The terrain is gently rolling. The various soil types range from fine sand to clay loam with loamy fine sand dominating the Birch Creek Unit, and fine sandy loam dominating the Nails Creek Unit. There are three main creeks that either run through or border the park, all of which drain into the reservoir. They are Nails Creek, along the southern border of Nails Creek Unit; Yegua Creek, which enters the lake on the north side of Nails Creek Unit; and Birch Creek, which forms the east border of Birch Creek Unit. Between the two parks, at the western edge of Lake Somerville, is Flag Pond and numerous other duck ponds, marshes and bogs.
The Somerville Trailway passes through dense stands of yaupon, post oak, hickory, blackjack oak, and water oak forests, past scenic overlooks and water crossings.
The trail has one of the best spring wildflower displays in the Texas State Park System. Many species of birds and wildlife may be observed and photographed, including white-tailed deer, fox, coyote, raccoon, rabbit, waterfowl and quail. A bird checklist is available at the Birch and Nails Creek headquarters. Visitors can enjoy fishing for white bass, crappie, largemouth bass and catfish.