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.
Gunpowder Falls At Glencoe, Md
Potomac River At Paw Paw, Wv
Monocacy River At Jug Bridge Near Frederick, Md
Youghiogheny River Near Oakland, Md
Patuxent River Near Bowie, Md
Youghiogheny River Near Mchenry, Md
North Branch Potomac River At Kitzmiller, Md
Potomac River At Harpers Ferry, Wv
Ches Bay At Mouth Of Susq R At Havre De Grace, Md
Susquehanna River At Conowingo, Md
Deer Creek At Eden Mill Dam Near Pylesville, Md
Mcintosh Run Near Leonardtown, Md
St Marys Lake Near Callaway, Md
Potomac River At Edwards Ferry Near Poolesville,Md
Little Deer Creek Reservoir Near Shawsville, Md
Potomac River At Burkittsville Rd At Brunswick, Md
Savage Riv Bl Savage Riv Dam Near Bloomington, Md
Cranberry Branch Near Westminster, Md
South Fork Jabez Branch At Millersville, Md
Octoraro Creek Near Richardsmere, Md
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.