The RiverCast forecasts the expected range of bacteria levels based on current rain, flow, and turbidity measurements,
compares it to U.S. Environmental Protection Agency (US EPA) guidance, and posts the corresponding rating.
The RiverCast and its water quality ratings are based on a historical analysis of relationships between water quality, stream flow, and rainfall data,
and on the US EPA’s 2002 Implementation Guidance for Ambient Water Quality Criteria for Bacteria.
This document identifies the maximum level of bacteria measured in a single sample that would be
within acceptable illness levels for recreational activities that may involve complete immersion in the water.
This graphic shows the RiverCast color ratings with corresponding ranges of allowable bacteria concentrations.
Historical bacteria and turbidity (water clarity) data collected by the Philadelphia Water Department and rainfall
and streamflow data provided by the U.S. Geological Survey near Fairmount Pool (Manayunk to Boathouse Row) from 1998 to 2000
were compiled for the analysis. The data were analyzed to identify flow, rainfall and turbidity conditions that
correspond to US EPA guidance for recreational water quality criteria.
The conditions and water quality ratings defined by the historical data analysis were tested on a set of data
from 2004 to 2005 to assess the accuracy of the relationships and RiverCast predictions.
The testing showed that the RiverCast predictions are very conservative. 65% of the time the RiverCast prediction was accurate.
35% of the time the prediction was conservative (higher bacteria levels were predicted than measured).
There were no examples of RiverCast-predicted levels lower than the measured levels.
A RiverCast rating will either accurately predict water quality or give a conservative recommendation to exercise more caution than might be required.
U.S. Environmental Protection Agency (2002). Implementation Guidance for Ambient Water Quality Criteria for Bacteria, EPA-823-B-02-003.