How is flow estimated?

Junction Kennet & Avon Canal ( River Kennet )
Left bank. The first mile is still administered as if it were the Thames.
The Kennet & Avon

1404:  Access up the Kennet in Reading -

An agreement between the Abbot and Convent of Reading of the one part and the Warden of the Merchant Guild and the commonality of the other part:
Vessels were to have passage to and fro between sunrise and sunset along this several water of the said Abbot and Convent upon condition that when they arrived at Brokenburghlok they should wait while they sent a man to the place within the said abbey called Blankport covent Keychyn and Westhey at one of which places shall be found a person deputed by the Abbot to open the said lock within two hours or less if possible. 
Toll: One Penny – strangers Two Pence.
Crews might make no play riott or noyse to the annoyance of the convent.

1794: Report of a survey of the river Thames between Reading and Isleworth ... John Rennie (the Elder)

Beginning near Reading there is a very bad and inconvenient stanch lock on the Kennet, near the leather mill, called Blake's Lock, this should be removed, and a proper pound lock and gauge weir put in its place.
The water [is of proper depth] from thence to the junction of the Kennet with the Thames, except in one place, where where there is only three feet nine inches of water.
There is another shoal at the junction of the rivers Thames and Kennet, where was only four feet three inches of water; this must also be ballasted out, and the entrance of the river should be rounded away so as to admit the water in time of floods to enter from the Kennet with less rapidity, and consequently prevent it from throwing up a sand bank in this place.

1810:  Ferryboat ordered across the mouth of the Kennet.
1823:  Ferry worked by chain.
1892:  Ferry replaced by towpath bridge.
1881: George Leslie, "Our River"

No one judging from the ignominious junction of the Kennet with the Thames would imagine what a pretty, cheerful stream it is nearer to its source.  In the vale of Newbury, and by Hungerford and Marlborough, it wanders about in most picturesque fashion, at times expanding into quite broad waters, at others spreading out into numerous small rills for the purpose of irrigating the meadows.  It is celebrated for its trout, and was beloved by Kingsley and many another of the angling fraternity.

Short term mooring, Left bank, supposed to be 24 hours. Do not leave valuables unlocked.
Was shopping for Tesco's but they may have moved?

1914: Hilaire Belloc -

That everything else of age in Reading has disappeared one need not say, but were the traveller in the middle ages here today, the one thing he would most seek for and most lack would be the bulk of the building at the farther end of town.
One can best say that Reading Abbey was like Durham.  It is true that Durham Cathedral stands upon a noble cliff overhanging a ravine, while Reading Abbey stood upon a small irregular hill which hardly showed above the flat plains of the river meadows, but in massiveness of structure and in type of architecture Reading seems to have resembled Durham more nearly than any other of our great monuments, and to emphasize its parallelism to Durham is perhaps the best way to make the modern reader understand what we have lost.

So let's try that -

View towards Reading Abbey from below Caversham Lock
View towards Reading Abbey from below Caversham Lock

But take Belloc seriously and visualize Durham Cathedral where Reading Abbey was and yes it dominates the place -

Durham Cathedral in place of Reading Abbey
And this is what Reading Abbey was once like, maybe, according to Belloc

[ Of course I have just had to guess the scale! ]
and here it is in an old print:

Reading Abbey
Reading Abbey as it was once.

1790s: and this is actually what the ruins looked like in the 1790s -

Reading Abbey Ruins, Belanger
Reading Abbey Ruins, Belanger

1881: George Leslie, "Our River" -

Just below Caversham Lock the river twists about, and on the tow-path side are very steep clay cliffs, worn into large concavities by the stream and its eddies, care being necessary in punting lest the disgrace should happen of a lot of crumbling clay getting lodged on the nose of the punt by bumping against the bank.