THE CONTROL OF RIVERS 257
action taken by one or other party to improve their own lot at the
expense of parties with opposing interests. Sometimes action was
violent, with loss of property or life and limb. In the Middle Ages,
those estates and interests usually triumphed which had the back-
ing of the Church, such as milling. In later years, the navigation
and public water supply interests usually had their way. It is only
quite recently, with the recognition of the vital importance to this
country of a vigorous agriculture, that land drainage has become
an important aspect of river control. The uses, or abuses, of a river
may be summarised as:
(a) Surface drainage of land primarily for agricultural purposes, but also
of houses and streets.
(b) Water supply for riparian farm lands and stock ; and irrigation of
meadows.
(c) Public water supply.
(d) Disposal of sewage and trade effluents.
(e) Navigation.
(f) Water power for milling, and the generation of electric power.
(g) Fisheries.
I propose to refer to these several matters quite briefly, and to
try to set out the conditions which each interest seeks to have
established.
LAND DRAINAGE
In order to provide for the effective disposal of surplus water
off the surface of the land, the average modern river requires large
works of improvement at places where the channel is constricted,
shoaled-up, or unduly winding in its course; and, furthermore, it
requires regular maintenance to keep at bay the annual growth of
weed and river plants which can arrest the flow of water to a very
serious extent. This is not to advocate such a straightening and
deepening of the channel that the land is thereby entirely dried
out, or else flooding of the valley downstream is merely aggrava-
ted, and the valley upstream parches badly in hot weather. In
these matters, experience has to be brought to bear upon theory,
or else much money may be wasted fruitlessly.
Where there is great variation in seasonal flow, it is necessary to
instal sluices and weirs to hold up water for land irrigation in dry
weather, and to allow water to be released in wet weather.
Generally, land drainage requires an adequate channel, kept as
empty as is permissible, to receive, and to convey away, any
sudden flood. It will be seen at once that this may well conflict
with such interests as milling and navigation. With increase in
labour costs, it is necessary to mechanise all processes as far as
possible; and certain items of plant, such as the dragline excava-
tor, have become standardised as the most serviceable devices
for cleansing watercourses of silt, mud, and vegetation. Such