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
Powder River Near Locate Mt
Yellowstone River At Glendive, Mt
Missouri R No. 4 Nr Nohly, Mt
Clark Fork Near Plains Mt
Yellowstone River At Forsyth Mt
Missouri R At W Frazer Pump Plant Nr Frazer Mt
Yellowstone River At Miles City, Mt
Flathead Lake At Polson Mt
Missouri River At Virgelle Mt
Prospect Creek At Thompson Falls Mt
Little Bighorn River Nr Crow Agency Mt
Nelson Creek Near Van Norman Mt
Lost Creek Near Anaconda Mt
Peoples Creek Near Hays Mt
Rock Cr Bl Canal Diversion, Near Darby, Mt
Rosebud C Bel Lame Deer C Nr Lame Deer Mt
Big Sandy Cr Ab Spring Coulee, Nr Laredo, Mt
Beaver Cr Bl Guston Coulee Nr Saco Mt
Prickly Pear Creek Near Clancy 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.