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Sinkholes are a natural and common geologic feature in areas underlain by limestone and other soluble rocks. Four major types of sinkholes are common to west-central Florida. They include limestone-solution, limestone-collapse, cover-subsidence, and cover-collapse sinkholes. The first two occur in areas where limestone is bare or is thinly covered. The second two occur where there is a thick cover (30 to 200 feet) of material over limestone.Limestone-solution sinkholes result from subsidence of overlying materials that occurs at approximately the same rate as dissolution of the limestone. The sinkholes reflect a gradual downward movement of the land surface and development of funnel-shaped depressions. Limestone-collapse sinkholes occur when a solution cavity grows in size until its roof can no longer support its weight, causing generally abrupt collapse that is sometimes catastrophic.Cover-subsidence sinkholes develop as sand in the cover material moves downward into space created in limestone by dissolution. Resultant sinkholes develop gradually and are generally only a few feet in diameter. Cover-collapse sinkholes occur where clay layers that overlie limestone have sufficient cohesiveness to bridge the developing cavities in the limestone. Eventual failure of the bridge results in a cover-collapse sinkhole that may develop suddenly.Large withdrawals of water for various uses may provide a triggering mechanism for sinkholes to occur. Loss of water's .buoyant support of unconsolidated deposits that overlie cavities can cause the materials that bridge the cavity to fail and sinkholes to appear. Conversely, loading of the land surface by construction, such as impoundments, may cause collapse of materials that bridge cavities and sinkholes to develop. Impoundments may also provide continuous sources of recharge water and hasten development of cavities in limestone.West-central Florida was divided into seven zones based on geology, landscape, and geomorphology and the relationship of these factors to the types of sinkholes that occur in each zone. The zones are: (1) areas of bare or thin cover that experience slow developing limestone-solution sinkholes; (2) areas of thin cover, little recharge, high overland runoff, and few sinkhole occurrences; (3) areas of incohesive sand cover of 50 to 150 feet that have high recharge and generally experience cover-subsidence sinkholes; (4) areas that have 25 to 100 feet of cover, many sinkhole lakes, and cypress heads and experience predominantly cover-collapse sinkholes; (5) areas of 25 to 150 feet of sand cover overlying clay that experience cover-collapse and cover-subsidence sinkholes; (6) areas with more than 200 feet of cover, numerous lakes and sinkholes, and high land-surface altitudes that experience numerous cover-subsidence sinkholes and occasional large-scale cover-collapse sinkholes: and (7) araas with cover greater than 200 feet that have 100 or more feet of clay with high bearing strength and low leakance that preclude infiltration of corrosive water and develop...

Map showing major drainage basins in southwest Florida-48 Map showing precipitation during the 1978 water year-Graph showing median and 1978 water-year precipitation-Map showing precipitation patterns during the dry season (October through April

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Federal and non-Federal agencies. These programs involve: 1. Collecting, on a systematic basis, data needed for the continuing determination and evaluation of the quantity, quality, and use of the Nation's water resources. 2. Conducting analytical and interpretive water-resource appraisals that describe the occurrence, availability, and the physical, chemical, and biological characteristics of surface and ground water. 3. Conducting basic problem-oriented research in hydrology to improve the scientific basis for investigations and measurement techniques, and to predict quantitatively the response of hydrologic systems to stress. 4. Disseminating water data and the results of investigations and research through reports, maps, services, and other forms of public Coordinating the activities of Federal agencies in the acquisition of water data for streans, lakes, reservoirs, estuaries, and ground waters. Providing scientific and technical assistance in hydrologic fields to other Federal, State, and local agencies, to licensees of the Federal Power Commission, and to international agencies on behalf of the Department of State.