I agree with Bill. If you just drop gravel or sand over the muck, it won't take tons, it'll take tens of tons, and there still isn't any guarantee that the lighter muck won't rise to the top of the heavier material.
In a local lake that has a muck bottom, there is enough disturbance (read that as people swimming) that they keep the muck stirred up and away from that area. That area has a sand bottom, and is about 24" deeper than where the people don't swim. In the Spring, there is a 2"-4" layer of muck on top of the sand, but after a month of use, it's back to sand again.
