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 (<smile> 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 impessive 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.
The tour will not take you into the generator buildings, but you will see the pumps that pump water up to Banks Lake. A few of the pumps can be run in reverse to generate electricity from the water in Banks Lake at times of higher demand.
The tour will also take you across the top of the dam. Before the increased security, you could drive across the dam. But no more. However, take the tour and you will get to stand atop the dam and look down the spillway to the water below. This photo shows the spillway and the third powerhouse in the distance.
Below is a 1-minute video of water flowing over the dam from various angles, including a shot from the top of the dam.
While you're visiting Grand Coulee Dam, be sure to stay through the evening and watch the laser show on the dam (summer only). This show started in 1989 and played through 2012. There will be a new laser show beginning in 2013. The photos below were shot with water running down the spillway. The white water makes the show more spectacular than when shown on the darkened concrete dam. You can see the spray from the water near the bottoms of the photos.
Here is a 3-minute video showing a small part of the 2012 laser show on the dam with water flowing over the dam. The effect to the eye is much better than what it looks like when captured on video.
In the days before the laser show, colored flood lights were used to light the water at night. The photo below was taken in 1970. Different than the laser show, the colored floodlights playing on the dam were still quite spectacular. You can see the bank of floodlights in the lower right of the photo (the bright area). When you visit the dam now, you can still see the bank of floodlights below the picnic area.
Grand Coulee Dam Locator Map