"Basil Jet" wrote

[snip]

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?

Interesting question. I haven't the slightest idea of the correct
answer, but I'm prepared to guess.

The gap where brooks meet might be to keep the volume or level equal
on each side of the wall

Salmons Brook and Pymmes Brook have a fairly tarmac'd over watershed,
and so are probably subject to flash flooding whenever there's heavy
rain. Perhaps two small flows would do less damage than one big one.
Perhaps the wall creates some fundamental change in the flow, laminar
flow as compared to turbulent, for example.

It seems the tradition among water engineers that rather than slow
the water flow down in wet weather, as would have occurred naturally
in prehistoric times, when the watershed was all trees, instead they
encourage the water to whoosh down as quickly as possible so that it
leaves their patch, and becomes somebody else's problem. Admittedly
that seems rather illogical if the Environmental Agency controls
everything, but still ...

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?

If the water comes down like a flushing toilet when it rains, that
might be a reason.

Stream routes in the flatter parts of Britain are not very natural.
There were already about 6000 water mills at the time of the domesday
book, needing channels to and from the mill. With eating fish
compulsory on Fridays, fish farming in stream fed ponds was
widespread, as were ponds with decoy ducks for duck hunting. I'm not
sure when irrigated water meadows came in, but that caused more
stream diversions.. There were other events, too, as when the Saxons
diverted the River Lea, to leave a Viking fleet, moored there, high
and dry, and attackable. The Lea navigation added bits of canal.
Things got moved for the reservoirs, and an extra River Lea was built
as a sort of storm drain

Doing strange things to rivers was an issue in Magna Carta

Jeremy Parker