Grassy Lake is a 213-acre lake with a maximum depth of 15 feet. It has very soft (unmineralized) water. Aquatic plants grow to a depth of 6 ft and consist mostly of waterlilies, milfoil, spikerush, and bulrush. The lake bottom along the shoreline consists mostly of muck, ledgerock, and boulder. The main inlet is from Tee Lake and the outlet is to the Range River.
Grassy Lake's littoral zone (that portion of the lake that is less than 15 feet in depth, where the majority of the aquatic plants are found, is a primary area used by young fish, and provides the essential spawning habitat for most warmwater fishes) is 213 acres. The lake's numerous fish species include black crappie, bluegill, hybrid sunfish, largemouth bass, northern pike, smallmouth bass, walleye, white sucker and yellow perch.