You might call up the provincial park and ask them about group camping. I tried searching the Sask Parks website but the website is terrible and the search isn't working? Based on the info here it must be the Sawmill Bay Campground.
http://www.townofcreighton.ca/accommodationsSK.html There are other spots to the south of Denare Beach along the lake, near the south end.
That map is great! Unfortunately, I never learned enough geology to interpret it as well as I would like. I used to have a Canada-wide version hanging on my office wall. Time to put that up on the lime green walls of my new place. Some 11 years ago after running the S-W my family (3.5 of us at that time since my youngest was
in utero) camped on an island to the south of Missi/Big Island. That island was made up of black rock that was smooth as glass and some of the broken corners were as sharp as glass as I recall. Or maybe it was as sharp as eroded glass, which is to say that it wasn't so sharp that we were cutting ourselves. It shows up on the geological map as "vb" - basic volcanic.
My older daughter celebrated her 3rd birthday on that island and we baked her a birthday cake over the fire. Good times.
