The Locks

Why are they narrow boats?
To fit into narrow locks!

How do locks work?

    A gate at each end of the lock controls the boat and the water, with stanchions on land to control both. The drawing to the left shows its parts.
    Say we are going "uphill", that is, from a lower water level to a higher one. The low water level is in the lock, and the "top" gate is closed. Once the boat enters the lock, the person on the ground uses a windlass to close the "bottom" gate. Then the "top" paddle opens at its bottom to let water in. Once the water rises to the level of the higher water, the person rotates the handle, opens the gate, and the boat chugs out of the lock.

Lock drawing from:
Canal Guide 2: Llangollen & Montgomery Canals