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.
Mississippi River At Grafton, Il
Kress Creek At West Chicago, Il
East Branch Dupage River At Short St At Lisle, Il
Tyler Creek At Elgin, Il
Ohio River At Smithland Dam, Smithland, Ky
Fondulac Creek Near East Peoria, Il
Skokie River Near Highland Park, Il
North Fork Embarras River Near Oblong, Il
Midlothian Creek At Oak Forest, Il
Lake Fork At Atwood, Il
Robinson Creek Near Shelbyville, Il
Aux Sable Creek Near Morris, Il
Mill Creek Near Batavia, Il
Salt Creek At Elmhurst, Il
Sugar Creek Near Bloomington, Il
South Branch Kishwaukee River At Dekalb, Il
East Fork Kaskaskia River Near Sandoval, Il
Des Plaines River At Lincolnshire, Il
Blackberry Creek Near Montgomery, Il
Boneyard Creek At Urbana, Il
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.