10 REPORT OF INVESTIGATIONS No. 50
in others. Surface streams in this region either go dry or recede to very low flow after relatively short periods of drought. The lowlands are ground-water discharge areas and contain few lakes except in the mainstem of the St. Johns River. Streamflow is more sustained than in the other regions because of water stored in the lakes along the mainstem of the St. Johns River, spring flow, and seepage of ground water from both the water-table and artesian aquifers.
CLIMATE
Orange County has a subtropical climate with only two pronounced seasons-winter and summer. The average annual temperature at Orlando is 71.50F and the average annual rainfall is 51.4 inches. (See table 1.) Summer thunderstorms account formost of the rainfall. Thunderstorms occur on an average of 83 days per year, one of the highest incidences of thunderstorms in the United States (U. S. Weather Bureau, Annual Report 1960).
SINKHOLES
Sinkholes are common in areas such as Orange County that are underlain by limestone formations. Rainfall combines with carbon dioxide from the atmosphere and from decaying vegetation to form weak carbonic acid. As the water percolates through the limestone, solution takes place and cavities of irregular shape are gradually formed. When solution weakens the roof of a cavern to the extent that part of it can no longer support the sandy overburden, sand falls into the cavity and a sinkhole forms on the surface. (See figure 4.) Most of Orange County's natural lakes, ponds, and closed depressions probably were formed in this manner. Sinkholes range in size from small pits a few feet in diameter to large depressions several square miles in area. Large depressions are usually formed by the coalescence of several sinkholes.
Sinkholes may form either by sudden collapse of a large part of the roof of a large cavern or by gradual infiltration of sand through small openings in the roof of the cavity. The latter condition is illustrated by the formation of a sinkhole in Canton Street in Winter Park in April 1961. The sink was first noted as a depression in the graded road. By the following day a hole about 6 feet in diameter had formed. In the next 2 days the hole gradually