Page 61 - Studio International - May 1974

Man's impact on life in

inland waterways

HA Hawkes

Since his earliest days man has probably used with the natural flow in the river will affect allowed to enter the rivers. Coloured effluents
water for cleansing and processing food and the river organisms, they find it difficult to from dye manufacture and dying processes
other materials. The availability of a good predict exactly how. Further changes in ecology may have a similar effect.
source of water was paramount in the selection are likely to result from the proposed transfer Another form of physical pollution may result
of his sites for settlement. To satisfy his needs of water from one river system to another, as from the discharge of heated effluents, as from
for water man interferes both quantitatively envisaged in the scheme to supply water to the electricity generating stations. Fortunately,
and qualitatively with the hydrologic cycle. To drier and more heavily populated areas of the because vast quantities of water are needed for
ensure adequate supply he has impounded south-east from the rivers in the north-west. such purposes, power generating stations are
rivers to form reservoirs. In contrast, to Although it is appreciated that some usually situated on our larger lowland rivers.
prevent flooding of the land, he has canalized canalization of rivers may be necessary to The life in these rivers, unlike in upland
some rivers to speed the water on its way to the prevent the flooding of agricultural and some streams, is capable of withstanding a wide
sea. Much of the used water from his different residential areas, the effect on the life in the range of temperatures. It is only when the
activities, substantially changed in character, rivers is detrimental. By replacing the variety temperature in the river water reaches below
returns into rivers and lakes. of habitats in a natural river, of alternating riffles 30ºC that adverse effects result. If the river is
Although some of the quantitative changes and pools, by a uniform channel, often with an already polluted organically the effect of the
produce secondary benefits to man e.g. the inhospitable concrete bed and banks, the river raised temperatures on the deoxygenation of
amenity value of an impoundment, qualitative life is impoverished. the river may be more serious. In upland
changes in the character of the used water are The return of man's used waters to rivers and streams the cold-water species may be
rarely beneficial and mostly seriously lakes affects aquatic life to a degree, depending seriously affected by increased temperatures in
detrimental. upon the relative amounts of effluent and river stream water caused by the removal of shade.
The impoundment of a river and the water and the concentration of the impurities it Trout streams in America have been seriously
consequent flooding of the river valley to form contains. By examining the communities living affected in this way. In some cases increased
a reservoir changes the aquatic community in the river bed along a length of river, biologists temperatures in the lowland rivers have resulted
from one of flowing waters (lotic) to one of are able to detect such changes and thereby in increased productivity of fish, and in other
static waters (lentic). The over-all effect monitor the water quality and detect sources of cases during the colder winter periods the fish
is to increase the population of aquatic pollution. The bottom dwelling communities in have been attracted to the warmer waters.
organisms, including fish. Unlike in natural rivers, because they are subjected continuously Because of these potentially beneficial
lakes the water level in reservoirs alters to the water flowing over them, are better consequences of heated water discharges it
appreciably and sometimes rapidly, and since monitors of quality than infrequent water has been suggested that the term 'thermal
many of the invertebrate animals, including samples taken for chemical analysis. Having enrichment' is more appropriate than 'thermal
fish food, occupy the littoral or marginal zones, detected sources of pollution by biological pollution'.
they are adversely affected, as are the fish which monitoring, however, intensive sampling and Chemical Pollution. Natural waters as they
lay their eggs amongst the vegetation in this analysis of the water is needed to identify the arrive on the earth's surface already contain
zone. Barrages may also affect the passage of polluting substances. dissolved chemicals and in passing over and
migratory fish. Although fish passes can be Different types of effluent affect aquatic life through the soil add to the concentration of
constructed, in barrages concerned with in quite different ways; although most dissolved materials. Because different natural
hydro-electric schemes it is the downward effluents affect organisms in more than one of waters contain different concentrations of these
migration of the fish which is affected as they these ways, for convenience we shall consider substances, it is difficult to define water quality
are pulled through the turbines by the greater them separately as (i) physical; (2) chemical in purely chemical terms. Many effluents,
flow of water. and (3) biotic. however, contain such substances or other
In general the creation of impoundments is Physical Pollution. The discharge of effluents substances not occurring in nature in unusually
regarded as being beneficial to man; apart from containing suspended mineral matter, even high concentrations. In the former category
the primary purpose of supplying water or though non-putrescible and non-toxic, can effluents may contain high concentrations of
power, the amenity value may be appreciable. seriously affect the life in the river by producing salt in which case the community of organisms
Tropical impoundments such as the 175 mile turbid conditions and blanketing the stream downstream is changed to one tolerant of saline
long Lake Kariba, which was formed by the bed with silt when it settles. The discharge of conditions. If, however, the discharge is
damming of the Zambesi, has provided china-clay wastes to many rivers in Cornwall intermittent, then neither the natural
commercial fisheries with the production of has this effect. The turbidity prevents the community nor the replacement community
much needed protein although causing initial penetration of light and suppresses algal and are able to exist.
problems with weed growth. plant growths which form the basis of food The most common form of chemical pollution
Present British water supply policy, chains upon which fish eat. Although at higher is caused by chemicals which are directly toxic
formulated by the recently disbanded Water concentrations the suspended particles affect to stream life. Many metals are toxic at
Resources Board, is for communities to the fish directly by abrasion of gill membranes, concentrations lower than i mg. per litre.
abstract water from the rivers in which the fish can withstand high concentrations but are Many of the synthetic organic substances which
flow is maintained throughout the year by flow affected by the sparsity of food. Similar effects man is increasingly producing and using are
regulation using flow-regulating reservoirs. occur in coal mining areas such as South toxic at even lower concentrations. It is the
Although ecologists feel that this interference Wales where coal dust from coal washeries is occasional discharge of such toxic chemicals
247