Life in Lakes and Rivers. T. Macan T.

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       CHAPTER 5

      RIVERS

      It is possible to entitle a chapter ‘a typical lake’ and to fill it with an account of physical and chemical changes which follow an annual cycle in a great many lakes. Another chapter is entitled ‘different kinds of lakes’, though here a certain degree of accuracy has been sacrificed to obtain a short title, and the account includes pieces of water that are too small to be regarded as lakes in the strict sense. In neither chapter is there much about fauna and flora, the plan being to describe freshwater animals and plants in chapter 6 and then to pass to an account of the various communities found in different biotopes. A biotope is a region in which the conditions are of such uniformity that the plant and animal communities do not vary much; the stony substratum of a lake, weed-beds and the open water are examples of biotopes. The purist would prefer the term biocoenosis for the assemblage of animals and plants that inhabit a biotope, but here the more general term ‘community’ is used.

      It is impossible to treat rivers in the same way. Lakes are all recent in geological terms, and many of what were called lakes in the preceding chapter are recent in historical terms, having been made by man. Each lake was formed by one event taking place in a limited area. Water courses are much older and have continued their existence in spite of the events which formed lakes. For example the rivers of the English Lake District tend to rise near the middle of the area and radiate from it. This pattern was presumably established at a time when the mountains exposed today were covered by a dome of younger rocks. The watercourses cut down through this and later eroded the underlying rocks in the same direction, though these originally had the form of a ridge not a dome. The disappearance of the covering layers left the old rocks scarred by valleys whose direction bears no relation to the way in which they were laid down. Each watercourse is, therefore, modified today by many geological strata.

      The plan of this chapter is to describe the few rivers about which something is known; how far they are ‘typical’ only further work will show. It has also been found necessary to include some information about the plants, and about human activities. Whereas these have brought whole new bodies of standing water into existence, they have modified rivers. These modifications, however, have been extensive and have affected every British river of large size. Waste disposal, water supply, water power, drainage and navigation have been the main activities through which man has altered water-courses.

      The history of investigation of rivers fits more easily into the account of their misuse in Chapter 14. It suffices to record here that important surveys were carried out between the wars by the late Mr F. T. K. Pentelow, Dr R. W. Butcher and colleagues in the Ministry of Agriculture team. Since the war knowledge about small stony streams has advanced greatly, investigations having been made in most of the upland areas of England, Wales and Scotland. One Lake District river has been investigated by Drs R. Kuehne and W. Minshall working in England during the tenure of a year’s fellowship from America. Of recent years the number of biologists on the staffs of the River Authorities has increased considerably. It seems to be envisaged, however, that their task is to analyse as many samples as possible from as many stations as possible in order to keep a check on the condition of the river and its tributaries. For this purpose it is often reckoned that identification to species is not necessary. For basic information about the lower courses of rivers and their inhabitants, we still have to turn to the old surveys.

      The property of water important to the study of lakes is its density at different temperatures; for the study of rivers, its flow downhill. This has a direct influence on organisms that live in it and a possibly more important indirect one through its effect on the substratum. Table 3 is taken from Tansley (1939) but its original source is a text-book on river and canal engineering. An engineer contrives even gradients and neatly regulated bends; Nature does not. The irregularity of a natural water-course produces a mozaic that confounds the systematic mind at the start and ensures that any scheme of classification is no more than a rough general guide. What does happen in nature? In attempting to answer that question, we shall take an imaginary river, but admit at once that our imaginations owe much to familiarity with the Lake District. The rocks there are hard, impermeable and often steep. In places water flows a long distance over a flat sheet of rock, but generally it has eroded a gulley of some kind. In this rapids generally alternate with pools in parts of which conditions are surprisingly quiet, especially if the stones and boulders are large. In the rapids large stones and boulders tend to jam in the gulley and hold up a bottom that is far more stable than might be expected on so steep a gradient. Conditions in this zone must obviously depend on the relation between the dip of the strata and the angle at which they are exposed, and on the size and form of the fragments which break off the rock. Where the gradient becomes less steep, moderate-sized stones plucked from the rocks above and washed down begin to come to rest. In the Lake District they tend to be flat and accordingly they have an inherent stability. Further breaking-up is taking place all the time and as the smaller pieces are rolled downstream their edges are rounded. This produces the unstable bottom of round stones that is often found some distance down the valley, if the gradient falls evenly. An outcrop of rock is a fresh source of flat stones, and it, or any other obstruction, produces a striking alteration of the flow pattern, confining swift current to the surface layers. Settling of gravel, sand and finer particles becomes possible, and, as these fill up the interstices between the larger stones, they produce a remarkably stable bottom. This happens also during a spell of low water. Percival and Whitehead refer to it as a ‘cemented’ bottom. It is one which occurs almost everywhere, but generally it is covered by loose stones. Occasionally some new obstruction halts the downward flow of loose stones and then the stream is floored by substratum of this type.

      Table 3. Relation of current speed and nature of river bed