Complex sentence: In Roman times and for a long time after the River Lea was tidal as far as Hackney Wick where the Hackney Brook met the Lea and the confluence was very wide when flooded.
Split into simpler sentences: In Roman times and for a long time after, the River Lea was an estuary, tidal as far as Hackney Wick. Here is where the Hackney Brook met the Lea; the confluence was very wide when flooded.