On 28 January 1907 James Robinson Kilroe [near the bottom of the page] of H M Geological Survey read to the Royal Irish Academy a paper on “The River Shannon: its present course and geological history” [Proceedings of the Royal Irish Academy Vol XXVI Section B No 8 Hodges, Figgis & Co Ltd, Dublin; Williams & Norgate, London 1907]. I thought that Plate V was interesting.
It will be perceived that instead of the river being shallow over the unyielding Silurian slate-rock, set almost vertically, and striking across the river-course, it is deeper than over the limestone of Lough Derg, and much deeper than over the comparatively easily eroded Old Red Sandstone at Killaloe. The river-bed actually drops below the datum line above the town, while at the town it is 100 feet above datum.
Old Red Sandstone strata are here to be seen in the river-bank, and Silurian rocks in situ in its bed. A barrier is thus formed, partly of Silurian, and partly of Old Red Sandstone rocks, which without the artificial impounding weir would retain the waters of Lough Derg to a depth of some 104 feet opposite Derrycastle — two miles above Killaloe.
One might have expected to find a fairly level shallow bed from Killaloe northward, a sudden drop from slate-rock to the sandstone floor, and a pronounced wide, well-formed valley in the limestone district southward to Limerick.
None of these elements exist; instead, we have the formidable barrier at Killaloe, naturally damming up a considerable depth of water in Lough Derg, and the river falling away southward by a series of rapids which correspond with drops in the canal, south of O’Briensbridge […], along an alternative course, possibly one used by a branch of the Shannon.
Here is an extract from the Plate V map, showing the steps of the (pre-Ardnacrusha) Limerick Navigation between Lough Derg and the sea.
Kilroe wrote of Lough Ree:
The waters of Lough Ree stood some 10 feet higher within recent times than they now do, as proved by evidence of solution, with under-cutting of limestone blocks, to be seen about five miles north-west of Athlone, close to the railway, in the townland of Cornaseer.
Under these conditions the lake must have been, perhaps, twice its width, and for a considerable period. Its ancient surface-level is clearly indicated by the caps of the mushroom-shaped blocks.
And of the Shannon between Lough Ree and Lough Derg:
The extreme flatness of the river between Athlone and Meelick is such that, consequent upon the completion of the Suck Drainage-works in 1892, it was found that the callows along the Shannon above the confluence of the Suck at Shannonbridge were much more liable to sudden and frequent floodings than they previously had been.
The more rapid discharge of the Suck waters into the Shannon, before ordinary extra water had time to pass away, had the effect of modifying the regimen of the main stream to an extent which resulted in an action at law [La Touche -v- The Suck Drainage Board].
I have found only one account of that case, in the Freeman’s Journal of 1 July 1893. The plaintiffs, Messrs Harrison and La Touche, owned land at Cappaleitrim, on the west bank of the Shannon above Shannonbridge. They said that the actions of the Suck Drainage Board had caused their lands to be flooded:
[…] that the defendants brought water from the Suck into the Shannon, containing a drainage of 40 miles, with such velocity and such volume that the Shannon was penned back, and that the back water caused the damage to the lands complained of.
[…] The jury disagreed and were discharged.
I don’t know whether the matter ever again came before a judge.
Yes, I had noticed the deepest water in the Shannon appeared to be at Deer Rock. About 120 feet I seem to remember according to the depth finder. Good place to dump a well weighted body, no ?
I’ve never had occasion to try …. bjg
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