When the Romans founded the Londinium settlement, was the Thames approx. 2 times wider at that location than it is today?

I found an image on wiki, but I'm not sure how accurate it is.



Also, has anyone explored the three cemeteries indicated on the map? I suppose they're covered with buildings today, so that might be a non-starter.

Could you not google map this, i think it would of got narrower at a guess.
Atb

Measuring from your diagram and the scale gives just over 200yds, taking the scale on google maps and measuring at Southwark Bridge or London Bridge (about the same place as the bridge on the diagram) the width is just over 200m.

So as a rough estimate it hasn't changed more than a few yards either way.

ETA
Incidently the walled area is what is now approximately the City of London, the rest of the metropolitan sprawl is the City of Westminster.

According to the diagram, during the 2nd and 3rd century AD, the north bank of the Thames came up almost to the Roman wall. And on the south side, it appears to be marshland or something, because there is no distinct south bank. That means the river has narrowed at least 100 meters. The light blue line that parallels the north bank of the river adjacent to the Roman wall is labeled "Waterfront 2nd/3rd century AD." If the current river is 200 meters wide as you stated, they must have shored up the south bank in such a way that the loss of 100 yards on the north bank was compensated for by increasing the width on the south bank. Either that or I am retarded at reading maps.

I find it interesting how cities such as London and Paris have expanded since Roman times. What began as small settlements not that long ago in comparison to the ancient cities of the Levant and North Africa, grew faster than far older cities such as Jerusalem and Memphis/Cairo (31st century BC). I suppose I find it interesting because we have nothing like it.

Having looked at your map again I now think the blue overlay is the current river and the old bank boundary is the much wider thin line.

Not realising the overlay was the current state of play it is no wonder that when I measured it against the aerial photos they came out the same.

The river has had many buildings and businesses along the banks over the years and with wharves and docks being build and destroyed and rebuilt it is no wonder the artificial bank has moved.
The river is less likely to flood now too so the banks don't change like they could I guess, and the old map may show the maximum extent of the banks as they could be during the high tide or after a flood, buildings would be further back as they cannot be protected from the rising waters like they can these days.

I suspect the width of the Thames measured at low water mark would be about the same.  The Thames is tidal with a rise that can be up to 24 feet and at low water historically there would be a wide "beach" of stinking mud. The Thames is now embanked through the city which restricts its width at high water.
You access the beach at low tide in many places and you can the remains of clay smoking pipes and other small historical items.  Looking for these is called mudlarking.