Hydrologic influences on water levels were investigated at Three Oaks Recreation Area (TORA), a former sand-and-gravel quarry converted into recreational lakes in Crystal Lake, Illinois. From 2009 to 2015, average water levels in the lakes declined nearly 4 feet. It was not clear if these declines were related to variations in weather (precipitation or evaporation) or other hydrologic influences such as municipal supply pumping or nearby quarry operations. Data were collected using three approaches to determine the possibility of such hydrologic influences. First, water levels were collected at 15-minute intervals at three wells equipped with pressure transducers from April 14 through September 27, 2016. The continuous data allowed assessment of lake and well water-level responses to precipitation, pumping influences, and quarry operations. Second, a single-day synoptic water-level survey was completed to create a water-table map to determine groundwater flow directions. Third, single-well aquifer tests (slug tests) were completed on the three data-collection wells to estimate the aquifer’s horizontal hydraulic conductivity. Collectively, these data were used to estimate the velocity and volume of water entering and exiting TORA. The single-well aquifer tests (mechanical slug tests) were conducted on monitoring wells 421317088182301--43N8E-9.5f (South Lake), 421244088180201--43N8E-9.2a (Rakow Road), and 421301088191501--43N8E-8.2c (Canterbury Park) within the vicinity of Three Oaks Recreational Area (TORA), in Crystal Lake, Illinois, about 35 miles northwest of Chicago. The tests were conducted April 26, 2016, for monitoring well 43N8E-9.2a, and May 20, 2016, for the remaining two wells. This data was used to support the project work described in: Gahala, A.M., 2018, Hydrologic influences on water levels at Three Oaks Recreation Area, Crystal Lake, Illinois, April 14 through September 27, 2016: U.S. Geological Survey, Scientific Investigations Report 2018-xxxx, xx p., http://dx.doi.org/XXXXXXX