Leeches are very very common in lowland lakes in the Northwest and simply aren't a real problem for the lake or people. While they might be icky, they are pretty harmless and just part of a healthy lake's ecology.
The rainbow trout and largemouth bass in our lake eat leeches - open up a fish's stomach and you may see them. This is no known effective and safe way to control leeches.
We do have issues to manage with our lake, but leeches aren't one of them.