The Potholes Reservoir is formed by the earthen filled O'Sullivan Dam. The reservoir is located just south of Moses Lake. The reservoir was created as part of the Columbia Basin Irrigation Project and is used to irrigate the nearby land. Because of this the water levels can fluctuate throughout the year. The reservoir holds a variety of fish species. The most popular are the walleye and rainbow trout. The rainbows are stocked yearly and can grow over 20 inches. The smallmouth and largemouth bass also provide good fishing opportunities with the smallies typically holding in the deeper sections of the reservoir and the largemouth preferring the shallower sections. The perch fishing can also be productive. There is excellent shore access on this reservoir and it occasionally freezes to provide ice fishing in the winter months.