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.
Yellowstone River At Billings Mt
Tongue R Bl Brandenberg Bridge Nr Ashland Mt
Tongue River At State Line Nr Decker Mt
Smith River Bl Eagle Cr Nr Fort Logan Mt
Madison River Bl Hebgen Lake Nr Grayling Mt
Dearborn River Near Craig Mt
Silver Bow Creek At Opportunity Mt
Silver Bow Creek At Warm Springs Mt
Ruby River Near Twin Bridges Mt
Lost Creek Near Galen, Mt
Missouri River Near Landusky Mt
Sun River At Simms Mt
Big Hole River Bl Big Lake Cr At Wisdom Mt
M F Flathead River Near West Glacier Mt
Yellowstone River Near Sidney Mt
Willow Creek Near Harrison Mt
W Rosebud Cr At Emerald Lk Campground Nr Roscoe
S F Flathead River Nr Columbia Falls Mt
Missouri R At E Frazer Pump Plant Nr Frazer Mt
Silver Bow Cr Bl Blacktail Cr, At Butte, 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.