The conditions for a day are determined from River flow and recent rain.  Conditions for TOMORROW and the NEXT DAY are based on National weather Service flow projections and weather forecasts. Most E. coli in the Rivers are washed off the land by rain.

The posted conditions apply only to the NF, SF and main stem Shenandoah River, not to tributaries or any other water body.

All River E. coli data from 2007 to date collected by VA DEQ, USGS and FOSR support the observation that E. coli measurements are less than 235 cfu/100ml when river flows  in cfs are less than the drainage area in sq miles. (1 cfsm) .  Of 1123 measurements  made  at less than 1 cfsm 1036 were less than 235 cfu/100ml. Of the 87 over 235cfu /100ml 48 were collected within 2 days of rain exceeding 0.5 inches  most the remaining 39 were collected within 2 days of some rain.

The median flow at the Front Royal 01631000 gage  has been less than 1 cfsm every day from May 15 to September 15 for 98 years. The average flow for the same period was less than 1 cfsm 80% of the time. see the duration curve 

Days posted green mean the day is similar to conditions when the bacterial were less than 235 cfu/100ml. On green days the river flow at the site in cubic feet per second (cfs) is less than the drainage area of the site in square miles (sq mi) and it has not rained more than 0.5 inches.  235 cfu/100ml is a standard developed over years of study showed that only about 30 out of 1000 swimmers would experience illness when the E. coli were less than 235 cfu/100 ml.

On yellow colored days the bacteria on similar days in the past have often had more than 235 cfu/100ml. Yellow colored days may have dots. The more dots the similar days in the past had more E. coli.  On yellow days with dots the flow cfs exceeded the drainage area sq mi. Yellow days with no dots were preceded by rain.

On red colored days the bacteria on similar days in the past have always been more than 235 cfu/100ml. Red days have very high River flow and are not safe due to currents, bacteria and muddy conditions. Red days normally occur 2% of the time during the swimming season  (mid May to mid September) after heavy rains.

This site is not a health advisory. You use the Rivers at your own risk.  While 235 cfu/100 ml is a standard used by many jurisdictions, Virginia currently has no standard for freshwater primary contact recreation (swimming).  Virginia Department of Health suggestions for swimming safety are here.