The water temperature varies depending on the solar radiation and air temperature. During the day and when the sun is high as in June and July the water absorbs almost all the solar radiation that strikes the surface of the water. At night the water cools a little depending on how low the air temperature drops. Once the lake temperature exceeds about 20 much of the energy goes into evaporation. 

 The water temperatures shown in the charts below were taken at the surface but in a lake as shallow as Pelican the temperature doesn't change very much with depth after sometime in  May.

In September the sun doesn't rise as high, more radiation is reflected and days are shorter resulting in lower water temperatures.