States are required to submit reports of water quality to EPA in even numbered years. Because the IWI supplements the state and tribal water quality assessments with additional information, the two reports may not be exactly the same. The IWI is an excellent example of how the government can provide information for the public's use in watershed protection and restoration. The Index not only categorizes watersheds based on their overall aquatic health, but also provides more detailed information on 15 separate indicators at both national and local scales. Interested citizens can, for instance, see where the contamination is, what the major sources are, whether discharge permit holders are in compliance, and where data are missing, and how one watershed compares to others.
Detailed text describes the significance of the data, and it can be downloaded from the Internet.
For each condition indicator, values were selected which, in EPA's professional judgement, represent an appropriate basis to describe the aquatic resources within the watershed as having good quality, fewer problems or more problems. All other indicators are weighted equally. Where there is insufficient data for a particular indicator we will display that on the map and present it in the Profile. At least 10 of the 15 data layers must be present to calculate the overall index for any given watershed. The IWI includes a separate source water assessment data layer which combines three data sets into a single index to characterize source water condition.