Where is the ORIGINAL London Bridge?

As of yet there are no signs that the Roman built a bridge on the site of London Bridge.

Yes they did ford the Thames but further up stream.

It fell down           . 

It fell down and had to be rebuilt.

its at lake havasu city ...and no it was no demolished ...it was taken down stone by stone ..every piece numbered ..was sold to america ...they thought they had bought tower bridge ...it opens to allow shipping through ..but it was a different bridge they bought ...after they erected it they redirected the colorado river to pass underneath

If you mean the one the Romans built, it hasn't been seen for a couple of thousand years.

Which one?  It's been rebuilt several times.

The original London Bridge was pulled down by Ethelred the Unready, in 1014,to stop a Viking army. 

It's at Lake Havesu in Arizona.

The original was most likely a Roman Pontoon bridge, so would have likely fallen into the Thames. The first permanent one was destroyed during a battle around AD60.

https://en.wikipedia.org/wiki/London_Bridge

Demolished in 1832.

Its replacement bridge is in Lake Havasu, Arizona.