Landscape of the River

Ireland resembles a pie which has been baked without benefit of the necessary egg-cup to support the centre. The coastal areas are high and reasonably dry, whereas the centre of the country is low and extremely soggy. One-fifth of the land area of Ireland drains inward through various lakes, small streams and rivers, reaching the system known as the Shannon.

Geological Formation
The landscape of the central lowlands of Ireland through which the Shannon flows reflects the underlying geology: this is predominantly carboniferous limestone (formed 350 million years ago), overlaid by varying thickness of glacial drift deposited during the Ice Age (which commenced two million years ago). Diversity in the landscape is provided by the small areas of higher ground formed from younger rocks on top of the limestone, such as the hills surrounding Lough Allen, or of older rocks of shale and sandstone, projecting through the limestone as a result of folding. Examples of the latter are Slieve Bawn, north of Lough Ree, a range of hills running southwest to northeast interrupted by the Shannon, and the Slieve Bloom moun tains, or those mountains west and south of Lough Derg. Any other diversity in the landscape is the legacy of the last glaciation (100,000—10,000 years ago): the drumlins of South Leitrim; the moraines and eskers in the central Shannon area; the topography suitable for later bog formation; the major lakes, Lough Ree and Lough Derg, which were probably formed through chemical solution of the underlying limestone and denudation by ice and running water.

The source
Traditionally the Shannon rises in the Shannon Pot, a round pond on the slopes of Cuilcagh Mountain in Co Cavan, from which a small trout stream emerges, but there is no visible water support entering the pool. In recent years, however, potholers have discovered what is thought to be the true source of the river much further uphill, where a small stream disappears into a sink-hole. This, in fact, is across the Border in Co Fermanagh. The whole upper part of Cuilcagh Mountain consists of a porous limestone and is full of sink-holes and risers. From the Shannon Pot, the river receives a number of tributaries, some of which are larger than itself, and emerges into the head of Lough Allen.

Mineral Wealth
Lough Allen, the third largest of the Shannon lakes, lies between the heather covered Arigna hills and Slieve Anierin. These are Upper Carboniferous in age, made up of shales, flagstones and sandstones, and contain coal seams. Iron occurs in the form of nodules of ironstones within the shales—particularly on Slieve Anierin (the mountain of iron)—and small local smelting works existed in many places such as Arigna, Boyle, Dromod, Drumshanbo and Drumsna, utilising the coal seams and earlier charcoal from the forests around Lough Allen. Until recently the horizontal bands of coal were exploited almost exclusively in the nearby Arigna Power Station. Over the centuries, silt brought down by the Arigna river gradually blocked the Shannon’s exit, raising the level of the lake. Since the 1920s it has been used as a storage reservoir for the Shannon hydro-electric scheme, causing variations in levels of up to 6m.

Ice Age Influences
South of Lough Allen, the tremendous influence of the various ice ages becomes obvious. This is the area which is covered with drumlins, small rounded hills of blue clay, containing boulders of various sizes. These succeeded in blocking whatever drainage pattern existed prior to the ice ages, ponding back water into lakes. It has been suggested that Lough Allen at one time flowed into Donegal Bay. Further south, the evidence of blockage is very clear in many places: one of the most obvious is at Rosebank, just south of Carrick-on-Shannon, where the present high ground on either side of the river was obviously connected right across the river at one stage, ponding back the water right up to Lough Key. The water reaching these lakes carried a great deal of blue clay in suspension, and this was dropped in the still water to make very extensive, completely flat, boggy fields on either side of the river. The height of the flood water is clearly indicated by small cliffs on the southern shore of Lough Drumharlow, eroded by wave action when the lake was much higher than its present level. At Rosebank, immediately below this restriction, the river bottom has been scoured to a very considerable depth; this type of scouring could be evidence of a waterfall in the area at some time in the past. Where the restriction actually occurred the river bottom consists of very large boulders, which are too big for the current to move; this area had to be dredged in the course of constructing the present navigation.

