ok this has been keeping me awake for a bit:
Canal Locks.
Canals have no natural flow. Each section between locks is isolated. Check out this link and click "down".
http://www.starling101.btinternet.co.../lckworks.htm#
If there is a lot of canal traffic going down, more than there is going up, the balance of water won't be equal on both sides. How do they maintain levels of water regardless of which way traffic is going?