on entering it are deflected downstream. Why?

The aggrading streams by which flood plains are constructed
gradually build their immediate banks and beds to higher and
higher levels, and therefore find it easy at times of great floods
to break their natural embankments and take new courses over the
plain. In this way they aggrade each portion of it in turn by
means of their shifting channels,

BRAIDED CHANNELS. A river actively engaged in aggrading its valley
with coarse waste builds a flood plain of comparatively steep
gradient and often flows down it in a fairly direct course and
through a network of braided channels. From time to time a channel
becomes choked with waste, and the water no longer finding room in
it breaks out and cuts and builds itself a new way which reunites
down valley with the other channels. Thus there becomes
established a network of ever-changing channels inclosing low
islands of sand and gravel.

TERRACES. While aggrading streams thus tend to shift their
channels, degrading streams, on the contrary, become more and more
deeply intrenched in their valleys. It often occurs that a stream,
after having built a flood plain, ceases to aggrade its bed
because of a lessened load or for other reasons, such as an uplift
of the region, and begins instead to degrade it. It leaves the
original flood plain out of reach of even the highest floods. When
again it reaches grade at a lower level it produces a new flood
plain by lateral erosion in the older deposits, remnants of which
stand as terraces on one or both sides of the valley. In this way
a valley may be lined with a succession of terraces at different