THE CONTROL OF RIVERS
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uplands run intermediately between low level drainage rhynes,
the flow between the two systems, through and between the
meadowlands, being individually controlled.
In south-eastern England, the principal irrigation crop is water-
cress, for which raw spring water is usually desired, rather than
feed from a river which may have suffered contamination, and
alteration in temperature.
Riparian farmers rely on streams and rivers to a great extent
for watering their stock. It is only recently that piped supplies of
fresh water have been made available to farms, thereby obviating
the use, for dairy and other purposes, of water that was frequently
of a rather doubtful quality.
PUBLIC WATER SUPPLY
In the north of England, and in the impervious and mountain
areas of Wales and Scotland, valleys have often been dammed to
hold up, in artificial lakes, the very large supplies of clear, soft
water that flow off their upland catchments. As this water is in-
tended for human consumption, great care has to be taken to
eliminate contamination, and, in order to ensure purity of the
water, this has often meant the demolition of dwellings and farms
near the intended lakes, in addition to those drowned by the deep
water. The alternative provision of ideal sanitation systems is
expensive to instal and maintain, but is, I feel, socially desirable.
Frequently, however, these upland catchments are uninhabited
and open moorland. Their afforestation, to make better use of the
land economically, and to conserve water by reduction of evapora-
tion is in my view a good thing; and it is to be hoped that as time
goes on, the present accent on conifers will be shifted instead to
such hardy deciduous trees as can survive in the uplands. In all
such cases of impounding reservoirs, a certain proportion of the
original flow, referred to as compensation water, must always pass
the dams to keep an agreed minimum flow in the river.
In southern England, the porous nature of many valleys, and
the low rainfall, together, often preclude the effective damming of
a valley. Abberton Reservoir, on the Layer Brook in Essex, is for
the reception and storage of water pumped from the River Stour
and from boreholes. The reservoirs in the Lee Valley, and in
south-west Middlesex beside the Thames, are large artificial
basins into which water from those rivers is pumped for the use of
London and district. In the case of these two rivers, legal safe-
guards exist for the maintenance of sufficient residual water for
navigation, the balance being available to the Metropolitan
Water Board. So great is the demand from this Board that
restrictions frequently have to be imposed in dry weather, to
conserve the supply. In the summer of 1949 the Lee almost dried