The U.S. Geological Survey‘s (USGS) Hydro-Climatic Data Network (HCDN) is a subset of all USGS streamgages for which the streamflow primarily reflects prevailing meteorological conditions for specified years. These stations were screened to exclude sites where human activities, such as artificial diversions, storage, and other activities in the drainage basin or the stream channel, affect the natural flow of the watercourse. In addition, sites were included in the network because their record length was sufficiently long for analysis of patterns in streamflow over time. The purpose of the network is to provide a streamflow dataset suitable for analyzing hydrologic variations and trends in a climatic context. When originally published, the network was composed of 1,659 stations (Slack and Landwehr, 1992) for which the years of primarily "natural" flow were identified. Since then data from the HCDN have been widely used and cited in climate-related hydrologic investigations of the United States. The network has also served as a model for establishing climate-sensitive streamgage networks in other countries around the world.
After nearly two decades of use without undergoing a systematic revalidation, questions have arisen as to whether many of the original stations still maintain their climate-sensitive status or even remain operational, as some are known to have closed. Some watersheds had been altered to the point that stations no longer meet the minimal disturbance criteria set forth in the original HCDN report. In addition, some sites that did not qualify as HCDN sites in 1988 (the last year of data evaluation) because their records were too short now have sufficiently long streamflow records for climate-sensitivity studies. Accordingly, a review of the existing network was initiated in 2009 in order to drop old stations and add new ones as appropriate.