is then it gradually fills up, and so on;
coming back from time to time if permitted, after a long cycle of
years, to its first course.

In evidence of the vast quantity of sediment which rivers deposit, I may
mention that the river-deposits at Calcutta are more than 400 feet in
thickness.

In addition to temporary "spates," due to heavy rain, most rivers are
fuller at one time of year than another, our rivers, for instance, in
winter, those of Switzerland, from the melting of the snow, in summer.
The Nile commences to rise towards the beginning of July; from August to
October it floods all the low lands, and early in November it sinks
again. At its greatest height the volume of water sometimes reaches
twenty times that when it is lowest, and yet perhaps not a drop of rain
may have fallen. Though we now know that this annual variation is due to
the melting of the snow and the fall of rain on the high lands of
Central Africa, still when we consider that the phenomenon has been
repeated annually for thousands of years it is impossible not to regard
it with wonder. In fact Egypt itself may be said to be the bed of the
Nile in flood time.

Some rivers, on the other hand, offer no such periodical differences.
The lower Rhone, for instance, below the junction with the Saone, is
nearly equal all through the year, and yet we know that the upper
portion is greatly derived from the melting of the Swiss snows. In this
case, however, while the Rhone itself is on this account highest in
summer and lowest in winter, the Saone, on the contrary, is swollen by
the winter's rain, and falls during the fine weather of summer. Hence
the two tend to counterbalance one another.

Periodical differences are of course comparatively easy to deal with. It
is very different with floods due to irregular rainfall. Here also,
however, the mere quantity of rain is by no means the only matter to be
considered. For instance a heavy rain in the watershed of the Seine,
unless very prolonged, causes less difference in the flow of the river,
say at Paris, than might at first have been expected, because the height
of the flood in the nearer affluents has passed down the river before
that from the more distant streams has arrived. The highest level is
reached when the rain in the districts drained by the various affluents
happens to be so timed that the different floods coincide in their
arrival at Paris.

FOOTNOTES:

[52] Darwin's _Voyage of a Natural