This is "the" playspot for Southern Illinois.
Low Flows (3-6 inches): Class II
Medium Flows (6-12 in): Class II+
High Flows (1 foot and up): Class II+ with arguable class III or IV hole at the bottom.
Little Grassy generally runs from December through Mid-Spring following a good rain event. The water spills over the dam on Little Grassy Lake and flows across 100 yards of bedrock. In that stretch are two holes. The top hole, even at high flows, is relatively mild. The bottom hole has a large undercut boulder situated just downstream. Most of the flow goes river-right of the boulder so there is not much water pushing into the undercut. However, it is something to be aware of.
At medium flows, this hole is at its best. The water covers the undercut and the hole grows wider and steeper. It is still too shallow for vertical moves but it allows for all day spins. At high flows, this hole is river wide and is quite retentive. If you are playing the top hole and are blown downstream, punch the lower hole river-right. If you do not hit this at the correct speed or angle you may be in for a long surf! There are exit points to the left and right, but the trough is deep and it will take some effort to get out. Very few have had the nerve to play this hole intentionally at high flows. There are no eddys below the top hole at high flows. So, if you don't feel like you want to try the bottom hole, don't try the top hole. If you wash out of the top hole in mid-stream, you will have no choice but to try the bottom.