The Alberta River Water Quality Index is a way to summarize physical, chemical and biological data into a simple descriptor of water quality. The Index provides a simple snapshot of annual water quality conditions in major rivers of the province.   The River Water Quality Index formula incorporates three factors representing key aspects of water quality:

Scope: the number of variables not meeting water quality objectives
Frequency: the number of times objectives are not met
Amplitude: the amount by which objectives are not met
The result is a number between zero and 100, where 100 represents the best quality relative to objectives. The numbers are further divided into 5 categories ranging from Excellent to Poor.

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