I was in CO for a week last year at just this time. Platte and Poudre were both blown out by snow melt, so I went to a high lake. I found Chambers Lake at about 8000', where the ice off was only about a week old. The locals told me that the Lake Trout (Mackinaw) cruise within 10 or 20 feet of the bank and bite aggressively. I went out in a pontoon, but a float tube will work if you have some serious insulation. Anyway, for once they weren't lying to a flat lander, because I hit a bunch of them along rocky shores with an intermediate line and cone head buggers. That's an option if the rivers are high.