This report describes the hydrogeology of the Genesee Valley and the effects of the ceiling collapse of the Retsof salt mine on the aquifer system. It discusses the origin and character of glacial sediments; the occurrence, flow directions, and chemical quality of water in the aquifer system before the collapse; the effects of the collapse on the aquifer system in terms of land subsidence, water-level declines, changes in water quality, and exsolution of natural gas, and design and calibration of the ground-water-flow model. It also presents results of flow-model simulations, including an estimated ground-water budget and graphs showing the simulated water-level recovery; and results of subsidence simulations, including maps and graphs showing the extent of land subsidence.