The Potholes Reservoir is part of the Columbia Basin Project and formed by O'Sullivan Dam. Potholes refers to the many glacial depressions in ancient sand dunes here. These potholes and small islands make this area one of the best in Washington for birding. The best birding is from March through October: Sandhill cranes and waterfowl in late February, raptors in April, Sage Thrasher, Lark Sparrow, Burrowing and Long-eared Owls and shorebirds in May. With the August reservoir drawdown, the exposed mudflats attract shorebirds such as a variety of sandpipers, stilts, plovers and curlews.Area is open year-round. Additional wildlife information can be found here. Potholes State Park is also located on the reservoir and is administered by Washington State Parks. Reservations are available by calling (888) CAMPOUT or (888) 226-7688, or by clicking here. Access to the reservoir is adequate except west side is limited. Fish species include yellow perch, crappie, largemouth bass, rainbow trout, and walleye. Fishing is year-round.