Water-quality variations in the Bull Run Watershed, Oregon, under 1978 to 1983 management conditions
During the period October 1978 to September 1983, the U.S. Geological Survey, in cooperation with the City of Portland (Oregon) Water Bureau, conducted a study in the Bull Run River basin to define the hydrologic characteristics of the basin, and to examine relations between basin characteristics (both natural and man-made) and stream water quality and quantity within the basin. Hydrologically, the 1978-1983 period can be characterized as representative of the long-term average, with no records of extreme events. Likewise, water quality constituent concentrations affected by quantity of streamflow are representative of average values and ranges and exclude values that would be obtained during periods of extreme events. Ranges of concentration of major anions and cations for surface water collected October 1978 to September 1983 are similar to values collected historically. The ratio of constituent to chloride values determined for precipitation data collected during the period June 1980 to September 1981 indicated that other sources besides seawater contributed to its composition. In ratios of constituents in precipitation, Bull Run values are similar to those of other remote sites in Alaska, Washington, and California. Comparisons of storm-related suspended sediment load to annual suspended sediment loads indicated that 62% to 78% of the total annual loads occurred in 3 to 4 days during an average year. Multiple-linear regression analysis using discharge, suspended sediment and specific conductance indicated that most of the variation in the annual values could be explained by naturally occurring processes within the basin. A nonparametric time-trend analysis of 24 water quantity and quality constituents showed no statistically significant trends with estimated slopes large enough to be readily measurable for a particular year. Four constituents that were sampled weekly (turbidity, specific conductance, silica, and phytoplankton) had statistically significant trends with slope indicators that might be measurable after 6 years. However, trend analysis on daily mean specific conductance and suspended sediment values does not confirm the weekly constituent trend results.