http://maps.google.co.uk/maps?hl=en&...03433&t=h&z=19

Salmons Brook and Pymmes Brook both have dividing walls in their centre. The
walls end where the two brooks meet and then a new dividing wall starts
after the junction. Why is this?

If you track the combined brook southwards, it nearly meet the Lea at
Stonebridge Locks, but has its own channel alongside it to just past
Tottenham Locks. Why is the brook deliberately kept apart from the Lea
Navigation?