The Elk Creek basin in west-central Wisconsin was studied during the 1980 water year to define the water quality in relation to streamflow. Agricultural nonpoint-source discharges were suspected of contributing significantly to degraded water quality. Two water quality and streamflow gaging stations were established--one on Elk Creek near Independence and the other on Bruce Valley Creek near Pleasantville.
Streamflow for the 1980 water year was about 28% greater than average, and precipitation was about 36% greater than average. At Elk Creek, base flow was about 68% of the total stream discharge and, at Bruce Valley Creek, base flow was about 56% of the total stream discharge. Streamflow at Elk Creek during the study ranged from 43 to 1,020 cu ft/sec; Bruce Valley Creek discharges ranged from 2.8 to 306 cu ft/sec. The low flow discharges are significantly greater than the 2-year, 7-day low flow, whereas the peak discharges were between a 2-year and a 5-year flood recurrence interval.
Suspended-sediment yields were greatest at Bruce Valley Creek, and was 215 ton/sq mi/yr compared with 184 ton/sq mi/yr for Elk Creek. Double mass-accumulation curves, however, indicated that loading rates were similar for both stations. Suspended sediment and nutrient yields were slightly above the long-term average.
Total phosphorus , ammonia nitrogen, and organic nitrogen yields also were highest at Bruce Valley Creek. Phosphorus yields at Bruce Valley Creek were 1,6000 lb/sq mi/yr compared with 1,350 lb/sq mi/yr at Elk Creek. Because base flow was a greater percentage of total discharge, nitrite plus nitrate nitrogen yields and loads were highest at Elk Creek.
All phosphorus concentrations at Bruce Valley and Elk Creeks exceeded levels recommended by EPA to prevent the formation of biological nuisance growths. Only one sample, collected on March 18, 1980, at Bruce Valley Creek may have exceeded the Wisconsin Department of Natural Resources criterion of toxic levels for un-ionized ammonia (0.04 mg/L). No samples from Elk Creek extended the criteria. (
