RESERVOIRS. These may be divided into two classes, "impounding
reservoirs" and "service reservoirs," the latter being concerned
with the distribution of water (see WATER SUPPLY). Impounding to
the fact that the flow of streams and rivers varies greatly
throughout the year, it is necessary to provide works to store
water if any substantial use is to be made of the annual
discharge. Such works are known as impounding reservoirs, their
function being to store water when the stream flow is ample for
the purpose of augmenting the natural flow in dry weather. The
urgency for the construction of such reservoirs must have become
apparent in very early times in countries where the climatic
conditions were such that the streams ran dry for a portion of
the year, and records exist of one being made in Ceylon as early
as 504 B.C. Anciently reservoirs were formed by an embankment
across the valley through which a stream flowed, and were
sometimes of vast extent, the Padavil-Colan Tank in Ceylon, for
instance, having an em