Fresh Water Wetlands

Regions that are covered permanently or for long periods with shallow water are known as wetlands—a term that includes marshes, swamps, fens, and shallow lakes. The existence of many wetlands is very precarious. Some disappear relatively quickly and become dry land; others, such as those in the Nile Delta, are gradually disappearing beneath the waves because of land subsidence. Wetlands, in fact, are dwindling faster than any other ecological system—a process that man has accelerated during the last century by drainage and reclamation. As a typical example of the rate at which the world’s wetlands are diminishing, there are 74 million acres in the U.S.A. today, compared with 127 million a century ago.

To many people, wetlands are just wastelands that should be drained or filled in to provide land for agriculture, or for building, industry, and roads. For others, wetlands have an educational and scientific value, or are places for recreation. The benefits of reclamation, however, are easier to assess in economic terms than the value of leaving them intact, so the economist and engineer are generally allowed to have their way. Such a conflict of interests exists, for example, in Florida. Nearly half the state is wetland, which includes vast areas of cypresses and the largest mangrove swamps in the world. The Everglades between Lake Okeechobee and the sea consists largely of a 70-mile-wide belt of grass interspersed with channels one to four feet deep. This area of 4000 square miles-supports the most varied animal life in the U.S.A.—alligator, deer, panther, bear, snake, heron, ibis, spoonbill, etc., all of which make the area a tourist attraction.

On the other hand, Florida’s main source of income comes from agriculture, on which the northern states depend for early lettuce, tomatoes, and other cash crops. Much of today’s fertile land was once wetland that has been drained. This has involved diverting water by canal from Lake Okeechobee to the sea, or into storage reservoirs for irrigation. The Everglades is consequently rapidly drying up with the result that its wild life is threatened with extinction, its facilities for sport are dwindling, and some of its fisheries put at grave risk.

However, the importance of conserving certain wetlands is being increasingly recognized. For example, many wetlands provide a large flat area over which floods can spread, and so have an obvious economic value; even numerous small ponds in a catchment area can have an important effect in smoothing out peak floods. Many authorities have found to their cost that destroying wetlands in order to develop a flood plain commercially increases the rate of runoff and the intensity of floods. Other wetlands, such as those of deltas and estuaries, are often the most fertile areas in the world—more productive, acre for acre, than an ordinary wheat field. This is very important in a world that is short of food; already parts of the East have found it well worth while to ‘farm’ fish in special ponds. But the profit of a wheat field is easier to assess than that of a wetland, and as yet few have seriously investigated the potential yield of food from these fresh-water areas.

The recreational benefits of wetlands have also been under-estimated until recently. It is now recognized that many ailments are caused or aggravated by boredom and the unrelieved tensions of city life. The opportunity for recreation and a change of scene is therefore important, and wetlands provide one of the best places to go. In America, over £1300 million are spent every year on fishing for sport and waterfowling. Eighty per cent of American waterfowl breed in the wetlands of Canada, Alaska, and the northern central states of the U.S.A.; they also spend time in wetlands in the southern U.S.A. during their migration. There are also a million waterfowlers and amateur anglers in Europe, and many of the waterfowl winter in the Camargue wetlands of southern France. Wetlands are also used for scientific and educational study, for they often contain a greater variety and profusion of life than anywhere else. And because wetlands are in a precarious state, often changing rapidly, they provide a valuable opportunity to study the effects of such change on the flora and fauna.

A wise approach seems to be to drain those wetlands that have no great merit, and to leave intact those of exceptional interest. It takes a long time for a wetland to become established; once destroyed it can no more be replaced than, say, a historic monument like Abu Simbel. And if people can spend enormous sums of money on rescuing Abu Simbel from the rising waters of the Aswan Lake, surely they can leave an interesting wetland alone.

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Man’s greatest success in reclaiming wetlands is the Netherlands, where for centuries people have toiled to convert water to dry land. At one time, the whole of what is now the west of the Netherlands was submerged by the sea. Gradually, however, the winds and tides built up a range of low sand dunes, while silt from the numerous rivers turned the watery stretches behind the dunes into marsh. The Romans, who came to the country around 50 B.C., wrote of a wilderness of marshes and shalldw lakes, with a network of rivers, estuaries, and creeks that continually changed their course. At that time the inhabitants lived on mounds of earth built above high tide level. About the 11th century, mounds were connected by dikes to enclose land for agriculture.

