Well, you've already discussed liability. Whatever you do put up signs, add a gate or at least chains. Enforce it too. tell people to stay off. Stuff like that is a sort of an attractive nuisance.

At 40' this would be a major engineering project. A span that long would take some sizable elements and material to anchor it on. You are going to have to get some assistance with not only engineering it, but with the construction.

Also check to see if you CAN do it. Many places creeks are protected and you may have to get a permit or permission from a government office to build on, over or even within so many feet of the banks. A place I used to line I had a friend who's father owned a lot on a creek. He could not mow right up to the bank and was warned and fined one year after doing so.