Five miles east of Kennewick [1] off Finley Road, Two Rivers County Park is open daily for boating, swimming, and picnicking along the Columbia River/Lake Wallula.
The U.S. Army Corps of Engineers operates Hood Park (four miles southeast of Pasco, 877/444-6777, May–mid-Sept., $18–20), just east of Pasco [1] for boating, swimming, and picnicking. You can also camp here, on the Snake River near its confluence with the Columbia.
Make reservations at 518/885-3639, 877/444-6777, or www.recreation.gov [2].
Links:
[1] http://www.moon.com/destinations/washington/columbia-gorge-and-wine-country/tri-cities
[2] http://www.recreation.gov