Waterfall Glen is a forest preserve in Downers Grove Township, DuPage County, Illinois, between the towns of Darien and Lemont, covering 2,492 acres (1,008 ha). It contains several waterfalls on Sawmill Creek as it empties into the Des Plaines River. Rocky Glen Falls, the largest waterfall in the preserve, is actually a natural dam in the valley glen. In the 1930s, the Civilian Conservation Corps helped restore the waterfall to its naturally tiered state, after years of erosion due to runoff from a nearby mill. It completely surrounds Argonne National Laboratory. It is also home to a popular model airplane field, located in the southwest section of the forest preserve. Hikers can also make the interior trip to St. Patrick Cemetery nestled deep in the preserve. Waterfall Glen's tallgrass prairies, bogs, and Midwestern oak savannas contain 740 native plant species, 75 percent of all the plants known to grow naturally in DuPage County. Over 300 species of mammals, birds, fish, amphibians, and reptiles and another 300 species of invertebrates use the forest preserve, either year-round or during their migrations.