I'm getting to know a little bit about the river from people who've lived by it. It's maybe my favorite way to learn about things. Part reality, part imagined, part who knows what, it's terribly exciting.
What I've heard:
There are pill boxes along the river. It was thought that Germans might send submarines up the river to attack. (Risky business on a tidal river.)
Impressively large ships used to come up the river to the Colchester docks before the dam was put in.
In high tides and heavy rains the water front of Wivenhoe used to flood. It was considered a regular enough occurrence that people would just move into the upper floors of their houses temporarily.
(This will grow as I remember what I've heard and hear more.)