Rapid population growth in La Plata County, Colorado, has increased the demand for ground water in the Florida Mesa area. This report was prepared in cooperation with La Plata County to provide needed information about the geology, extent, thickness, and depth of the aquifers in the area; sources of ground-water recharge and discharge; direction of ground-water movement; water-level changes; and water quality in the alluvial and bedrock aquifers. Ground water in the study area is present in bedrock formations and in terrace deposits on Florida Mesa. Porous or fractured sandstone beds that contain bedrock aquifers are present near land surface along the northern margin of the study area and are present at depths less than 3,000 feet throughout the study area. Terrace deposits as much as 200 feet thick and consisting of gravel, sand, silt, and clay are present on Florida Mesa. The terrace deposits and the upper part of the underlying Animas and Nacimiento Formations form the principal aquifer under the mesa. Ground water under the mesa is supplied from precipitation and irrigation water. A small part of the precipitation and irrigation water on the mesa percolates to depth in the soil and recharges the aquifer. Irrigation water is the largest source of this recharge. Water levels in the aquifer can decline because of a reduction in irrigation recharge, or because of an increase in well pumping. Because irrigation recharge is so much larger than pumping, changes in recharge can have a much larger effect on ground-water levels than can changes in pumping. Factors that tend to increase ground-water recharge and thereby increase or maintain ground- water levels include: maintaining large rates of surface-water diversion onto Florida Mesa, reducing surface flow off the mesa, increasing use of ponds and spreading basins to promote infiltration, and irrigating by use of flood irrigation. The general direction of ground-water movement on the mesa is from the northern part of the mesa to the south, southwest, and southeast. Most ground water discharges from the mesa to the Animas and Florida Rivers through seeps and springs along the margin of the mesa. Winter water levels in wells generally are lower than summer water levels because of the lack of irrigation recharge during the fall and winter. Potable water of low dissolved-solids concentration is present in the shallow parts of most aquifers.