This report is an index of all surface-water, ground-water, and quality-of-water data which have been or are now being collected on a continuous or periodic basis, by the U. S. Geological Survey and the cooperating agencies in the Delaware River basin, to September 30, 1951. The index is divided into two principal sections, the surface-water section relating to streams and the ground-water section to wells and springs. Each section contains an index of records on quality of water. All surface-water data in this report are classified by stream name and state. The ground-water data are classified by state and county inasmuch as most ground-water studies and compilations are made on the basis of a county as a unit. Lists of selected publications relating to surface and ground waters appear at the end of the respective sections. These lists include not only most of the publications issued or prepared by the Geological Survey, but also a representative group of publications prepared elsewhere, particularly reports concerned with large parts of, or the entire, basin. Those Federal, State, and municipal agencies now engaged in cooperative water-resources investigations with the Geological Survey are listed in a section following the ground-water portion of this report. In addition to the data indicated in this index, the Geological Survey has obtained information at many miscellaneous localities throughout the Delaware River basin. Much of this information is available to the public upon application to the Survey field offices, and to facilitate the use of these miscellaneous data, a list of the field offices having