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 6000_l._, being 10_l._ 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.
_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.