Running Water - The Changing Earth: Exploring Geology and Evolution - page 307

converted to kinetic energy , which is the energy of motion.
Much of this kinetic energy is used up in fl uid turbulence,
but some is available for erosion and transport. You already
know that erosion involves the removal of weathered mate-
rials from their source area, so the materials transported by
a stream include a load of solid particles (mud, sand, and
gravel) and a dissolved load.
Because the dissolved load of a stream is invisible, it is
commonly overlooked, but it is an important part of the to-
tal sediment load. Some of it is acquired from a stream's bed
and banks where soluble rocks such as limestone are present,
but much of it is carried into waterways by sheet fl ow and
by groundwater. A stream's solid load is made up of parti-
cles ranging from clay sized (>1/256 mm) to huge boulders,
much of it supplied by mass wasting, but some is eroded di-
rectly from a stream's bed and banks (

manifestation of abrasion (Figure 12.7b, c). These depres-
sions form where swirling currents with sand and gravel
eroded the rock.
Once materials are eroded, they are transported for
some distance from their source and eventually deposited.
The dissolved load is transported in the water itself, but the
load of solid particles moves as suspended load or bed load .
The suspended load consists of the smallest particles of
silt and clay, which are kept suspended above the channel's
bed by fl uid turbulence (

Figure 12.8). It is the suspended
load of streams and rivers that gives their water its murky
appearance.
The bed load of larger particles, mostly sand and gravel,
cannot be kept suspended by fluid turbulence so that it is
transported along the bed. However, some of the sand may
be temporarily suspended by currents that swirl the stream-
bed and lift grains into the water. The grains move for-
ward with the water, but also settle and fi nally come to rest
and then again move by the same process of intermittent
bouncing and skipping, a phenomenon known as saltation
(Figure 12.8). Particles too large to be even temporarily sus-
pended are transported by traction; that is, they simply roll
or slide along a channel's bed.

◗

Figure 12.6). The di-
rect impact of running water, hydraulic action, is suffi cient
to set particles in motion, just as the stream from a garden
hose gouges out a hole in soil.
Running water carrying sand and gravel erodes by
abrasion, as exposed rock is worn and scraped by the
impact of these particles (

◗

Figure 12.7a). Circular to
oval depressions called potholes in streambeds are one

◗

◗ Figure 12.6 Hydraulic Action

and Erosion

This stream acquires some of
its sediment by eroding its banks.

a

Image not available due to copyright restrictions