74 REPORT OF INVESTIGATIONS NO. 50
channels. Lakes occur in all parts of the county, but the vast majority of them are in the western half.
SURFACE AREAS
The surface areas of lakes in Orange County range from less than one acre for some sinkhole lakes to 31,000 acres for Lake Apopka. The area of a lake continually changes. If the range in stage of a lake is large and its shores slope gently, changes in its area are large. If the range in stage is small or if the shore is steep, changes in area are small.
DEPTHS
The shallowest of the permanent lakes in Orange County is Lake Poinsett. This lake was only 2-feet deep when it was at its lowest level in 1945. The deepest body of water in the county is Emerald Spring, a sinkhole near Little Lake Fairview. Emerald Spring was sounded to a depth of 334 feet.
Depth contours for 73 selected lakes in Florida were shown by Kenner (1964). Six of these lakes are in Orange County.
ALTITUDES
At its lowest level, Lake Cone, a widening of the St. Johns River, was only about 2 feet above msl. A small lake near Tangerine is shown on the U. S. Geological Survey topographic map to be at an altitude of 158 feet.
The altitude of a lake's surface seldom remains constant very long. Figure 16 shows the percent of time that specific altitudes were equalled or exceeded for selected lakes in Orange County.
SEASONAL PATTERNS IN LAKE-LEVEL FLUCTUATIONS
Lake levels fluctuate in response to the net differences between rainfall and evaporation with modifications by surface- and ground-water inflow and outflow. Table 1 shows the monthly averages and extremes of rainfall at Orlando and figure 34 shows the estimated monthly averages of evaporation from lakes in Orange County. Average monthly evaporation from lakes in Orange County was computed by multiplying average pan evaporation at Orlando as determined by the Weather Bureau by monthly coefficients determined from evaporation studies at Lake