The southwestern part of Seminole County--in east-central Florida-is characterized by sinkholes formed by the subsidence of surficial deposits into solution cavities in the underlying limestone deposits. The area includes three sinkhole basins created by such subsidence: Cranes Roost, Palm Springs, and Grace Lake.

Cranes Roost basin (drainage area, 5.02 square miles) contains a closed drainage system of lakes and swamps--including Lakes Adelaide, Florida, and Mobile--that terminates at Cranes Roost sink. It also contains Lake Orienta and two unnamed sinks which do not overflow into Cranes Roost sink.

Palm Springs basin (drainage area, 1.77 square miles) includes Lake Marion, Eleventh Hole Pond, and several small unconnected sinks.

Grace Lake basin (drainage area, 1.64 square miles) includes Island Lake which overflows into Grace Lake.

The recent spread of urban development has tended to encroach on the flood plains of lakes in these sinkhole basins and cause concern over the flood hazard that results. An investigation was made of the area to document the highest known lake levels, to examine possible effects of urbanization with regard to increasing the flood hazard, and to appraise the possibilities of controlling lake levels to reduce or limit the flood hazard.

The highest lake stages of record in the three sinkhole basins occurred in September 1960. Analyses of the hydrologic relations between lake stages, ground-water levels, stream discharges, and rainfall in the area indicated that the lake stages of September 1960 probably were the highest attained since at least 1895.

Cultural development that increases the percentage of a sink basin covered by impervious materials will cause more rapid runoff of a larger part of the rainfall in the basin. Thus, as development progresses, lake stages seldom reached under natural conditions may be reached more frequently unless the lake levels are controlled.

In Crane Roost basin, the levels of Lakes Mobile, Adelaide, and Florida could be controlled by enlarging their surface outlets and adding control structures; however, such measures might be ineffective unless the level of Cranes Roost also was controlled. The level of Cranes Roost could be controlled by removal of water by pumping or by providing a surface outlet; pumping might be ineffective during extreme wet periods because of the potential for inflow from upstream lakes and the potential for ground-water inflow from the Floridan aquifer. The level of Lake Orienta could be Controlled by removal of water by pumping or by providing a surface outlet.

In the Palm Springs sink basin, the levels of Lake Marion and the several small sinks could be controlled by removal of water by pumping or by providing surface outlets. For the several sinks, however, providing a surface outlet would require an extensive excavation and pumping might prove impractical because of seepage induced from the Floridan aquifer.

The level of Island Lake--in Grace Lake sink basin-could be controlled by enlarging its surface outlet and providing a control structure. The peak stage of Grace Lake could be reduced by lowering its outlet to the north, but release of water during wet periods might cause downstream flooding.