The Master Water Data Index (MWDI) is a computerized data base developed and maintained by the National Water Data Exchange (NAWDEX). The MWDI contains information about water-data collection sites. This information includes: the identification of sites for which water data are available, the location of these sites, the type of site, the data-collection organization, the types of data available, the major water-data parameters for which data are available, the frequency at which these parameters are measured, and the media in which the data are stored. This document contains a definition and description of each component of the MWDI data base. (Woodard-USGS)