* NOTE: Hambleden Lock figures are estimates, they are not available online! So the three locks diagram is centred on Shiplake. The flow estimates are unaffected.

NB the footway from the right bank is only to the lock island and then back to the right bank. No access to left bank.

How is flow estimated?

EA MARSH Downstream graph
EA MARSH Upstream graph

from Environment Agency Guide 2012-2013

Left Bank, tel: 01491 572992, length: 135'2", width: 21'1"
Marsh Weir for canoists

1404:  John Dayton indicted for not properly maintaining, at Rotherfield Pypard, locks and winches in aid of the navigation. 
(The site of Marsh Lock might have been counted as in this parish at the time)
1585:  Bishop - New Mills, Right bank

1698: Marsh Lock and Mill, detail in Jan Sibrecht's painting:
"Henley from the Wargrave Road II", 1698 (River and Rowing Museum, Henley) -

Marsh Weir (lock) and mill, detail from Jan Sibrechts painting 1698
The two barges are coming upstream
and the narrow section of weir is being removed
on the left is the spur of land on which a winch is mounted
which will pull the barges up against the flow through the weir.
Note top right the small mill and water wheel.

See Henley Bridge for more details from Jan Sibrechts

1715:  Overton’s Map – New Mills, Left bank
1746: Griffiths – “Mash Lock”
1773:  Pound lock built.  The Revd Humphrey Gainsborough, the artist’s brother, minister of Henley Congregational Church, was involved.  He was an inventor and friend of James Watt.
1776:  Humphrey Gainsborough died in the Lion Meadow “whilst conversing with some gentlemen about the locks he had constructed, having about £70 belonging to that useful work in his pocket.”

1780:  Marsh Lock was

decaying fast.  The gates want to be new plank’d.  The mill stream  waste water setting against the eyett below the pound, cut it away in such a manner that a row of piles & rails must be set up, as well to secure the bank as to prevent barges driving against it by the strong current.

1786:  Weir “broken down by ice & water to the extent of 100 foot”.
1787:  Poundlock rebuilt

1793: Marsh Lock, Boydell -

Marsh Lock, Boydell 1793
Marsh Lock, Boydell, 1793

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

I found [Marsh Lock] five feet two inches deep on the upper sill of the lock, and five feet on the lower sill, the water in the pond below being five inches under pen.
There are two corn mills on the Berks side, and a corn and paper mill on the Oxford side. In the middle is an old flash lock, or stanch the property of Marshall Conway, in very bad repair. There are also two sets of fishing bucks, one the Marshall's, and the other Mr. Stevens's as with so many mills and fishing bucks uncontrouled, as they seem to be, one cannot wonder that the water in the pond above should often be worked very low. This I found literally to be the case, for when I arrived at the lock night prevented my further Observations ; next morning I found the mills had lowered the pond fifteen inches, which stopped the navigation till a new head was penned.
The lower sill of Marsh Lock; should be sunk about six inches. The sþace between the ayts, at Mr. Stevens's, and the Berks. side of the lock, should be shut up by a weir; and the space between town ayt, and another small one under the mill, should be contracted. These works being done, and the barge channel being ballasted [dredged] from near the tail of the lock to the bottom of town ayt, will remedy this part of the channel.
From Bowney [Bolney?] Flatts to Marsh Lock, the water is good;

1813:  Lockhouse built
1814:  “the entrance to the Lock being extremely inconvenient and dangerous for barges.”
1823:  “Putting up new bridges over Mr. House’s millstream”
1825:   Swingbridge repaired
1843:  The Lock was in very bad condition, and was thought to be on the wrong side of the river.  Navigation into it was difficult, and in high water dangerous.
  (Fred Thacker, 1920, says “Even now it is a troublesome place to hold up, at either end.”) 
Plans were drawn up, but never executed, for its removal to the Right bank.
1867:  The flooded state of the country above Henley, when the mills stopped working on Saturdays, was complained of.
1870: Mill offered for sale to the Conservancy “for the improvement of the river”  They did not buy.
1877:  A boatslide was ordered but never built.
1879:  Conservancy tried to buy mill.  It was not for sale.  A new weir was built.

1878: Marsh Lock, Henry Taunt -

Marsh Lock, Henry Taunt, 1878
Marsh Lock, Henry Taunt, 1878
© Oxfordshire County Council Photographic Archive; HT2408

1879: Marsh Mill photo –

Marsh Mill, 1879
Marsh Mill, 1879

1881: George Leslie, "Our River"

At Marsh Lock there are two Mills, one on each side, and the opposing currents from these Mills render the approach to the lock from below always rather difficult.
The tow-path passes along to the lock, over the weir and mill waters, by a long, wooden bridge, most picturesque in its line and character, and which I am grieved to say is all doomed to be removed, as the whole weir is now being reconstructed.
I think it is quite possible the construction of this bridge and the arrangements of the weir and lock were carried out under the direction of a brother of Gainsborough the artist...
In winter time, when there is a flood, the long line of this bridge and the weir, as seen from the high ground on the Wargrave Road, is very singular and striking, especially when the meadows and tow-path are under water...
A disagreeable smoky chimney spoils the look of the Mill on the Oxfordshire side, but the other, though lately rebuilt, is by no means ugly;  some little outbuildings in red brick above the weir are not out of harmony with their surroundings, and the long walls of the Mill garden are suggestive of peaches, nectarines, and all manner of wall fruit, which the situation and aspect favour in the highest degree.

