In cooperation with Dallas Water Utilities (DWU), the U.S. Geological Survey (USGS) measures nutrient and major-ion concentrations in selected reservoirs in north Texas. This data release documents water-quality data obtained from Ray Hubbard Lake, Grapevine Lake, Ray Roberts Lake, and Lake Lewisville during water years 2016–20. In addition to nutrient and major-ion concentrations, water-quality field properties and other selected constituents measured in the water-quality samples collected from these reservoirs are documented. The study began in water year 2016 (WY16) with monthly sampling of Ray Hubbard Lake. A water year is defined as the 12-month period from October 1 through September 30 and is designated by the calendar year in which it ends. During this first year, water-quality samples were analyzed for nutrients only (nitrogen, phosphorous, and multiple species of each nutrient). In water year 2017 (WY17), the study design changed to quarterly sampling, expanded to include Grapevine Lake, and added the analysis of major ions (Calcium, Chloride, Fluoride, Magnesium, Potassium, Silica, Sodium, and Sulfate). In water year 2018 (WY18) to present, the study design further expanded to include quarterly sampling of Ray Roberts Lake and Lake Lewisville. For this study, each reservoir currently (2021) has eight sampling sites. Water years 2018, 2019, and 2020 also include water-quality data collected for the USGS Zebra Mussel Monitoring Program for Texas for all four reservoirs. Water-quality samples are collected at a minimum of five equidistant depths, including top and bottom depths, at each site. Samples are analyzed for nutrients, major ions, and bromide by the USGS National Water Quality Laboratory (NWQL) in Denver, Colo. Water-quality field properties (dissolved oxygen, pH, specific conductance, temperature, and Secchi depth) are also measured at each sampling site. This dataset includes all 4,756 samples from the original data release for the previous water years, plus 1,933 additional samples collected during the 2020 water year. NOTE: While the previous versions are available from the author, all the records from the previous versions can be found in version 3.0. First posted: September 16, 2019 (available from author) Revised: September 30, 2020 (version 2.0 - available from author) Revised: September 2021 (version 3.0)