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.
Big Hole River Near Glen Mt
Gallatin River At Logan Mt
Yellowstone River Near Livingston, Mt
Big Hole River Near Wise River Mt
Madison River Near Norris Mt
Clark Fork Ab Little Blackfoot R Nr Garrison Mt
Clark Fork Above Missoula Mt
Big Hole R Bl Hamilton Ditch Nr Twin Bridges, Mt
Beaverhead River At Twin Bridges, Mt
Madison River Bl Ennis Lake Nr Mcallister Mt
Big Hole River Near Melrose Mt
Marias River Near Loma Mt
Jefferson River Near Twin Bridges, Mt
Blackfoot River Near Bonner Mt
Jefferson River At Parsons Bdg Nr Silver Star, Mt
Clark Fork Near Galen Mt
Madison River Near West Yellowstone, Mt
Madison River At Ennis, Mt
Big Hole River At Maiden Rock Nr Divide Mt
Jefferson River Near Three Forks 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.