Rating System

The River Bacterial Index formula incorporates two factors representing key aspects of water quality. They are: the number of times bacterial density in these samples exceeds a guideline (frequency) and the extent to which the guideline is exceeded (amplitude). The result is a number between 0 and 100, where 100 represents the best quality, relative to objectives. The numbers are further divided into 5 categories:

• Excellent (96-100) - Objectives almost always met
• Good (81-95) - Objectives occasionally not met, but usually by small amounts
• Fair (66-80) - Objectives sometimes not met by moderate amounts
• Marginal (46-65) - Objectives often not met, sometimes by large amounts
• Poor (0-45) - Objectives almost always not met by large amounts