This U.S. Geological Survey data release provides surface-water quality, streamflow, and groundwater-elevation data collected within the Central Pine Barrens (CPB) Region of Suffolk County, New York, from October 1, 2019 through September 30, 2020. The data were collected in cooperation with the Central Pine Barrens Commission and the Town of Brookhaven as part of a five-year comprehensive water-resources monitoring program. Water quality and quality-assurance data from seven sites on two rivers (Carmans River- five sites and Peconic River- two sites) in the CPB are included. Carmans River sites were sampled four times throughout the year (fall, winter, spring, and summer) and Peconic River sites were sampled twice throughout the year (fall and spring). Water-quality field parameters measured during each site visit are included. All samples were analyzed for nutrients, major inorganics, and alkalinity. An annual set of organic samples for pesticide and pharmaceutical analysis were collected at one Carmans River site and two Peconic River sites. Periodic groundwater-elevation data collected from six CPB wells are included. Continuous (15-minute interval) groundwater-elevation data are included for one of the six wells.