well somebody who actually knows will answer this for you I'm sure. But I will tell you that Ameren has some of the most comprehensive rules and regulations on docks I have ever seen on a lake. By the time an owner completes all the requirements they probably do feel like they should own the water. But I do not believe there is a minimum anchoring distance I've seen. Ameren discourages people from cutting off shore access also.

On Lake Egypt in Southern Illinois the Power company there owns the Lake and has a specific set of rules governing all activity on the Lake. And there it is prohibited to anchor within 100 ft of a dock. And you cannot cast within 10 ft.