Benthic macroinvertebrate samples representing 151 taxa were collected in August 1995 to examine the linkage between land use, water quality, and aquatic biointegrity in seven tributaries of the Blackfoot River watershed, Montana. The tributaries represent silvicultural (timber harvesting), agricultural (irrigated alfalfa and hay and livestock grazing), and wilderness land uses. A 2.4 km (1.5 mile) reach of a recently restored tributary also was sampled for comparison with the other six sites. A geographic information system (GIS) was used to characterize the seven subwatersheds and estimate soil erosion, using the Modified Universal Soil Loss Equation, and sediment delivery. The wilderness stream had the highest aquatic biointegrity. Two agricultural streams had the largest estimated soil erosion and sediment delivery rates, the greatest habitat impairment from nonpoint source pollution, and the most impoverished macroinvertebrate communities. The silvicultural subwatersheds had greater rates of estimated soil erosion and sediment delivery and lower aquatic biointegrity than the wilderness reference site but evinced better conditions than the agricultural sites. A multiple-use (forestry, grazing, and wildlife management) watershed and the restored site ranked between the silvicultural and agricultural sites. This spectrum of land use and aquatic biointegrity illustrates both the challenges and opportunities that define watershed management. (KEY TERMS: macroinvertebrates; aquatic ecosystems; land use; GIS; biomonitoring; nonpoint source pollution; sediment delivery; soil erosion; watershed management.)1Paper No.

assisted with field work. Peggy Bahls performed all statistical calculations relating to periphyton community structure. Rob Greene assisted with algal assays. Chemical analyses were conducted by the Chemistry Laboratory Bureau of the Department of Health and Environmental Sciences.

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Values for 31 biologically-related water quality parameters were measured seasonally at 16 stations on 11 streams in northcentral Montana from September 1977 to April 1978. Mean values for 15 key indicators were used to develop a composite water quality rating based on biological conditions. Three stations had poor water quality from the standpoint of stream biology: Big Sandy/ Muddy/ and Pondera Creeks. All three suffered from heavy silt loads resulting from accelerated stream bank erosion/ poor irrigation practices/ and natural causes. Also/ nutrient levels were seasonally very high at these stations due to agricultural runoff. Big Sandy and Pondera Creeks were affected to a lesser extent by municipal discharges. Eleven other stations were ranked as fair and were affected to varying degrees by non-point source pollution. Two of these 11 stations-Milk River at Chinook and Teton River near Dutton-also receive miinicipal discharges in need of upgrading. Only two streams were rated as good: the Dearborn River and the Missouri River at Cascade. On this basis / it was concluded that non-point source pollution is the most serious / biologically debilitating water quality problem at stations on the Northcentral Loop. Survey results probably can be considered representative of overall water quality in the lowland portions of northcentral Montana because of similar water and land use practices.