I put this here to avoid the GD tomfoolery about treadmills and aliens and the lambasting I would surely receive. but its a question I cannot wrap my mind around so please explain in laymans terms. I am neither a mathematician nor scientist.

I was at Disney a couple weeks ago and was fascinated by a water bridge they have. Basically its 2 huge lakes near the Contemporary and Grand Floridian hotels. Between these large lakes is a "bridge" that allows  vehicle traffic to pass underneath it while the water and boat traffic can navigate across the bridge and go between the 2 lakes. Aside from the fact that they some how keep the water in the bridge channel and manage to support the weight of what has got to be a metric shit load of water weight, I am curious as to how the weight of boats passing across the bridge effects the load. This is the part I cannot get my mind to understand. Does the weight of the boats and all their contents add to the load on the bridge or because the boats are floating mean their is no additional load on the bridge as the boat floats across it?  

The boats add no extra weight because the displace an amount of water equal to their weight.

Google ship displacement.

A ship (or boat) has an unloaded water line. Add cargo and the ship needs to displace more water, so it settles deeper...

A freighter or tanker might sit 20' deeper loaded than unloaded be cause of displacement differences.

Ok so that makes sense. So if the boat and all the people and crap on it weighed say 25000 lbs then then it would displace 25000/8= 3125 gallons of water, or there would be 3125 less gallons of water in the bridge channel as the boat went thru it. amiright?? So knowing that the engineers who designed the boat bridge just need to make sure its strong enought to handle the weight of the water it could hold, basically, if it can hold the weight of the water in it, it can hld the weight of anything that could pass thru it and still be floating. If theres enought water to float whatever, then the bridge will support it. fascinating. yes, I am a simple guy.

Your summary is correct.

Water leaves the bridge as the boats go onto it.  You could fill it with boats, there would be less water, but the water that left would equal the weight of the boats that were sitting there.

Technically, adding boats will raise the height of the water in both the lakes and the bridge and thereby increase the load, although the amount is pretty much insignificant for the case of the bridge, unless its surface area makes up a significant percentage of the area of the combined bridge plus lakes.  Also, the location of the boats being in either lake or "on" the bridge has no effect on the total.  The effective load on the bridge of any boat anywhere on the body of water would be (displaced weight of boat) * (surface area of the bridge) / (Total surface area of bridge and lakes).

You will also get some transient changes in the load carried by the bridge due to movement - wakes, wind-blown waves and other water movement will affect how much water is actually being held up by the bridge at any given instant, but will also (most likely) be very minor compared to the total static load.

Mike

This ^^^

Adding any "load" to the lake system, be it boat, human on a boat, fuel in a boat's fuel-tank, displaces water, raising the water-level of the entire lake system thus evenly distributing the load across the entire surface area of the water system. The amount of load change on the bridge is proportional to the load change of the lake multiplied by the relative surface area of the bridge compared to the entire lake system. IE, Add 1,000 lbs to a lake system that covers 4 acres and the bridge is .05 acres: 1,000*(.05/4) = 12.5 lbs added to the entire bridge.

In theory, I suppose a ship could displace a little water, causing the water level to rise VERY slightly, which would increase the pressure at the bottom a little. But in engineering that would be the rounding error of a rounding error.

Tell a green puke that boats raise the height of the ocean.

They actually do but it is way out in the decimal places of ocean volume.

The whole thing will probbaly drive them nuts.