If I well understand your message (I'm french...) your problem is easy to solve. Let's resume : you have a DEM file where the lake is not taken into account as datum of height, and the other one where the lake is taken into account. (I hope it is correct english, or at least understandable... :-)) It I'm right, and if both files coincide well , take the one with the lake, apply on it the water materiel you want, and make it coincide with the other one. Then, lower it slightly so that only the lake is visible... I hope that can help you...