Just after the fall at the begining of dragonspire.
What is needed to operate the wheel to lower the bridge?

You need a Bridge Crank.

You'll find it in another area of the dungeon. Try "sailing down the river" a bit and look for any openings you haven't seen before.

Sazerac

A crank. Can be found in another part of the dungeon in a chest. Backtrack to the lake. Swim with the current down it and jump on the only ledge you will see to the right. Follow the tunnel up to a dead end and a chest. Then go back and where you climbed up the boulders the tunnel will continue on across the hole...Run and jump using the zero right number pad. Looking up helps you to jump better. This opens up most of the rest of the tunnels with the crank along in there someplace.