Space Needle is a shite target. The Columbia Tower is a better bet.The Space Needle in Seattle. The US Bank Tower in Los Angeles. Wells Fargo Center in Sacremento. The Golden Gate Bridge and TransAmerica Pyramid in San Francisco. Symphony Towers in San Diego.
If they hit something vital, it might knock out power production (which would be a serious problem for the Northwest and California), but the dam is not exactly easy to fly an airliner into (it sits inside a bit of a valley, for one thing, it's not exactly easy to spot from altitude, and you'd have to approach it from the north, which presents a major set of problems in and of itself).What if they hit the Grand Coulee Dam?
And the Hoover Dam is even harder to fly at- there's a bridge in the way and the valley is only barely wider than the wingspan of a mid-size airliner.