Life in Lakes and Rivers. T. Macan T.

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СКАЧАТЬ with intensity of rainfall. At intervals exceptional downpours, often restricted to a comparatively small area of the mountains, make considerable alterations to a river bed, which may remain comparatively unchanged until the next downpour. But change never wholly stops; a boulder may stabilize a stretch for many years but all the time it is being chipped away by the smaller stones washed past it until the day must come when it is no longer large enough to withstand a flood. Away it goes and a considerable section of the adjacent bottom with it until a new pattern is established.

      For the biologist the important distinction is between the upland reaches, where erosion is taking place, and only plants, such as mosses, that can attach themselves to flat hard surfaces provide cover for animals, and the lowland reaches where a plain is being built (or would be if the drainage engineers permitted) and rooted vegetation grows. Dudley Stamp (1946) recognizes three zones; mountain, foothill, and plain. Butcher has proposed a classification of rivers according to which of these zones they rise in. In mountain areas with hard rocks the rivers will traverse all three zones, but where there is chalk or other pervious rock the river may spring from the foothill or the plain region. His scheme, however, has not caught on, and most workers agree that an entire river may be so diverse that it will not fit with other rivers into a category. Schemes for recognizing zones within a river have, in contrast, been popular. The best known goes back a long way and has been elaborated in recent years especially by fishery workers. It is based on the species of fish found, which appears to have a fair correlation with the slope. One drawback is that some of the fish do not have a wide geographical distribution. Dr Kathleen Carpenter (1928) has adapted it for British waters:

      1. The Headstreams and Highland Brooks are small, often torrential, and without fish. Temperature conditions vary greatly. Low temperature is common, but a stream that arises from shallow soil may be warm, and a slow-flowing stretch may soon reach a high temperature on a sunny day.

      2. The Troutbeck is larger and more constant than the headstream. Torrential conditions are typical and the bottom is composed of solid rock, stones, and boulders, with perhaps some gravel. The trout is the only permanent fish of the open water though the miller’s thumb (Cottus gobio) is found sheltering among stones. These first two zones together correspond roughly with Stamp’s upper or mountain course.

      3. The Minnow Reach is still fairly swift and patches of silt and mud are only to be found in a few places protected from the current, but higher plants, notably the water crowfoot, Ranunculus fluitans, are able to gain a foothold. This is roughly the middle course of Stamp. It is the Thymallus (grayling) zone of the continental workers, but Carpenter rejects this name because the grayling is not a widespread species in Britain.

      4. The Lowland Reach is slow and meandering, with a muddy bottom and plenty of vegetation. Coarse fish are characteristic, and on the continent of Europe it is known as the bream zone.

      Tansley classifies rivers into five zones, basing his system very largely on the work of Butcher (1933).

      Zone 1 is described as very rapid. Where vegetation is present at all, the important plants are mosses and liverworts; higher plants are often absent altogether and never dominant. This class includes all Carpenter’s headstreams and highland brooks and part, at least, of the troutbeck.

      Zone 2 is moderately swift with a bottom of stones and boulders, but with occasional patches of finer material in which a small number of higher plants can gain a foothold. Ranunculus fluitans (or sometimes R. pseudofluitans), the water crowfoot, is the most important.

      Zone 3 has a moderate current with a gravelly bed. The list of higher plants is much longer. The water crowfoot is still the most important, others are the simple bur-reed, Sparganium simplex, several species of Potamogeton, and the Canadian pond-weed, Elodea canadensis.

      Zones 4 and 5 are medium to slow, and very slow or negligible respectively. The list of higher plants is long and, as it varies a good deal from river to river, confusion rather than clarification would be the result of reproducing it here; but it may be noted the water crowfoot is usually not an important constituent. It is impossible to equate this classification exactly with that of Carpenter, but Zone 2 and part, at least, of Zone 3, correspond with her minnow reach, and Zones 4 and 5 and perhaps part of Zone 3 correspond with her lowland coarse fish reach.

      Against this background a few British rivers which have been studied in detail may now be examined. The Lake District, as was described earlier, is drained by rivers which radiate from the centre. Their valleys were enlarged by glaciers during the Ice Age and generally deepened in such a way that a lake was left when the ice retreated. The Duddon is one of the few valleys in which there is no lake. Its highest tributary, Gait-scale Gill, rises at an altitude of 735 m. (2400 ft.) in a flat area covered with bog and small pools of open water. Beyond this it tumbles steeply down the fellside, dropping 300 m. in 900 m. (33%). At the foot of this slope there is a delta of large stones under which the water disappears in dry periods. Exceptional rainfall towards the end of the year during which Kuehne and Minshall were at work enlarged this delta, and incidentally carried away every one of about twelve maximum and minimum thermometers which they had buried in various parts of the system. The analyses made by these two workers showed that the calcium ranged from 0.57 to 1.10 parts per million in Gaitscale Gill, a low value, even for the Lake District, but after the flood the concentration below the delta rose to 3.0 p.p.m. This illustrates the point, stressed earlier, that freshly exposed faces yield more nutrients than those which have been leached for some time. The highest temperature recorded in the gill was 17.2° C., 5.6° C. lower than the highest temperature recorded elsewhere in the system.

      Several streams run down the fellside parallel with Gaitscale Gill and feed the main river which here runs roughly westwards. Beside it runs a road, originally made by the Romans, probably as a line of communication through the area to facilitate the subjection of the natives (Rollinson, 1967). It is now used mainly by tourists, for there are no dwellings beside it between Langdale at one end and Eskdale at the other. The river swings round to take a southerly direction in an upper valley with a comparatively slight incline. There were four farms in this valley, but only two are used for farming today. In autumn a few green fields around them stand out among the predominant greyish-yellow of the poorer pastures, probably as a result of liming. Sheep, which range far and wide over the surrounding fells, particularly in summer, are the chief product. A few conifers, planted recently, are the only trees.

      The slope steepens to separate the lower from the upper valley. The slope in the lower valley is 1 %, the river is floored with round stones of all sizes, and it flows torrentially down to the estuary. There is no plain region, and only the first two of Carpenter’s four zones and the first of Butcher’s five can be recognized. There are two small villages in the lower valley, residences scattered outside them, and some twelve farms on which cattle as well as sheep are reared. Deciduous woods cover extensive areas of the valley sides. The upper valley comes to an end about 175 m. above sea level, the lower at sea level. The difference in climate between the two is obviously great but no figures are available. The temperature of the swift river gives no indication of it. The valley walls rise steeply on both sides but to the west there is a plateau over which flows the longest tributary.

      The River Duddon is about 11 miles (18 km.) long. We pass from it to the River Tees (Butcher, Longwell, and Pentelow, 1937) which is about 100 miles (160 km.) long. It rises in the Pennines and flows to the North Sea. The gathering ground drained by the headstreams is fairly large. It is high above sea level, it receives a relatively heavy rainfall of about 60 inches (1520 mm.) a year, and the rock is impermeable. The result of these four factors is a severe scour in time of flood, and this has carved out a deep bed. Consequently the energy of a flood is not dissipated in inundating the surrounding country and the effect is concentrated on the river-bed. On one occasion some carts were being filled with gravel at the water’s edge when the river rose so suddenly that the carts had to be abandoned and were swept away. This illustrates a most important condition affecting the plants СКАЧАТЬ