How much did the Thames separate London before the building of the Blackwall tunnel?

Lets start by saying that for a long period of time London was quite smaller than the sprawl it is today. There is now an area called the City of London, which used to be walled. Parts of those walls exist today and the places of the entrance gates are still marked, so if you ever visit, you can see for yourself how small that area actually was - just over one square mile, or two square kilometres. In fact, it is still called "the Square Mile".

The settlement maintained compact size from its beginning in the 1st century AD to the Middle Ages, despite the rise in population numbers.

Most of that early London was actually situated only on the north bank of the Thames, so the river did not quite bisect the city but was more of a border.

Before the name London even existed (but there were little settlements in the area) it was possible to cross the river via a ford when the tide was low, or by ferry when the tide was high. The first bridge was build by the Romans.

The main bridge was a wooden structure build in the middle of the 1st century, around the place where today you can nowadays find the so called London Bridge. And that was plenty enough for the townsfolk to cross the river.

Mind you, in the early Medieval times the Thames became border between two warring kingdoms, Mercia and Wessex. So townsfolk would rather have less crossings available for possible invaders to come raid their place.

A stone bridge was commissioned in the 12th century by the English King. It took over 30 years to complete. Standing some 26ft (8m) wide, with a drawbridge part allowing the passage of tall ships and defesive gatehouses at both ends, it made quite a dent in the royal treasury. In order to get back his money, the king allowed shops and houses to be build on the bridge, so he could tax them. By 16th century there were some 200 buildigs on the 250m (800ft) long bridge, some 7 stories high. The road was only 4m (12ft) wide and when it congested it would take up to an hour to cross the bridge.

The large bridge would also obstruct the flow of the Thames, cause rapids and otehr problems for ships.

In the times of Shakespeare, the population of London reached half a million people. New districts were build and the south bank of the Thames became well populated as well (the borough is still named South Bank). This led to the building of several new bridges, although the London Bridge remained the main one. But the south side of London had a slower and a more unfavourable development. During the Middle Ages it hosted mostly entertainment such as theatres and prostitution. Only in the 18th century south London became more refined. Even today, South Bank is a place for entertainment and culture.

By the 18th century the London bridge was in dire need of replacement and so, the new London Bridge was build. This soon became the busiest part of London, with 8000 pedestrians and 900 vehicles crossing every hour and had to be widened only 60 years later.

In 1831 London became world's largest city and maintained its status for nearly a century, which led to the creation of the first local urban rail network, soon to become also a subway. Soon after, of course, came the Blackwall tunnel.