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.
Tobacco River At Eureka, Mt
Milk River At Eastern Crossing Of Int Bndry
Sun River Bl Diversion Dam Nr Augusta Mt
Belt Creek Near Monarch Mt
Tongue River At Tongue R Dam Nr Decker Mt
Red Rock R Bl Lima Reservoir Nr Monida Mt
Clarks Fork Yellowstone River Nr Belfry Mt
Big Horn Canal Near St. Xavier Mt
Birch Cr Bl Heart Butte Road, Nr Heart Butte, Mt
Musselshell River Ab Big Coulee Creek At Lavina Mt
Missouri River At Cascade Mt
South Fork Sun River Near Augusta Mt
Missouri River Near Great Falls Mt
Mission Creek Ab Reservoir Nr St. Ignatius Mt
Ruby River Below Reservoir Near Alder, Mt
Marias River Near Shelby Mt
Bitterroot River At Bell Crossing Nr Victor Mt
North Fork Sun River Near Augusta Mt
Muddy Creek At Vaughn Mt
Teton River Bl South Fork Nr Choteau 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.