Thanks, Martin. I knew I wasn't going crazy reading that about all of those waterways being connected... Any idea when they permanently separated the two waterways? (Or perhaps they're still connected, but they're using that dam? I've never actually noticed it, but you can bet I'll look for it on my next trip)
And Edward, you're right that the Rivers of America are connected to the Seven Seas Lagoon via the canal to the west of the park. There's a lock that keeps the Rivers a few feet higher than the Lagoon. The Rivers may just use water out of the lagoon.
As for the Disneyland "dark water" system that was referred to earlier, I know (thanks to The E-Ticket magazine, which I'll have to drag out at some point to double-check this, but not tonight) originally almost all of the waterways in Disneyland were connected as part of one very large system.
The lowest point in the system was the Rivers of America, and the highest point was somewhere in Fantasyland. (I'm presuming it was for the Storybookland canals and perhaps the Phantom Boats)
Water would flow from Fantasyland, past the Castle the Hub, down a stream from the Hub to the Jungle Cruise, down past the Treehouse, and then to the RoA. I know that some part ot eh water system between the Jungle Cruise and the Rivers it went through an underground pipe.
The genius of this system was that the entire waterway system used only one pump (or set of pumps) in one location to pump water from a low point to a high point. From there, gravity did everything else.
Over the years, things may have gotten segmented up into smaller systems, with the large-scale additions like New Orleans Square and the Indiana Jones entrance.
-Rob