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.
Tongue R Bl Brandenberg Bridge Nr Ashland Mt
Tongue River At Miles City, Mt
Powder River Near Locate Mt
Tongue River At State Line Nr Decker Mt
Yellowstone River Fish Bypass Channel Nr Intake Mt
Madison River Near West Yellowstone, Mt
Madison River Near Norris Mt
Madison River Bl Ennis Lake Nr Mcallister Mt
Milk River At Nashua Mt
Beaverhead River At Twin Bridges, Mt
Madison River At Ennis, Mt
Jefferson River Near Three Forks Mt
Clark Fork Ab Little Blackfoot R Nr Garrison Mt
Jefferson River Near Twin Bridges, Mt
Gallatin River At Logan Mt
Clark Fork At Deer Lodge Mt
Big Hole River Bl Big Lake Cr At Wisdom Mt
Big Hole R Bl Hamilton Ditch Nr Twin Bridges, Mt
Big Hole River Near Wise River Mt
Madison River Near Cameron 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.