During the 13th century, the sea broke through parts of the coastal sand dunes and flooded large areas of land to form the Zuyder Zee. From then on, floods were frequent, and dikes were often breached by heavy seas and high tides. The first dikes were made of earth, and planted with grass to resist erosion from waves. Later, they were strengthened with reeds, straw, and hurdles woven from willows. In the 14th century, dikes were constructed of wooden piles backed with compressed seaweed, and then these were replaced by dikes made out of wood. But because these wooden dikes had vertical faces, they broke under severe wave action, and in the 17th century they were finally abandoned after they became riddled with holes by a boring mollusc {Teredo. After several hundred years of dike building, the Dutch learnt that the best way to combat wave action is to build sloping dikes faced with stone—a lesson that other countries learnt 200 years later after bare earth dams failed through gradual erosion.

During the 17th century, the Dutch started to drain large areas of dike-enclosed land, called polders. This was, literally, an uphill task, and they made full use of the only power then available—the windmill. Relays of windmills pumped water from ditch to ditch, to canals, to rivers, and finally into the sea. However, the polders were subsiding, and the Dutch had to heighten hundreds of miles of dikes. Towns like Rotterdam and Amsterdam had to build levees along the river near to where it flowed into the sea to prevent flooding of the land.

In spite of improvements, occasional floods continued well into the 20th century. After the flood disaster of 1916, the Dutch revived an old plan that would mean that dikes around the shores of the Zuyder Zee would never have to be raised again. The plan proposed building a 25-mile dam to enclose the Zuyder Zee, so reducing the length of coastline exposed to tides by as much as 200 miles. The dam, finished in 1932, provided the only long-term solution to the problems of the northern Netherlands, for it is much easier to strengthen and raise the existing dam, if the sea continues to rise, than it is to reinforce and lengthen 200 miles of dikes along the former sea-shore. The dam is also invaluable in several other ways, as the Zuyder Zee is now a fresh-water lake, known as the Ijsselmeer, or Lake Ijssel. Sluices in the dam keep back sea water at high tide, and open at low tide to let through fresh water from the river Ijssel, at the same time flushing away salt water from the soil. The lake is now a vital fresh-water reserve for domestic and agricultural use during drought, and the former herring fishery has been replaced by fresh-water fishing. Moreover, there is no longer any threat of sea water intruding into the soil and canals, which previously ruined much of the agricultural land.

Large parts of Lake Ijssel have been turned into polders. The first stage in the creation of a polder is to build an encircling dike, and to pump out the water. The old sea-bed is then drained by criss-crossing the land with ditches, which empty into a deeper ditch around the polder inside the dike. Reeds are often grown at this stage to help dry out the soil, to prevent weeds from taking root, and to stop the topsoil from blowing away. After a few years, underground drains are laid, from which water empties into ditches, and is finally pumped up and out into Lake Ijssel. Although most of the soil is basically fertile, it always needs careful treatment before it is ready for agriculture. Most of the land is used for arable farming, and the rest for dairy farming and market gardening.

Today, half the people of the Netherlands live below sea-level, relying on dams, dikes, and sand dunes to protect themselves from inundation. But there is always a risk of flooding, and during the last 200 years each flood has been worse than the last . The conditions that lead to these disasters are complex, and are basically: very low barometric pressure over the southern North Sea, which allows the sea-level to rise; spring tides; northerly gales in the North Sea together with a westerly gale in the English Channel. A combination of all these conditions is fortunately rare, but when a major flood does occur, immense damage is done to property as well as agricultural land. After the disastrous record flood of 1952, the Dutch wisely planned their new sea defences with a long-term view on the assumption that future floods will be even higher.

The new defences centre around the deltas of the Rhine, Meuse, and Scheldt, in the south-west of the Netherlands, where the shore-line is about 600 miles long. Rather than go to the enormous expense of reinforcing or rebuilding existing defences, the Dutch have conceived one of the world’s great works of civil engineering, called the Delta Plan . This consists of raising the seaward dikes of a number of islands and joining the islands by a series of dams, thus creating a complete barrier to the North Sea. The dams are being equipped with sluice gates up to 186 feet long; these gates open at low tide to release river water and winter ice, and close at high tide to exclude the sea from the branches of the estuary. What was formerly estuary will become fresh-water lakes, so that the problem of salt-water intrusion will be solved. Also, the sheltered water behind the dikes will be kept at a constant level and will help to satisfy a problem that is urgent in a heavily populated country like the Netherlands—that of providing opportunity for recreation, such as sailing, fishing, and water-skiing.