Data Collection


Continuous water-quality data were collected by in situ (left in place) water-quality monitoring devices and discrete water-quality data were collected using manual sampling methods. Four streamflow and temperature stations were in operation in the Johnson Creek basin during water years 2007–10, three on Johnson Creek, and one on the largest tributary, Kelley Creek. Of the three on Johnson Creek, two stations contained turbidity probes. The dates and locations of temperature, turbidity, and streamflow monitoring are shown in table 1. All data are available through either the USGS National Water Information System (NWIS) web site (U.S. Geological Survey, 2011b) or the project web site (U.S. Geological Survey, 2011c).


Station Instrumentation


Turbidity was monitored using a McVan Analite^® NEP 395 probe at each station. The maximum upper range of the Analite^® probe is 1,000 Formazin Nephelometric Units (FNU) (International Organization for Standardization, 1999; Anderson, 2005). Recorded turbidity values compared favorably with a calibrated field turbidity meter and never exceeded the upper range of the instrument. Periodic cross-sectional measurements were made with field turbidity meters. These measurements showed little variation throughout the cross sections at Gresham and Milwaukie, suggesting well-mixed channels. Temperature data also were collected at each station and used for quality control. Streamflow data were collected in accordance with the maintenance and evaluation protocols outlined by Rantz and others (1982) and Kennedy (1984).


Data at the Milwaukie station (14211550) were collected in 30-min increments. The Gresham station (14211400) is prone to more rapid changes in streamflow and turbidity than the Milwaukie station. Consequently, data were collected at this station in 15-min increments. The data from both stations were uploaded to the USGS NWIS several times per day by phone or satellite telemetry.


Suspended-Sediment Sampling


Suspended-sediment samples were collected periodically from water year 2005 to 2010 at the Milwaukie and Gresham gaging stations. Samples were collected primarily during large storm events, when SSCs were highest, using the standard USGS method for the equal-width-increment (EWI) method (Edwards and Glysson, 1999). Most samples were collected using a US DH-48, DH-59, DH-74 or DH-77 sampler (fig. 2). SSC samples were analyzed at the Cascade Volcano Observatory laboratory (Guy, 1969), and the results were reported in milligrams per liter. Additionally, most samples were analyzed for the percentage of silt and clay, reported as a mass percentage of sediment finer than 62 µm in diameter. The resulting data were used to investigate the relation between the optical property of turbidity and the concentration of suspended sediment in the stream (appendixes A and B).


First posted October 3, 2012