Grand Coulee Dam is definitely one of the Seven Wonders of Washington, and is quite spectacular to visit. Completed in 1942, its original purpose was for irrigation. However, it was completed just as the United States was entering World War II, and the power generation capability of Grand Coulee Dam was immediately harnessed to help with the war effort.
You can see all three powerhouses in the photo above. The left powerhouse is on the right ( they are named as seen from upriver from the dam). The right powerhouse is just to the left of the spillway in the photo. And on the far left, at an angle to the rest of the dam, is the third powerhouse which was completed in 1974.
Here is a photo from 1952 or 1953 showing the dam before the third powerhouse.
The dam is quite impressive when spilling water. The roar can be heard from miles away. However, it is now rare to see the water flowing over the spillway. Usually all of the water is used to generate power, but in cases where too much water is entering Lake Roosevelt, excess water is spilled. According to the tour guide I had in July, 2012, this has only happened a few times in recent years. Most of the time the dam looks more like the photo below with dry spillways.
When you visit the dam, be sure to allow time to go through the visitor’s center and to take the tour. This used to be a self-guided tour, and when the dam was built, marble floors were installed throughout much of the dam. However, most of those floors are now only seen by security personnel and maintenance workers. After 9-11, security has been stepped up. When you take the guided tour you will never be out of sight of security personnel.