The
conditions necessary for the existence of a pool are not alone those of
the rocky substratum of the river-bed, but more especially, the
stratifications of mud and clay left after each flooding. For instance,
an extensive bed of sand, enclosed between two layers of clay, would
remain moist, and supply well-water during the dry season; but a trivial
variation in the force and Amount of the current, in different years,
might materially affect the place and the character of the deposition of
these clay strata.
In searching the beds of partly dried-up watercourses, the fact must
never be forgotten, that it is especially in little tributaries at the
point where they fall into the main one, that most water is to be found;
and the most insignificant of these should never be overlooked. I presume
that the bar, which always accumulates in front of tributaries, and is
formed of numerous layers of alluvial deposit, parallel to the bed of the
great stream, is very likely to have one, at least, of its layers of an
impervious character. If so, the bar would shut in the wet sand of the
tributary, like a wall, and prevent it from draining itself dry.
When a river-bed has been long followed by a traveller, and a frequent
supply of water found along it, in pools or even in wells, say at every 5
or 10 miles--then, should this river-bed appear to lose itself in a
plain that is arid, there is no reason why the traveller should be
disheartened; for, on travelling further, the water will be sure to be
found again, those plains being always green and grassy where the water
in such river-beds entirely disappears.
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