Little Grassy Lake is a 1,200-acre (4.8 km²) reservoir in southern Illinois, created by the damming of Little Grassy Creek, a tributary of Crab Orchard Lake and the Big Muddy River. Most of the lake is located in Williamson County, southeast of Carbondale, Illinois. The lake is accessible from Giant City Road out of Carbondale, Illinois. Two arms of the lake penetrate into adjacent Jackson County. The lake is managed by the U.S.