258 THE ESSEX NATURALIST
machines may work from the bank, in the case of smaller water-
courses, or from a moored pontoon in the case of wide streams.
In the case of urbanised districts, where much of the land in sur-
faced with concrete, tarmacadam, house roofs, and other im-
pervious materials, the absorption by the ground is reduced and
the run-off into streams, after rainfall or a thaw, is very quick.
The streams of north Middlesex, steeply graded in the natural
state, derive much of their flow from urbanised country, and now
have the characteristics of a Pennine stream, rising often to a
depth of three or four feet in twenty or thirty minutes after the
incidence of a storm. These streams seek to enlarge their courses,
by erosion of the banks, to the cost and dismay of riparian front-
agers who are not prepared for such happenings. In the common
interest, it is intended ultimately to provide such streams with
walls and floors in concrete, to such dimensions as will contain
adequately any anticipated flood. Where the flow is too rapid, and
now results in the underscouring of walls and bridges, weirs may
be introduced to check the flow. It is generally desirable to keep a
stream open, and not to roof it over. Otherwise, cleansing and
maintenance become difficult and expensive; and no margin
whatever is allowed for the occasional big flood which may over-
whelm such a culverted stream and do great damage by seeking
overland relief. An exception may, however, be made for an
artificial channel of a purely relieving character, for which dis-
charges have been calculated.
IRRIGATION AND CONSERVATION
It is undesirable to convey water away too swiftly, for two
reasons. Firstly, the surface of the ground will become parched,
and meadow land will deteriorate. Secondly, in the case of porous
strata such as ballast or chalk, the percolation of water downwards
will be reduced so that the yield of springs and wells farther down
the valley will diminish. It is therefore desirable to conserve all
water where that is possible.
Irrigation, as such, is not widely practised in this country, as the
mild climate and frequent rainfall render it generally unnecessary.
In those tropical countries where rainfall is inadequate to support
crops, irrigation is the most vital social service. In England, it
used to be practised more widely than at present for the flooding of
water meadows, to produce small local artificial floods. Modern
meadow practice seeks to control the level of the ground water by
keeping it at desired levels in adjacent ditches, rather than to inun-
date the actual meadow surface. The latter practice requires a high
level feed channel, and necessitates the labour of attending to
many individual sluices. It is still possible to do this in such areas
as parts of Somerset, where high level carrier streams from the