A water quality index provides a single number (like a grade) that expresses overall water quality at a certain location and time based on several water quality parameters. The objective of an index is to turn complex water quality data into information that is understandable and useable by the public. This type of index is similar to the index developed for air quality that shows if it’s a red or blue air quality day. The use of an index to "grade" water quality is a controversial issue among water quality scientists. A single number cannot tell the whole story of water quality; there are many other water quality parameters that are not included in the index. The index presented here is not specifically aimed at human health or aquatic life regulations. However, a water index based on some very important parameters can provide a simple indicator of water quality. It gives the public a general idea the possible problems with the water in the region.

BASIN has researched several types of water quality indices and has found one that is used by many communities for characterizing overall water quality. This index was originally developed by the National Sanitiation Foundation (NSF) (Brown and others, 1970). BASIN may revise the NSF index in the future to better suit Boulder area water specifically.