The Chicago River runs backward. It used to go into Lake Michigan, but using a lock, the flow was reversed! The flow was changed because the dirty river was dirtying the city’s clean drinking water. The Chicago Harbor Lock is important to the people in Chicago.
A lock helps boats going down rivers when there is a change of elevation. One of the v-shaped gates open. The boat goes into the lock. The gate, which is facing upstream, closes. The boat is ‘trapped’! If you were looking straight down at it, it would look something like this:
If the boat is going upstream, the water level in the thirsty lock rises, the other gate opens, and the boat drives out! If the boat is going downstream, the water level decreases, the other gate opens and then the boat is free.