From EIA

Ten Largest Plants by Generating Capacity in Washington State in 2009″

1. Grand Coulee, Hydro, 7,079 MW

2. Chief Joseph, Hydro, 2,456 MW

3. Transalta Centralia Generation, Coal, 1,632 MW

4. Rocky Reach, Hydro, 1,254 MW

5. Columbia Generating Station, Nuclear, 1,131 MW

6. Wanapum, Hydro, 1,044 MW

7. Boundary, Hydro, 1040 MW

8. Priest Rapids, Hydro, 932 MW

9. Wells, Hydro, 840 MW

10. (3-way tie) Lower Monumental & Little Goose & Lower Granite, Hydro, 810 MW

Eight of the top ten are hydro plants while one is nuclear and one is coal. Nine of ten are considered CO2 free (with respect to electricity generation). Finally, Grand Coulee is also has the largest power capacity for ANY electricity generating station in the U.S.