US Water Quality Index
Our composite Water Quality Score combines temperature, reservoir levels, and flow rates to rate conditions for swimming, fishing, and recreation.
How We Calculate Water Quality Score (0–100)
Maximum points when water temp is near ideal 71.5°F (22°C) for most recreational activities.
Full points when reservoir is at or near full pool capacity. Low levels reduce score.
Moderate, stable flow (100–5,000 cfs) earns full points. Extreme highs or lows reduce score.
Shields River Bl Sf Shields River Nr Wilsall, Mt
Sun River Bl Diversion Dam Nr Augusta Mt
Thompson River Near Thompson Falls Mt
Little Bighorn River Nr Crow Agency Mt
Marias River Near Chester Mt
Milk River At Eastern Crossing Of Int Bndry
Hyalite C At Hyalite R S Nr Bozeman Mt
Beaver Creek Near Hinsdale Mt
W Rosebud Cr At Emerald Lk Campground Nr Roscoe
Belt Creek Near Monarch Mt
Red Rock R Bl Lima Reservoir Nr Monida Mt
Middle Fork Rock Cr Nr Philipsburg Mt
Whitefish River Near Kalispell Mt
Birch Cr Bl Heart Butte Road, Nr Heart Butte, Mt
Musselshell River Ab Big Coulee Creek At Lavina Mt
West Fork Bitterroot River Nr Conner Mt
South Fork Sun River Near Augusta Mt
Little Bighorn River At State Line Nr Wyola Mt
E Gallatin R Ab Water Reclamation Fa Nr Bozeman Mt
Boulder River Near Boulder, Mt
Understanding the Water Quality Index
The US Water Quality Index is a composite score designed to give anglers, swimmers, boaters, and outdoor enthusiasts a quick, at-a-glance assessment of water body conditions. Unlike traditional water quality indices that focus on chemical pollutants (which require laboratory analysis), our index uses real-time USGS sensor data to reflect physical conditions relevant to recreation.
Temperature Component (40 points)
Water temperature is the most important factor for recreational fishing, swimming, and wildlife activity. The ideal range for most activities — comfortable for swimming, optimal for bass and walleye fishing — is approximately 65–78°F (18–26°C). Our scoring peaks at 71.5°F and decreases proportionally as temperature moves away from this ideal.
Reservoir Level Component (30 points)
Reservoir levels (measured as a percentage of full pool) reflect drought conditions, drought recovery, flood risk, and overall ecosystem health. A reservoir at full pool (100%) scores maximum points. Severely depleted reservoirs — common during drought — score fewer points. This metric is particularly relevant in western states where water storage is critical.
Flow Rate Component (30 points)
River discharge (measured in cubic feet per second, cfs) indicates safe boating, wading, and kayaking conditions. Moderate, stable flow rates between 100–5,000 cfs are typically ideal for most recreation. Extremely high flows indicate flood risk, while extremely low flows suggest drought stress and limited habitat for fish.