The Action of Rivers
Water resulting from precipitation on land, represented mainly by rain water, follows
several paths before reaching oceans or seas, or before being recycled back to the
atmosphere. Therefore, the equation:
Rain water = Running water (in streams and rivers) + underground water + water
absorbed by plants + water in lakes (part of which returns to the atmosphere by
is a reasonable representation of what happens to running water. Among these different
venues for water, it is clear that running water, or that in rivers and streams, has the
greatest effects on landscape, with its strong erosional and depositional powers. A river
or a stream is defined as a form of channelized flow of water on the surface of the earth.
The erosional and depositional powers of a river depend on a number of factors, the most
important of which are the amount of available water, the topography, and the nature,
size and amount of "sediments" or load this river can carry.
II- River processes:
River processes are known as "
" (e.g. fluvial deposits for deposits related to
running waters, .
.etc.). Like winds, rivers are capable of a number of processes
: Which includes the breaking up of rocks into particles of
different sizes before their transportation over variable distances. Weathering (
by the action of running water is known as
; erosion by running water that
leads to the deepening of the river channel is termed
, whereas that
which leads to the widening of the channel is known as
: Transportation takes place by traction, saltation, suspension
Material transported by a river and deposited in a place different
from where it formed is termed
In order to better understand these processes, and the geomorphological features that they
produce, we must first examine the factors that affect stream erosion and deposition.
III- Factors affecting the erosional and depositional powers of a stream:
: The velocity of a river or a stream affects the way it flows. In
general, fluid flow is either laminar (where flow lines remain parallel) or turbulent
(where such lines cross each other, Fig. 1). Higher velocities result in turbulent flow,
whereas slow moving rivers are characterized by laminar flow. Clearly, the type of flow
has a strong effect on the interaction between water and bedrock, and hence on the
erosional and depositional powers of a stream. However, keep in mind that velocity is
not the only factor affecting the type of flow. Other factors include the
stream bottom and the
of the stream channel.