KT, do you know the magnitude of your leak. I couldn't tell by your post the geographical location of your lake. However, depending on where its located you may have significant evaporation, especially during this part of the year. An easy way to test for leakage is to place a wide container (garbage can perhaps) in your lake and weigh it down with rocks or bricks and water. The lip of the container should only protrude from the surface of the water 1 or 2 inches; just enough to avoid wind induced splashing to transfer water from your container to your lake or vice-versa. The initial level of water in your container should be the same as the level of water in the lake. The larger the container and the lower the protruding lip, the better able you are in simulating both convective and molecular diffusion of water (evaporation) into the air space above. You should not add water to the lake during your experiment. Rain should be avoided or adjusted, because runoff from the rain into your lake will not be matched by direct rainfall into your container. On a daily basis measure the level of water outside your container as well as inside. You will then be able to quantify the magnitude of your leak. Good luck!