Thank you for your well-meaning advice, but it's not supposed to be a sandy beach. It's a depression in the ground that fills with water seasonally, or not... There's no time for the formation of a beach. Meltwater lakes are simply more or less large ponds, which can have quite "sharp" edges, as shown in the picture above of the Austrian lake. If I have time and leisure, I might have to deal with a sandy beach.