Like many of the tributaries of the North Platte in North Park, the Illinois river is a meandering meadow stream that holds a variety of trout.  Primarily browns the river fishes good with small streamers and dry dropper rigs.  The upper sections are good too but the river has closures due to a rare creek chub present in the river they are trying to help preserve.  I find as good of luck on this stream as I do Michigan, Canadian, Roaring fork and Grizzly creeks in the area.  Find some water that doesn’t look pressured and give it a shot.

There is a headwaters section that you can explore, and though beautiful, doesn’t seem to hold a ton of brook trout.  Nice place to camp in the area, but I found it underwhelming in the results.