. Candidate wells were identified from previous USGS studies and from the Oregon Water Resources Department (OWRD) well database (http://www.oregon.gov/OWRD/PUBS/ToolsData.shtml). A criterion for selection was that the water level in a well should represent unconfined or water-table conditions. Priority was given to relatively shallow, recently constructed wells that had complete location information. Well information, including location, construction, and groundwater level measurements, was entered into the U.S. Geological Survey National Water Information System (NWIS), and is available online at http://waterdata.usgs.gov/or/nwis/gw using the USGS site number (pl. 1; table 1). Groundwater levels relative to a measuring point on a well and the elevation of the groundwater relative to land surface were established using NGVD 1929 as the land-surface datum. The land-surface elevation at each well was determined from a topographic map or from a digital elevation model (DEM). The 2-m lateral resolution DEM covering most of the Portland metropolitan area was obtained from Metro (2002), the regional government agency serving the Portland metropolitan area. The vertical resolution of the 2-m DEM is reported as about 0.3 ft (Metro, 2002). The land-surface elevations for wells used in the study were estimated using the 2-m DEM and calculated as the median land-surface elevation within a 100-ft radius of the reported locations. The buffer was used because most locations of wells and springs only are known to an accuracy of 100 ft, depending on the method of determination. The elevation of some wells additionally was determined by leveling from a known point, using methods established by Kennedy (1990).

Surface Water

Surface-water data consist of records from continuous streamflow sites and individual streamflow measurements (pl. 1; table 1). The continuous streamflow sites are: Johnson Creek at Regner Road, at Gresham (map number 26), beginning in 1998; Johnson Creek at Sycamore (map number 30), beginning in 1940; Johnson Creek at Milwaukie (map number 41), beginning in 1989; and Kelley Creek at 159th Drive at Portland (map number 29) beginning in 2000. Streamflow records were computed according to methods described in Rantz and others (1982). Daily mean streamflow and instantaneous peaks are from the U.S. Geological Survey National Water Information System (NWIS), and are available online at http://waterdata.usgs.gov/or/nwis/sw by using the USGS site number to access the information (table 1).

Several streamflow statistics based on aggregated daily mean streamflow values were used to characterize mean and low-flow conditions. Thus, daily mean streamflow was averaged over an annual time scale to provide an indication of annual mean flow conditions. Low streamflow was characterized using the annual 7-day and 30-day minimum streamflow, which is the average of the lowest 7 or 30 consecutive daily mean streamflow values during a specified time period during each year, respectively. For example, the annual 7-day low flow during the months of May through October for each year of streamflow record would be calculated as the average streamflow of the lowest 7 consecutive daily mean streamflows during the months of May through October only. Base-flow separation, using the program PART, partitions daily mean streamflow to estimate the base-flow component (Rutledge, 1998). Flow-duration curves, which were developed by ranking daily mean streamflow over a given time period and calculating the percentage of time that a specified daily mean streamflow was equaled or exceeded (Searcy, 1959), also were used to characterize low streamflow.

High flows were characterized for streamflow sites with more than 10 years of record using annual peak streamflow data. Peak streamflow frequency, commonly termed flood frequency, indicates the annual exceedance probability associated with a particular magnitude of annual peak streamflow. For example, an annual peak streamflow with a 1-percent exceedance probability represents a large flood that has only a 1-percent chance of that streamflow being exceeded in any year. The reciprocal of the annual exceedance probability is the average recurrence interval of a particular annual peak streamflow. Thus, the flood with an annual exceedance probability of 1 percent has an average recurrence interval of 100 years and commonly is referred to as the 100-year flood. Flood frequency data for this study were determined by fitting the log Pearson 3 probability distribution to the recorded annual peak streamflow values at each site having sufficient record using methods described by the U.S. Interagency Advisory Committee on Water Data (1982) and Flynn and others (2006).

Streamflow measurements were used to determine temporal changes in groundwater discharge to springs and to identify gains or losses in streamflow from the stream as a result of interaction with the groundwater system using methods described in Rantz and others (1982). Streamflow measurements were made at many locations on Johnson Creek and tributary streams. Streamflow measurements at all locations described in this report are available online from the U.S. Geological Survey National Water Information System (NWIS), and are available at http://nwis.waterdata.usgs.gov/or/nwis/measurements by using the USGS site number. Spring discharge was determined by measuring the streamflow in the channel formed by the spring. Because springs in the Johnson Creek basin typically emerge at multiple locations, the streamflow was measured downstream where depth and uniformity of flow were sufficient for measurement by a current meter. Streamflow generally was measured during dry-weather periods to most accurately characterize spring discharge.

