Low reservoir and river levels, and low snowpack in the western United States portends low hydro generation for the spring and summer runoff season. The forecast has improved in the Pacific Northwest and the Colorado River Basin, but not so much in California and the Sierra Nevadas. There is still plenty of time for the situation to worsen or recover.
The PNW is by far the largest contributor to hydro power generation, and the Bonneville Power Administration (BPA) rules over it, and the Columbia River is the main driver.
Hydro generation in the BPA region has historically varied a lot. Here is a visual of this year's hydro output, and the comps for '12 and '13. The spring and early summer are prime time for BPA hydro, which exports most of that power to California. Forward power prices in Cali are currently indicating that hydro will not be abundant this summer.
The Colorado River Basin is fed by snowpack that was underperforming until recently. The Lake Powell Water Database maintains this lovely graph of snowpack, and the Blue Line (current season) is now above average, indicating a strong runoff season. Reservoir levels are quite low, so nothing will be spilling over the dam, but hydro should keep up.
And California hydro is off to a low early start, down about 1GW on average so far this year:
