Watch this video of our friend, Max Musselman (great name, eh?), tell you about how a lock works:
Why are dams built on the Mississippi River? How come dams are only on the Upper Mississippi (MN, WI, IL, IA, MO) and not on the Lower Mississippi? Does a dam impact fish migration, and if so how? What dangers do you think small boats such as canoes and sailboats face while going through a lock?