Eight stream sites (Fixed Sites) were chosen to describe the variability in the water quality of the Western Lake Michigan Drainages (WMIC) Study Unit of the National Water-Quality Assessment program. These sites were chosen in areas (Relatively Homogeneous Units) dominated by unique combinations of the environmental factors thought to be most important in influencing water quality; namely, land use, surficial deposits, and bedrock type. A study was designed to determine (1) the applicability of streamflow, nutrient, and suspended sediment data regularly collected at these eight sites describing the variability in these characteristics throughout the Study Unit during base-flow conditions and (2) the applicability of the interpretive results made from data collected at these few sites to streams throughout the Study Unit. This was done by sampling the Fixed Sites and an additional 83 sites in Relatively Homogeneous Units throughout the Study Unit during summer base-flow conditions.

Data collected at the Fixed Sites described the range in water-quality characteristics (stream-flow and concentrations of nutrients and suspended sediment) in the WMIC Study Unit and, in general, represented the water quality from the Relatively Homogeneous Units from which they were chosen. The result from the eight Fixed Sites agreed with those found for all of the sites; namely, that these water-quality characteristics in streams throughout the WMIC Study Unit during base-flow conditions are influenced primarily by the land use and surficial deposits in their drainage basins. General basin characteristics (bedrock information, topographic gradient, and basin size) were not important factors in explaining the variability in these water-quality characteristics during base-flow conditions, but may be important factors for other characteristics measured at Fixed Sites, such as major ions, and may be important during higher flow. In general, streams in agricultural areas had the poorest water quality; that is, they contained the highest concentrations of total phosphorus, total Kjeldahl nitrogen, and suspended sediment. Streams in urban and mixed agriculture/forested areas had moderate water quality, exhibiting the highest concentrations of total phosphorus, total Kjeldahl nitrogen, and suspended sediment, and the lowest base flow. In general, water quality in streams in areas with sandy/sand and gravel deposits and loamy deposits were very similar. Within the forested areas, streams in areas with a higher percentage of forested wetlands had lower base flow, higher concentrations of total Kjeldahl nitrogen, and lower concentrations of dissolved nitrite plus nitrate than streams in areas with a lower percentage of forested wetlands.

The variability in water quality throughout the WMIC Study Unit during base-flow conditions could be described very well by subdividing the area into Relatively Homogeneous Units and sampling a few streams with drainage basins completely within these homogeneous units. This subdivision and sampling scheme enabled the differences in water quality to be directly related to the differences in the environmental characteristics that exist throughout the Study Unit.