The Earne and Shannon rivers have three features which render them, in Mr. Lynam’s opinion, peculiarly easy to regulate their floods, and prevent inundations. They have large superficial areas of lakes. Their channels between the lakes are wide and deep, so capacious as to carry their floods with an inclination of less than an inch a mile. Their floods rise slowly, 4 inches to 8 inches in twenty-four hours, very rarely rising 1 foot in twenty-four hours. On the Shannon, all the mill-weirs and fish-weirs have been purchased and removed, and all the shoals have been deepened at a cost of 529,716l. The lakes in the Lough Earne basin have an area of about 50,000 acres. The shoals and straits, which obstruct the river and cause the inundations, have an aggregate length of merely 6 miles. Only one mill-weir (which is the only fish-weir) exists, and it is at the outlet, where there is a fall of 12 feet. The Shannon basin has lakes of the superficial area of 87,000 acres. In the length, from the Battle Bridge above Carrick-on-Shannon to Killaloe Bridge, of 128 miles, the lakes occupy 50½ miles; the broad, deep channel extends for 73½ miles; the confined portions of the channel occupy merely 4 miles; the portions of the channel confined so as to be visible obstructions are but 2 miles long. Neither mill-weir nor fish-weir stands in the way of the current. The floods scarcely ever rise 1 foot in twenty-four hours. The great floods are but 4 feet where deepest on the lands, and generally but 2 feet deep, and merely 18 inches deep over large areas. Many damaging floods are not more than 6 inches deep on the land.
From Lough Allen to the tide of the Atlantic Ocean at Limerick, a length of 149 miles by the sinuosities of the river, the Shannon has been made navigable for steamers with a depth of 6 feet of water. The river lies naturally in eight separate levels, but the lowest, at Limerick, is very small, and detached from the others by a length of 5 miles and a fall of 90 feet. The upper level, at the outlet from Lough Allen, has a fall of 20 feet in 6 miles. The lowest level, between Castleconnell and Killaloe, contains only 641 acres of lowland, rarely flooded in summer or autumn, and rarely covered by more than 1½ foot of water. To preserve the land from summer and autumn floods the surface of the floods must be lowered 2 feet nearly. A permanently solid embankment, used during many years for a navigation horse tow-path, extends along one side of the river, the only openings being four culverts for side drainage.
On the other side of the river there exists a natural ridge, which is a little higher than the highest floods. It is not continuous, but interrupted in five places. These circumstances are held by Mr. Lynam to “render it very easy to protect the lowlands from all floods.” Very favourable sites exist for back-drains to carry off rain-water and springs. The 641 acres of lowlands may be thus protected from summer and autumn floods at a cost of 6000l., being 10l. per acre. This would allow of winter irrigation also, which the occupiers of the lands particularly require. The system of river embankments is much objected to as dangerous, and properly so, when it is proposed to make high embankments. In this case the required embankments are in existence for seven-eighths of the required length, so permanently solid as to be absolutely safe, and the small portions to be built need not be more than 3 feet to 5 feet high. The obstructions are a rock-shoal near the middle of the length, an old bridge with narrow arches and thick piers, and a shoal of solid limestone rock at the outlet.
Minor Irish Rivers.
The Barrow River has been rendered navigable from the tideway below St. Mallins up to where it is joined by the Grand Canal at Athy Bridge, a distance of 43 miles, falling 172 feet. But from Athy to the mouth of the Barrow, in the estuary of Waterford Harbour, and through that to St. George’s Channel, the distance exceeds 60 miles.
The Blackwater River, county Cork, is navigable from its mouth at Youghall up as far as the tide reaches, or at most to Cappoquin.
There is another, and smaller Blackwater, connected with the Tyrone Canal, and flowing into Lough Neagh.
The Boyne River is navigable from the Bay of Drogheda for 22 miles, up to Trim, in the last 7 miles of which it ascends from Navan 189 feet by means of locks, which are from 80 to 100 feet long and 15 feet wide.
The Corrib River and Lough, or Lake, form a navigable line, commencing at the mouth of that river, in Galway Bay, and extending from Galway town in a north-westerly direction for about 24 miles.
The Earne River and Lough, or Lake, are navigable through the lake from the upper part, where the river enters it, below Belturbet, till it leaves it again at Enniskillen, where it is obstructed by weirs; but below the isle on which that town is built the river again expands into the lower part of the lake, through which it is also navigable. Thus far the entire distance is about 30 miles, and the navigation is terminated by a fall, from which the river has a rapid course of 9 miles to Donegal Bay.