These blockages occurred all the way downstream and were probably cleared one after the other, allowing plenty of time for the fine material from upstream erosion to be deposited on the bottom of the downstream lakes. The result is that the whole north Shannon between Cootehall and Lough Boderg is lined with low callow fields, with great depths of impermeable blue clay which grows nothing satisfactorily except field rushes. Here can be seen the typical drumlin landscape of south Leitrim, where 75% of farms are less than 12 hectares in extent, made up of small hedgerow-enclosed and ill-drained fields. While excavating for a slipway near Drumsna, we dug through 2m of blue clay, which contained neither sand, gravel nor stones. Underneath this there was 1m depth of woodland peat, containing sections of pine. This lay directly above the limestone bedrock. As the top of the peat was over 1 m below the present summer water level, this would infer that, at some stage prior to the ice ages, the water level must have been considerably lower, or perhaps there might not have been a river in this area at all.

The Upper Shannon
The bed of the Shannon and its lakes is very varied. Lough Key has an even-chequered pattern of hard shoals and islands. The smaller lakes in the alkaline waters of the upper Shannon all have deep deposits of white shell marl, up to 9m in depth in places which consist of the shells of myriads of small freshwater snails. The bottom depths are very even in these lakes, between 1.5 and 2m, except where the incoming river has scoured it in places down to 18m. Lough Forbes is peaty on the western side and alkaline on the eastern side, with a number of unexpected large rocks. The rocks in the shoals and on the shores of Lough Ree are reputed to be soft, being mostly covered with a very thick layer of lime. Some of the shoals on this lake are shown on the old maps as islands, but these have since been eroded away.

The Middle Shannon
Between Athlone and Portumna the landscape has changed. Here the Shannon is wide and sluggish and becomes extensively flooded in autumn and winter so that the river’s course is no longer visible. The major feature of the landscape are the esker ridges which cross the river approximately east-west. Eskers were formed as the ice was melting, when streams beneath the glaciers carried and deposited large amounts of sand, gravel and boulders. The most spectacular eskers are those low-lying hills at Clonmacnois. Between the esker ridges shallow lakes remained after the Ice Age and rapidly filled with marl (a deposit of calcium carbonate). The lakes became shallower as the environment dried out. Reeds and sedges, rushes and willows grew and partly decayed—thus peat accumulated and bogs were formed. Good examples of bog can be seen near Clonmacnois and Shannonbridge and many bogs are now exploited for electricity generation as at Lanesborough and Shannonbridge. Over most of the river bed, where there is any strong flow, it is scoured to large stones or boulders, or down to bedrock. In the stillwater sections it is filled with various silts. In the areas near the mechanised bogs the silt is largely milled peat and this peat has even encroached into the upper portions of Lough Derg, where it has seriously interfered with all forms of life in the lake.

Lough Derg
Lough Derg, in its northern section, resembles Lough Ree: headlands, islands and shoals composed of boulder-rich glacial drift appear, but the rocks in the lake are hard, with no protective coating of lime. On the mainland these soils support productive agriculture.

The southern end of the lake is different and very spectacular, with the Arra Mountains rising sheer on one side and Slieve Bernagh rising in the same way on the other side. The lake is of a considerable depth for more than half of its length: there is a continuous trench with depths of 24—30 m which runs almost to Killaloe.

Geologically, there is some speculation on the reasons for the course of the Shannon in this region. Why did the river not enter the sea at Galway Bay or eastwards, rather than meandering south where it ignored the obvious exit at Scarriff Bay and instead left the limestone and cut through the sandstone and slate hills at Killaloe before reaching the sea at Limerick? One explanation is that at one time the central plain of Ireland stood much higher and when it was denuded the river retained its original course, having carved a way through to plunge over a rock sill at Killaloe. This natural dam was removed in the 1 920s and a new artificial dam erected further downstream when the hydro-electric works were being carried out.

Farming Practices over the Centuries
Farming practices have changed the Shannon’s natural landscape over time. Apart from the better drained and productive soils of the higher land around Lough Ree and Lough Derg, traditional farming methods are still found over most of the area. Here farm size is on average about 12 hectares. Less than 5% of the land is tilled. The farmland south of Athlone, winterflooded and silt-enriched, is renowned for vegetable growing but summer flooding in some years causes problems. Geese are not uncommon, grazing in large flocks on the callows. These small farms cannot support a whole family and for most of them farming is now only a part-time activity. Additional income is sought from work in nearby towns, in the few local industries or with Bord na Mona.