MONITORING THE FRESHWATER THAMES

The monitoring of the River Thames is primarily done by the Environment Agency (EA) - much of it is automated - though I think some levels may still be reported by lock keepers by telephone.
Accessing it is not so simple - but basically there are the EA data sites which are the primary sources for various (often rather more user friendly) secondary sites.
Using Reading Bridge flow gauge and Caversham Lock levels as examples, these are the various sources with live examples:

EA READING BRIDGE:
https://environment.data.gov.uk/flood-monitoring/id/stations/2200TH.html

The above is the user friendly version of the EA data.
Behind it lies the json version which is quite definitely unfriendly to human users and designed for automated reading. To explain just how unfriendly this is it uses a coded version of no less than a five dimensional array! For those who need to know, using php, after downloading the file and using "json_decode($file,true);" the flow is at
$JSON["items"]["measures"][0]["latestReading"]["value"];
I tell you because it will take blood sweat and tears to find that out the hard way!
The json version is at:
https://environment.data.gov.uk/flood-monitoring/id/stations/2200TH.json

GAUGEMAP FLOW AT READING BRIDGE
The most user friendly secondary sites are the Shoothill Gaugemap sites. The flow reading with a nice graph (showing options of the last week, or last month, or last year) is at
https://www.gaugemap.co.uk/#!Detail/1070/1139 (Gaugemap links may need to be copied and pasted not clicked)

That site was 18 hours out of date when I wrote this which reminds me that the secondary sites can only be as reliable as the EA primary site and might sometimes be less reliable.

GAUGEMAP LEVELS AT READING BRIDGE
https://www.gaugemap.co.uk/#!Detail/1070/1138

There is a summary version at
https://www.gaugemap.co.uk/#!Detail/1070/1138
which names both flow and level (and most importantly gives us the stage datum which relates the level figure to Ordnance Survey level) so Reading bridge level will be 30 + gauged level metres above sea level.

RIVERLEVELS AT READING BRIDGE
https://riverlevels.uk/thames-reading

FLOOD WARNING SERVICE LEVEL AT READING BRIDGE
There is also the flood warning service level with line graph, but no flow
https://flood-warning-information.service.gov.uk/station/7110
That site will not allow itself to be shown on other pages such as this.

CAVERSHAM LOCK

Locks do not have flow gauges.
There are two levels shown. The prime level is the upstream level which is relative to the stage datum of the lock.
The downstream datum is different.
Its old name was "Tail Gate Pile Zero".

EA CAVERSHAM LOCK:
https://environment.data.gov.uk/flood-monitoring/id/stations/2201TH.html

Note that "level-stage" means the upstream level as opposed to "level-downstage"
Also that the two stage datums are different!
The "unfriendly" automated version is at
https://environment.data.gov.uk/flood-monitoring/id/stations/2201TH.json
Good luck if you have to get into that! One reason to get into it is that it gives the datum for the downstream level - at Caversham downstageScale: datum: 33.263
Note that the stage datum currently given as 36.509 (apparently under question at the moment) is not the same as at Reading Bridge.
So the weir drop here should be
(Upstream level + 36.509) minus (downstream level + 33.263)
The EA reference data is sometimes open to question. Do not be fooled by it being quoted to 3 decimals! In some reaches in summer the river flow according to the levels calculated would have to reverse!

GAUGEMAP CAVERSHAM LOCK:
NB "River level" means the upstream level.
https://www.gaugemap.co.uk/#!Detail/1071
(Gaugemap links may need to be copied and pasted not clicked)

THAMES.ME.UK FLOW PAGE

https://thames.me.uk/s00331.htm
If the sites above are primary and secondary then this is tertiary!
It is an attempt to see the whole flow. It attempts to use all the known flows and levels, and estimates them where there is no source available.
The Green cells are known sources and logical deductions from those sources.
If the sources are accurate then the green cell figures are accurate.
The pink cells are estimates which may or may not be correct, and can be very inaccurate (see comments in right hand column).
So, for example the flow at Caversham Lock is in a pink cell since it is not measured there - but we can be reasonably confident it will be the same as the measured flow at Reading Bridge.
By the time we reach Sonning Lock our estimate is inevitably a bit wilder.
At Sonning it must be at least Reading Flow plus Kennet flow. The Kennet flow is measured only at Newbury and the percentile excedance there can be found. Using that percentile applied to the Theale flow gives a logical value for the Theale flow.
But then there is nothing further to go on - so the Blakes Lock flow is a guessed 20% above the Theale flow. Then the lock flows are spread out to the next flow gauge which is Maidenhead and its bypassed Jubilee River flow.
That spread is inevitably not accurate and may be very inaccurate given that its starts with a guess at the Kennet flow. The sign of that may be that the flow estimate at successive locks is shown as declining!
But then that does actually happen sometimes, principally in the summer.
The other great unknown is water abstraction which becomes significant below Maidenhead as Londoners drink the Thames.
Often London's consumption exceeds the total flow and the river flow is highest at Maidenhead and declines very significantly down to Kingston.