Watershed Management and Nutrient Loading in the Newman Lake, Washington Watershed
by Douglas L. Robison
M.S. Thesis. Environmental Science and Regional
Planning.
Abstract
Since 1975, Newman Lake and the surrounding watershed have been the subject of several studies. Nuisance algal blooms occur seasonally in this meso-eutrophic lake, and community concern has resulted in the initiation of lake and watershed management efforts. Complex environmental conditions contribute to the intensity and frequency of the algal blooms; however, data collected suggests external nutrient loading from the watershed is significant. This 18,500 acre watershed has been subjected to over 100 years of logging in addition to agricultural impacts on the floodplain. Development and off-road recreational vehicles have also had an impact. The degree to which watershed management has been implemented and applied is uncertain.
In an effort to evaluate the impacts watershed disturbances and management efforts may be having on receiving water quality, total suspended solids data and components of phosphorus and nitrogen data were examined. Data was collected and analyzed for a twelve month period in 1996. In addition, data from 1975, 1986 and 1990 studies were re-evaluated. Nutrient loading was determined for each of these years and related to precipitation and hydrologic export from subbasins in the watershed. In 1996, the largest creek accounted for 76 % of total phosphorus loading, 72 % of total nitrogen loading and 82 % of total suspended solids loading. Data were
evaluated using regression analyses with nutrient and total suspended solids export values plotted against hydrologic export values. Regression analyses determined high correlation coefficients for parameters examined in the largest creek, but for smaller creeks, there were poor correlations and high variability for total phosphorus and for total suspended solids data. When precipitation was compared with total loading of phosphorus and nitrogen, for 1975, 1986, and 1990; a pattern was evident. As such, in 1996, loading values did not reflect this pattern.Improved watershed management strategies are recommended with an emphasis on best management practices for controlling erosion and nutrient loading. Incorporating the Spokane County Shoreline Program and Washington Forest Practices into a comprehensive watershed management strategy could improve current conditions in the watershed and provide improved in-flowing water quality to Newman Lake.