The Glen Canyon Dam blocks the Colorado River and forms Lake Powell. Lake Powell is the largest man-made lake in North America. When the lake is full, it has the capacity to hold 9,000,000,000,000 gallons of water. The Glen Canyon Dam visitor center is on the west side of the dam, and has good exhibits on the history and workings of this massive dam. 45 minute tours of the dam are provided by the Glen Canyon Natural History Assocation and cost only $5.00. There are discounts for children and seniors.
If you go:
Allow at least two hours if you plan to take the guided tour, and leave all security items in your locked vehicle. Call ahead for times and tickets.
Links:

Hwy 89
Page, Arizona 86040 USA
Mailing address:
Carl Hayden Visitor Center
Hwy 89 North
Page, AZ 86040 USA
Phone Numbers:
Glen Canyon Visitor Center
+1 (928) 608-6200
+1 (928) 608-6200
Glen Canyon Tours
+1 (928) 608-6072
+1 (928) 608-6072