Horse Bridge, Marsh Lock, George Leslie, 1881
Horse Bridge, Marsh Lock, George Leslie, 1881

Marsh Lock is a terrible one to pass through on a Regatta morning.  I shall never forget coming through from Wargrave on one of these occasions.  The gates could hardly open on account of the jam of boats against them, everybody as usual wanting to get in first, the ladies being by far the most eager and energetic in their endeavours.  The whole mass of boats shaped itself into the form of a huge arrow-head, and right down into the middle of the pack came slowly but surely a large tug-barge, called the “Spitfire” (since blown-up and wrecked at Sonning), with a crowd of Reading folks on board at a shilling a head.  As the gates at last opened, the wedge tightened up, and I was glad to remember that my punt’s sides were inch stuff, and the oak treads very strong, for I felt and heard the sides of other boats giving way like baskets, with many an ominous creak; outriggers and rowlocks got jammed and broken, and amidst cries and vituperation of every sort, the lock gradually filled.  I had no idea of how many boats a lock would really hold till then.  The excitement reached its pitch when the gun at Henley was heard announcing the start for the first race, and the instant the lower gates could be opened the whole crowd of boats rushed out, splash, dash, and away, like school boys out of school.
The return from the Regatta is never so bad, as people leave the course at various hours, and no one is in any very great hurry, but the poor lock-keeper has a very hard day’s work.

[ Except that in modern times (say 1990 to 2010), on the Saturday night of the Royal Henley Regatta, after the fireworks, there is a notorious jam of boats trying to get through towards midnight. Possibly wisely, the lockkeepers are not on duty then, though often they volunteer (and sometimes regret it!) There are always loud rows and river rage. Stay clear!
As a small manual boat owner I find these queues difficult. One doesn't want to queue jump - but sometimes it seems necessary for safety reasons. For some reason larger boats occasionally do not appreciate this ...
The ideal situation would be that small boats would queue on one side and large boats on the other. The larger boats would fill each lock and then the smaller boats would use the inevitable spaces left beside and behind them, thus maximising the use of the lock. However this is not an ideal world. ...
"The gun at Henley" - before VHF radios etc the regatta program was synchronised by the firing of a gun at the old finish on Henley Bridge. Thus it would have been audible at Marsh Lock.

1883:  A new pound lock was decided upon on the Right bank
1883:  The Mill was offered for sale; the Conservancy did not buy.
1884:  Right bank lock scheme abandoned
1886:  The Lock was rebuilt in timber.
1893:  Mill still working

1893:  Marsh Mill and Bridge, Francis Frith -

1893:  Marsh Mill and Bridge, Francis Frith
1893:  Marsh Mill and Bridge, Francis Frith

1895:  John Bickerdyke, Thames Rights and Wrongs -

A few years ago a picturesque old weirpool celebrated for its excellent fishing.  It was more or less an embankment in the shape of a horseshoe, with openings for water to rush through.  A restful thing for the eye, lying between two particularly ugly mills. This weir answered its purpose very well.  The Conservators however threw a series of immense iron watergates across from mill to mill, and levelled the old weir,  It pleased and brought grist to the millers, because in summer a better head of water could be maintained than with the old weir, which was a little leaky.  It immensely benefited the owners of property upstream, because they were less subject to floods than formerly.  But neither they nor the millers contributed anything toward the expense of making it.  The old inhabitants say there was formerly a way across the river at this spot:  along a horsebridge, then over the lock, and on to the road through from the mill yard.  But when these alterations were carried out this road was stopped.

A Tale of Henley, 1899, by Victor Whitechurch

1899: A Short Story from the Strand Magazine; real Victorian "daring-do" set at Henley Regatta involving an anarchist attack on Marsh Lock.
(To be taken with a pinch of salt!)

1899:  A law suit decided that Warborough millstream, running behind the lockhouse to the mill, is a private water.
[ BUT WHERE THAMES WATER FLOWS THERE IS A RIGHT OF WAY – and it flows there – of course there is little point going the few yards on it and certainly no powered craft should attempt it, but the principle remains.  Perhaps a lawyer could explain that decision? The Hedsor Water case (2004/5) has recently been decided the other way – and quite right too!]
Part of what the 1885 Act said was -

1. It shall be lawful for all persons, whether for pleasure or profit, to go and be, pass and repass, in boats or vessels over or upon any and every part of the River Thames, through which Thames water flows, between the Town of Cricklade and Teddington Lock, including all such backwaters creeks side-channels bays and inlets connected therewith as forms parts of the said river within the limits aforesaid. ...

1901: The Thames Illustrated by John Leland -

Marsh Lock, a short mile from Henley Bridge, is a point well known to those who come down the river to Henley Regatta. These will not forget the extraordinary crowding of boats, the grinding of one against another, the breaking of outriggers, the destruction of varnish and temper, at the lock on Regatta days, nor the eager and impetuous rush with which the boats escape into the freedom of the lower waters.
There are mills on each side of the river, but the lock and weir have lost a little of their picturesqueness. Some years ago, the towpath was carried over the lock, weir, and mill-water by a very quaint bridge. There is now a long, white, wooden structure, resting upon short piers - not vieing with its predecessor in picturesqueness - and yet, fortunately, far from being a disfigurement to the landscape; and the mills have the characteristic charm of most such structures that we meet by the Thames.
The surroundings are delightful, and the lock-house is a pretty place, known to all oarsmen as the headquarters selected by the Yale Club at Henley Regatta.

1955: Marsh Lock, Francis Frith -

1955:  Marsh Lock, Francis Frith
1955:  Marsh Lock, Francis Frith

1955: Above Marsh Lock, Francis Frith -

1955: Above Marsh Lock, Francis Frith
1955: Above Marsh Lock, Francis Frith