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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

Flowing onward, the river generally comes to a plain, often one of its own creating, the gradient approaches nearer and nearer to the horizontal and flow decreases. First gravel, then sand and finally silt settle to the bottom and provide a substratum in which plants can take root.

Probably few except those charged with the task of dredging it realize how much material is being deposited on river beds. It is not a continuous process and varies greatly 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 and animals of the river.

The small town of Croft is about 45 miles (70 km.) from the mouth and about 65 miles (100 km.) from the source, travelling by river, and is a convenient dividing point. Above Croft all the river is rapid with little rooted vegetation and few fish except trout, grayling, and minnows. The river downstream is still moderately swift but there is much more rooted vegetation and various coarse fish are plentiful. After flowing for 20 miles (32 km.) from Croft the river reaches the head of the estuary, which is some 25 miles (40 km.) long.

The Tees rises on Cross Fell at a point about 2,500 feet (760 m.) above sea level. Many small tributaries also rise just on the eastern side of the Pennine watershed, and some of them originate in thick peat beds. One, for example, drains the peat pools which were mentioned in Chapter 4. These little streams run down the hillside with a fairly though not extremely rapid flow, because the eastern slopes of the Pennines are not steep. About six miles from the source three main tributaries have coalesced and the river is some 12 metres wide with a fair flow over a bottom of stones and boulders. Then it enters a quiet stretch and for three miles the current is sufficiently slow to allow the deposition of some fine sediment, which provides a foothold for a few higher plants, Potamogeton alpinus, the alpine pond-weed, Callitriche intermedia, the water starwort, and Sparganium simplex, the simple bur-reed. The only other attached plants are the mosses, Fontinalis antipyretica, and Eurhynchium rusciforme, and, at certain seasons, the algae Lemanea fluviatilis and Cladophora glomerata.

This slow stretch provides a pretty example of the sort of exception to the general plan which is to be found in almost any river. It is caused by a stratum of hard rock, and at the end of it there is a fine waterfall. A little farther on, some 16 miles (20 km.) from the source and 1,000 feet (305 m.) above sea level, the river strikes a road and some human habitations, and comes to the end of what may conveniently be taken as the first part of its course. A chemical station was set up just here and the results obtained are shown in Table 4.

Table 4. Some dissolved substances in the Tees near the end of the first part, that is above any pollution

The average amount of calcium is about 12 parts per million, and so the water is soft although the river has been flowing over limestone. But there are big fluctuations in the concentration of all the substances except oxygen, which is plentiful at all seasons and at all times of day and night.

Trout occur in this part of the river and may be taken almost up to the source. They are plentiful but of small size, the average length being but six inches (15 cm.).

The next part of the course extends all the way down to Croft, which is at the point where the river changes in character. The river crosses several geological formations, which affect its nature, but it remains rapid throughout with a bottom of bare rock or stones. There is hardly any rooted vegetation.

The main difference between this and the preceding part of the river is that it receives sewage effluents from towns along its route. The first place of any size is Middleton-in-Teesdale, some twenty-two miles from the source, and the biggest is Barnard Castle, about eight miles farther on. The population connected with the sewage systems of the two places was 1,700 and 5,000 respectively when the survey was made. Below the outfalls there were changes in the flora, and there can be no doubt that these changes were directly attributable to sewage and the products of sewage decomposition. Below some of the larger works the dominant organism on the river bed was the sewage fungus. Father downstream there was a characteristic association of algae, but this finally gave place to the same association as was found in the upper waters, where there was no pollution. Below the smaller sewage outfalls there was no sewage fungus, but there was the characteristic change in the algal community encrusting the stones and boulders. In all this part of the river the amount of sewage was small compared with the volume of water into which it was discharged and pollution was not great. Oxygen concentration in the summer was lower than in the first part of the river (Table 4) but it never reached a seriously low level. Chloride rose, probably as a result of the sewage. The amount of calcium also increased and an average figure just above Croft was 24 parts per million; this rise was probably mainly due to the limestone over which the water had flowed.

Trout occurred throughout this part of the river and reached a greater size than in the first part, ¹/₄ to ³/₄ lb. (120–360 g.) in weight with a few specimens of 3–4 lb. (1.4–1.8 kg.). It contained minnows almost throughout, and grayling in the lower half; thus, although there are no marked physical changes, the river enters the third of Carpenter’s zones in this part of its course.

