Indiana's lakes are a valuable resource for both recreational use and their industrial potential. Some lakes are used for water supply.

The natural lakes are glacial in origin and are most concentrated in northeastern Indiana. Many of the lakes were drained by the early settlers. The natural processes of sedimentation ad accumulation of organic deposits tend also to reduce the number and size of lakes. The trend toward fewer lakes has been reversed in recent years by the construction of artificial lakes; the number of lakes has increased from a little over 500 in 1800 to 1,000 at the present time.

The recreational value of the lakes and the desirability of maintaining constant lake level led to legislation relative to the establishment of legal lake levels and to a program of lake-level observations, supplemented by the collection of hydrologic data affecting lake levels.

Observations of both surface temperature and temperature profile were made by investigates as early as 1875. The earlier data and data collected since 1946 have been analyzed to show the fluctuation of temperature with season, depth of water, size of lake, and other influencing factors. Surface temperature, which may vary several degrees within a 24-hour period, fluctuate with air temperature and reach highs of about 85°F in August. The bottom temperature of lakes more than 100 feet deep seldom rise above 45°F. Subsurface water temperatures below ice cover reach a uniform value of about 39°F except that just below the ice which remains closely 32°F. During summer, temperature stratification is established as the lake warms. Shallow lakes reach temperature extremes about 5°F above and below those of deep lakes. Size of the contributing drainage area, surface area of the lake, and flow through the lake have little effect on water temperatures below a depth of about 20 feet. Little aquatic life exists below depths of 30 feet from mid April to mid September owing to a lack of oxygen in the water. Large amounts of cool water, 50 to 10 degrees cooler than groundwater which averages 50°F to 55°F in northern Indiana, are available in the lower portions of the deep lakes. 

Evapotranspiration accounts for the disposal of about 70 percent of the precipitation in Indiana. During a year's time evaporation from the lake surface about equals the precipitation falling on the lake. During dry periods evaporation from a lake may exceed the inflow from small drainage areas and cause a lowering of lake levels. For the period April through October, evaporation as measure in a class A land pan of the U. S. Weather Bureau has averaged 44 inches at Evansville, 34 inches at Indianapolis, and 31 inches at Valpariso. A coefficient of about 0.7 is usually applied to yearly evaporation data from Class A land pans to obtain equivalent evaporation from lake surfaces.

Ice cover on the lakes extends from about December 15 to March 15 and reaches thickness of 24 to 30 inches during colder winters. Ice can be damaging to the lakeside installations by thrust action of by the wind action on ice cakes.

The stabilization of lake levels often requires the construction of outlet control structures. A detailed study of past lake-level elevations and other hydologic date is necessary to establish a level that can be maintained and to determine the means necessary for maintaining the established level. Detailed lake-level records for 28 lakes are included in the report, and records for over 100 other lakes data are available in the U.S. Geological Survey Office, Indianapolis, Ind. Evaporation data from the four Class A evaporation station of the U. S. Weather Bureau have been compiled in this report. A table showing the established legal lake level and related data is included.