OF THE THAMES VALLEY. 59
initial burden, which burden indeed forms practically its only
means of procuring additional debris for transport. Since we
are only concerned with the lower course of a river in this
paper—the "Plain-track" as Sir A. Geikie has termed it—we
need only investigate this process as carried on in such portions
of a stream.
Rivers in flowing over an approximately horizontal or gently
inclined surface tend to wind about from side to side in great
curves. The degree to which this winding is carried and the
magnitude of the curves increase with the volume of the river
and the nearer approximation to horizontality of the surface over
which it flows. On the convex or outer side of each bend the river
flows with increased velocity and carries with it the coarser
portions of its detritus. This degrades the bottom over which it
is pushed and gnaws into the outer bank against which it is
driven. On the inner or concave side of each bend, a process
exactly the reverse of that just described, is carried on. Here
the current is feebler, and part of the detritus which has hitherto
been pushed onward is dropped. The destruction which goes on
on the outer side is, as a rule, just counter-balanced by the
construction or shoaling up which goes on on the inner side, so
that as the outer curve is driven further and further outwards,
so in proportion the inner curve is built up and advances in the
same direction as that in which the other recedes.
Thus far we have dealt only with the lateral movements of
the stream. In the course of time the curves are slowly pushed
down the stream, so that we have a longitudinal movement in
each bank in addition to their respective movements of lateral
advance and recession. This longitudinal movement has been
aptly called the "travelling of stream curves," and this
"travelling" has a most important effect upon the degradation
of the channel in the lower part of the course of the stream.
Mention was made of the fact that on the convex side of each bend
the bottom of the channel, as well as the face of the bank, is eaten
into by the scouring action of the transported sediment. If
then the curves themselves are gradually shifting their
position down stream in addition to being accentuated, it
follows that the ultimate result will be that the whole channel is
gradually deepened and the river is slowly carried to a lower
level.