At Croft, 100 feet (30 m.) above sea level, there are several important changes in the river. It enters a great clay plain, laid down during the Ice Age, and flows across this in a meandering course, though with a fair flow. The stretch is too fast for a typical lowland course judged on purely physical grounds, and it is probably in the third of Butcher’s five zones, for Ranunculus fluitans is one of the commonest plants; but it is in the last of Carpenter’s four classes since coarse fish abound. Other changes are due to the confluence of the River Skerne, a large tributary, which is more calcareous and much more heavily polluted than the Tees. These three factors, different kind of bed, more calcium, and more sewage products, all influence the biology of the river below Croft. but it is not possible to measure exactly how great a part each one plays.

Most of the bed of the river is of medium-sized stones and gravel but there are occasional patches of sand. In water shallower than five feet typical plants are the water crowfoot, Ranunculus fluitans, and various species of pondweed, Pota-mogeton. These plants can colonize the gravel and sand, and when they have formed a large patch they cause a stagnant area on the downstream side. Silt settles here and accumulates rapidly if there is heavy pollution upstream. It is colonized by such plants as Nitella, the stonewort, Elodea, the Canadian pondweed, and Potamogeton crispus, the curly pondweed.

There was usually sufficient water in the Skerne to dilute the sewage it received to below the danger point, but in one summer there was a long hot dry spell as a result of which all the oxygen was used up, and the toxic products of decomposition without oxygen were liberated into the water. The Skerne itself had a thick coat of sewage fungus on its bed, and this organism extended for some distance down the Tees below the confluence of the two rivers. Its range varied widely according to the season of the year. In winter when, owing to the low temperature, the rate of decomposition of sewage is slow, it extended a long way downstream from the mouth of the Skerne, but in summer, when decomposition is more rapid, its range was less.

A green filamentous alga, Cladophora glomerata, the Blanket Weed, abounds where nutrients are plentiful, as they are below a sewage outfall where the organic matter has undergone the initial stages of decomposition. It appeared in the Tees towards the end of May and grew rapidly to form a thick carpet in the shallow water a long way down the river from Croft. Then the first flood in July would usually sweep it all away, and it would be seen no more until the following year. If it lasted long enough, it trapped a deposit of silt and enabled rooted plants to grow in places where otherwise the flow was too fast. Before the estuary was reached the algal community typical of the upper, unpolluted reaches had become re-established on the stones.

Nitrogenous compounds and other products of decomposition were brought into the Tees by the Skerne, and the calcium concentration was increased to some 30 parts per million. There was less oxygen in this stretch during the summer than there was farther upstream, and the lowest value was reached during the time when the development of Cladophora was at its height. The dense growth of this plant, respiring in the hours of darkness, used up much of the oxygen and reduced the concentration to between 50 and 60% of the saturation value. This is well above the point at which deleterious effects on fish are likely, and trout flourished in this, the last freshwater reach of the Tees, not uncommonly attaining a size of 1–1¹/₂ lb. (.45–.75 kg.). Coarse fish, chiefly dace and chub, were abundant, and fishing was a popular pastime.

Fig. 12 Longitudinal section of the Tees estuary showing the salinity at high and low tide

In the estuary, surveyed by Alexander, Southgate and Bas-sindale (1935), the most important natural phenomenon is the salinity. The fresh water tends to float on the sea-water and the result is a marked stratification. Figure 12 shows the average conditions at high tide and at low tide, but it gives rather a distorted picture because it is necessary to use such different scales. Horizontally an inch represents about three miles, but vertically it represents only about fifty feet. The surface current of fresh water draws up some water of higher salinity from below it, and to replace this there is an upstream creep of water of high salinity along the bottom. The whole mass moves up and down with the tide as shown in the figure. It is estimated that the mean time for all layers of a body of water to pass through the estuary is about six days in dry weather, decreasing to about two and a half under average winter conditions.

The estuary has been much changed by the hand of man, and it must be admitted with regret that the Tees is typical rather than otherwise of larger British estuaries. From about midway nearly to the sea there is an extensive industrial conurbation. This requires a navigable channel so that its products may be removed by sea, and accordingly the natural tendency of the river to drop silt where it is checked by its meeting with the sea is counteracted by the continual activities of dredgers. The river is a convenient main drain and, at the time of the survey, the sewage from rather more than a quarter of a million people was discharged into it untreated. So were a variety of industrial waste products, of which the most important were tar acids and cyanides. Both these decompose gradually in the water.

Much water is taken in to cool condensers and machinery, and this results in a slight rise in the temperature of the estuary. Oxygen, it need hardly be said, is not plentiful in solution in the water. The amount used up depends on the temperature and also on the salinity, being greatest at salinities of between 15 and 25 parts per thousand. The lowest concentration of dissolved oxygen recorded during the survey was 9% of saturation.