Seepage measurements are an indirect method of quantifying groundwater discharge (gains) or recharge (losses) at the streambed. Seepage measurements consist of a series of streamflow measurements made at numerous locations along a stream reach at nearly the same time. Assuming no tributary inflows or streamflow diversions from one location to the next, the difference in streamflow at the two locations indicates the net gain or loss attributed to interaction with the groundwater system. As such, a seepage study is a “snapshot” at a given time of the interaction of the stream and the surrounding aquifer. Seepage measurements are made during the summer low-flow period when streamflow is fairly constant and runoff from precipitation is minimal.

As a result of streamflow measurement uncertainty, the use of seepage measurements to determine streamflow gains and losses may not be reliable when the gains or losses are a relatively small percentage of the measured streamflow. The accuracy of a streamflow measurement is determined by several factors, including uniformity of velocity, channel characteristics, and limitations of the meter in use. Most streamflow measurements made during this study were rated as “good,” meaning they were within 5 percent of the actual value. The determination of gains and losses and the effect of measurement uncertainty are described in Lee and Risley, (2002, p. 22-23).

Spatial and temporal variations in hydrologic data were analyzed by using parametric and nonparametric methods. Spatial variations generally were analyzed using linear regression, a parametric method that relates one hydrologic variable to another with a best-fit line (equation). The strength of the linear regression relation commonly is measured by the R-squared (R²) value. Correlation was used to quantify the degree by which two variables are related. The strength of the correlation is described by the R value. A nonparametric correlation method, Kendall’s tau, was used to analyze temporal variations or temporal trends of hydrologic variables (Helsel and Hirsch, 2002). Kendall’s tau is the correlation between the ranked values of two variables, and was implemented using the S-PLUS^® data-analysis software (Insightful Corporation, 2005). Values of Kendall’s tau range from -1 to +1. The sign of Kendall’s tau indicates whether the relation is directly proportional (positive value) or inversely proportional (negative value). The magnitude of the deviation (either positive or negative), from zero indicates the strength of trend. The null hypothesis is that no temporal trend exists. The statistical significance of the trend was evaluated by the probability (p) that the observed trend happened by chance. A p-value of 0.05 indicates a 95-percent probability that the observed trend is not a chance occurrence. For this study, Kendall’s tau values having p-values less than 0.05 were considered significant.

Water Quality

Water-quality results consist of discrete stream samples analyzed for specific conductance and data from continuous stream-temperature recorders. Temperature data were recorded at long-term streamflow sites and at other locations for several weeks in conjunction with seepage studies. Continuous stream-temperature data collection in the Johnson Creek basin began in 1998 (pl. 1; table 1). Daily maximum, minimum, and mean stream temperature data are stored in the NWIS, and are available online at http://waterdata.usgs.gov/or/nwis/qw by using the USGS site number to access the information. Summer stream temperature was characterized using the 7-day average maximum value, a statistic commonly used for regulatory purposes. Samples analyzed for specific conductance and short-term, continuous temperature data collected during the seepage studies were used as a qualitative measure of groundwater discharge to the stream. The short-term temperature data collected in conjunction with the seepage studies are not published, but are available from the Oregon Water Science Center. Specific conductance (Wilde, 2005) and stream temperature data (Wagner and others, 2006) were collected according to set guidelines.

Meteorologic Data

The NWS site at the Portland Airport (site number 356751), about 10 mi north of the mouth of Johnson Creek, was the primary source of precipitation data. These data were provided by the NWS and served to the public by the Oregon Climate Service (Oregon Climate Service, 2007). Climate data at the Portland Airport began in 1938. These data were merged with unadjusted monthly total precipitation data at the NWS site in downtown Portland (site number 356761) from 1911 to 1937 (Oregon Climate Service, 2007). Some data gaps were filled with monthly precipitation data from Vancouver, Washington (site number 458773), about 10 mi northwest of the Portland Airport (National Oceanic and Atmospheric Administration, 2007), and Salem, Oregon (site number 357500), about 50 mi to the south (Oregon Climate Service, 2007). The precipitation data from Vancouver and Salem were adjusted proportionally, based on adjacent periods of concurrent data.

Precipitation data at the fixed location and the spatial distribution of precipitation were assessed using the Precipitation Elevation Regressions on Independent Slopes Model (PRISM) (PRISM Group, 2007). PRISM is a data layer of estimated annual precipitation in the State of Oregon in 800 × 800 m grid cells based on the period from 1971 to 2000.