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.
S Fk San Gabriel Rv At Georgetown, Tx
Nf Tolt River Above Yellow Creek Nr Carnation, Wa
Cumberland River At Williamsburg, Ky
Marais Des Cygnes R At Melvern, Ks
North Fork Spring River Near Purcell, Mo
Pataula Creek Near Georgetown, Ga
Le Sueur River Near Rapidan, Mn
Chena R Bl Moose C Dam Ak
Holmes Creek At Vernon, Fla.
Henderson Creek Near Oquawka, Il
Quaboag River At West Brimfield, Ma
Plum Ck Nr Luling, Tx
East Fork Big Creek Near Bethany, Mo
Sinnemahoning Creek At Sinnemahoning, Pa
Walker Rv At Snyder Ln Nr Mason, Nv
Maroon Creek Near Aspen, Co.
Niobrara River At Mariaville, Nebr.
San Bernard Rv Nr Sweeny, Tx
Croton R At New Croton Dam Nr Croton-On-Hudson Ny
Shenango River At Pymatuning Dam, Pa
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.