The curly pondweed, Potamogeton crispus, the starwort, Callitriche stagnalis, and the two mosses, Fontinalis antipyretica and Eurhynchium rusciforme, which are abundant throughout almost the whole length of the freshwater part of the river, penetrate a little way into the brackish water. A few seaweeds penetrate a short distance from the sea but only four extend beyond the fringe of the brackish water region. Fucus vesiculosus, one of the brown bladder wracks, extends to beyond the middle point of the estuary, growing on wharves and piles between tidemarks; and three species of filamentous green algae occur throughout the brackish region.

It is difficult to determine exactly which fish dwell permanently in the estuary, as so many of the species recorded are migrants passing through, or casual invaders, but the threespined stickleback appears to be a regular inhabitant, extending down to at least the upper reaches of the polluted part. The effect of the pollution on the fish, particularly the regular migrants, and on the lower animals is described in Chapter 14.

Reviewing the River Tees in the light of the classifications put forward at the beginning of the chapter, we find that it includes all of Carpenter’s classes, for the lowest reach, immediately above the estuary, is dominated by coarse fish. On the other hand the last two classes, numbers 4 and 5, of Butcher’s botanical classification are not represented, for the current is nowhere so sluggish that the water crowfoot ceases to be the dominant plant.

A contrast to the Tees is provided by the south country rivers rising in the chalk downs. Butcher has surveyed the plants of the Itchen, and there was a fisheries research station on the nearby Avon for several years before the war. Much of the gathering ground is chalk down. Rain falling on this sinks in and percolates relatively slowly so that it may not reach a hill-foot spring for months. The effect of heavy rain is, therefore, dissipated and it will not produce a marked flood wave as in the Tees. The other effect of the chalk is, of course, to render the water highly calcareous, and Butcher quotes a figure of 92 parts per million of calcium in the River Itchen.

Then the slope is not so steep. Moon and Green (1940) give a profile of the Avon and show that between Christchurch, which is at the mouth, and Salisbury the fall is about 150 feet in 39 miles, which is a little less than 4 feet per mile (0.075%). The river rises some 20 miles from Salisbury at an altitude of about 350 feet, so this upper reach, for which we have not been able to find accurate data, is somewhat steeper, and the figure for the whole river will be greater, but still far below that of 30 feet per mile (0.57%) for the Tees.

The springs giving rise to the Avon headwaters are usually at the foot of the chalk and often flow in wide valleys floored with gravel. Sometimes the streams have been broadened so that they flow over wide areas in which water-cress is cultivated. In dry weather the water-table often sinks below the surface of the gravel covering the impermeable stratum which is the true valley floor and the stream disappears. Sometimes, owing to the time which rain takes to seep through the chalk, there may be a long interval before the effect of a dry spell or a wet spell is manifest in the river.

Below Salisbury the Avon has been put to a variety of uses by man. One of the characteristic features is water meadows, although the method of farming under which they were engineered is now obsolete. The principle is to take water from the river in a main canal, which can be filled by the manipulation of sluices across its mouth and a barrage across the river. From this main canal the water is led into many subsidiary channels, from which it eventually runs over the land. It is gathered up in a complementary series of collecting channels and led back to the river at a lower level. The advantage of this system was that the grass could be watered at certain critical times of the year, and the farmer was independent of the capricious rainfall of this country. The significance of water meadows in the economy of the river today is that a great deal of flood-water finds its way on to them and runs back to the river slowly. This is a second reason why the effect of flooding is much less fierce in the Avon than in the Tees.

Dams and weirs are thrown across the Avon not only to deflect water for irrigation purposes but also to pen up a head of water to provide power for mills. Weirs and side channels to take excess water when the level of the river is high are usually to be found in connection with mills, and the result is that the river does not flow in a simple single channel but in a maze of anastomosing channels.

The water is rich in nutrient salts and, since there is no great scouring by floods, the rivers flowing from the chalk are heavily overgrown with a variety of aquatic plants. Butcher records that the commonest plants of the River Itchen are: Ranunculus pseudofluitans, water crowfoot, Sium erectum, the lesser water parsnip, and Apium nodiflorum, where the current is fastest; Hippuris vulgaris, the marestail and Sparganium simplex, the simple bur-reed, where it is somewhat less rapid; and Elodea canadensis, the Canadian pondweed, and Calli-triche stagnalis, starwort, in the slowest reaches. The vegetation forms such thick beds that it has to be cut and removed to let the water pass, and also to make fishing possible.

Besides the game fish, for which these rivers are famous, there is a plentiful and varied population of coarse fish.

The Avon has no torrential head-stream region nor a typical meandering lowland reach. The whole river occupies a place somewhere in between these two, but it cannot be made to fit exactly into any of the various schemes of classification. There is no steady loss of gradient from source to mouth, as there is in the theoretical river, but a mosaic of faster and slower reaches due to the various artificial obstructions which man has thrown across the river.

