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.
Flathead River At Columbia Falls Mt
Nf Blackfoot R Ab Dry Gulch Nr Ovando Mt
Mill Creek At Opportunity, Mt
Bighorn River At Bridge, At St. Xavier, Mt
Warm Springs Creek At Warm Springs Mt
Willow Creek At Opportunity, Mt
Milk River At Nashua Mt
Red Rock Cr Ab Lakes, Nr Lakeview, Mt
Silver Bow Creek At Montana Street, At Butte, Mt
North Fork Sun River Near Augusta Mt
Marias River Near Shelby Mt
Bitterroot River At Bell Crossing Nr Victor Mt
Muddy Creek At Vaughn Mt
Teton River Bl South Fork Nr Choteau Mt
St. Mary River Near Babb Mt
Flint Creek At Maxville Mt
Ruby River Above Reservoir Near Alder, Mt
Milk River At Havre Mt
Missouri River Near Ulm Mt
Thompson River Near Thompson Falls 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.