A third river worthy of notice is the Lark, another of those surveyed by the Ministry of Agriculture and Fisheries team in connection with pollution (Butcher, Pentelow and Woodley, 1931). It is a small river rising in the East Anglian heights and flowing in a west by north direction to join the Ouse. The water is highly calcareous like that of the River Avon.

Only a comparatively small portion was surveyed, but this stretch is fraught with interest because it illustrates yet another effect of human interference. It may be remarked here that no south country river of any size is in a ‘natural’ state, and any account of it must dwell at some length on the modifications imposed by man.

The River Lark was once navigable as far up as Bury St Edmunds, though the last few miles were kept open with difficulty because the gradient was rather steep and the amount of water available was small. Eventually river traffic ceased to pay, and the locks fell into disuse. They are now derelict and the river flows in a bed which, having been widened to take barges, is too large for the volume of water which flows down it. This disproportion is particularly marked in one stretch which is now heavily overgrown with two emergent reeds, Glyceria aquatica, reed poa, and Sparganium erectum, the branched bur-reed. These plants probably established themselves first on beds of silt in shallow water. Their gradual spread would impede the current still more and result in further deposition of silt, and the process has continued and was still active at the time when the survey was made. The dense growth of reeds tended to deflect the current to the side, where it encountered and eroded a soft sandy bank, and so made yet bigger the area in which conditions were suitable for reeds. A stage had been reached where, when the reeds began to grow up early in the summer, above them the river flooded even though there had been no unusual rainfall, and below them a miller was hard put to it to obtain sufficient head of water to drive his mill.

Beyond this stretch overgrown with reeds there is a stretch overgrown with submerged pondweeds. In parts of it the current is sufficiently strong to keep a gravel bottom clear of silt and the water crowfoot is the dominant plant. Elsewhere the current is sluggish, the bottom is muddy, and the chief plant is usually Potamogeton lucens, the shining pondweed. In some places it is replaced by a community in which Spar-ganium simplex and Sagittaria sagittifolia, the arrowhead, are the dominant species. There was no evident difference in the river to account for these two distinct communities and at first they provided something of a puzzle. But a study of the activities of the human beings interested in the river at length provided the clue, and it was noticed that the bur-reed-arrowhead community was found in those parts where weed-cutting was most frequent.

Finally the river runs through a stretch of fenland before joining the Ouse, but unfortunately the survey stopped at the head of this stretch. The fenland river offers the extreme example of the lowland course. Left to itself it would follow a tortuous channel beset with marshes and stretches of open water. Changes of course might occur and the stream might split up and lose its identity in a number of small channels as does the Euphrates today. Figure 6 shows the lower part of the Euphrates and the Tigris, and gives a good picture of a lowland course which has hardly been interfered with. In Britain no fenland river is left to itself. The fen soil is valuable for cultivating and the rivers are important as the means whereby the water pumped up out of the fens is got rid of. Vegetation, which would impede the flow of water, is removed and the channels are constantly dredged. Flood-banks are raised on either side, often at some distance from the river’s brim, so that an expansion in width is possible when the river rises above its natural banks. Water left behind by a flood stands for a long period in this land between the river and the flood-bank, and the resulting ‘washes’ are characteristic features of the fenland landscape.

Most waterways were not created by man, though he has modified some of them considerably, but there is one group that owes its existence to human effort – the canals. The Exeter ship canal was built in the sixteenth century and a few artificial waterways persisting to this day date from even earlier times. But the title of ‘father of inland navigation’ is usually bestowed upon the third Duke of Bridgwater, at whose instigation a canal from Worsley to Manchester was built and opened in 1761. The commercial possibilities of this new means of transport were quickly exploited, and in the next forty years nearly 4,000 miles of canal were put into operation. After about 1800 the activity began to wane as the challenge from rail and road became ever greater. Today some of the canals have disappeared and others, though still containing water, are no longer used.

Even a used canal is surprisingly rich in animal life and an unused one is highly productive. Canals are almost confined to the lowlands and so their water is usually hard and rich in nutrient salts. There is sufficient flow to keep these replenished; but there is no danger of excessive flow after heavy rainfall of the kind which may wash away so much plant and animal life from the canal-like stretch of a river.

Furthermore canals link up all the main river systems draining central England. Boycott (1936) writes: ‘And about the middle of last century a snail could start in the Thames at London and travel in uninterrupted water to Norfolk or Leeds or Kendal or Newtown in Montgomery or Hereford or Trowbridge, or by slipping into the upper waters of the Avon in the Vale of Pusey even to Christchurch or Southampton.’