Geology Reference
In-Depth Information
usually increases downstream as smaller streams (tributaries) join the main channel (or
trunk flow), adding more water to the stream.
Carrying a heavy load
In order for streams to carry sediment particles, they must move quickly, gathering
enough energy to lift a particle into the flow. How much energy is needed depends on
the size of the particle: Larger particles require faster moving water (which has higher
energy) in order to be lifted and carried along. The particles being carried by a stream
are called its
load.
The type of load carried by a stream is determined by the size of the
sediment particle and the energy of the stream. Here, I describe the three different kinds
of load.
Suspended load
Sediments that a stream carries in its flow are called the
suspended load.
These sedi-
ments are lifted away from the bottom of the stream and suspended in the moving water.
When a stream appears “muddy,” that means the stream has a suspended load of sedi-
ment particles. Suspended loads are most often composed of the smallest particles such
as clay and silt. (See Chapter 7 for details on grain sizes, clay, and silt.)
Bed load
Sediments that are too heavy to be lifted and suspended are moved along the bottom of
the stream channel in the
bed load.
A stream must move quickly enough to move these
particles, but they are not picked up and carried in the flow. Sediments and rocks in the
bed load move along by rolling or bouncing as the water pushes them.
There are two types of bed load movement. Some sediments (usually sand-sized) move
along by
saltation
or bouncing along the bottom. They are picked up by the stream just
long enough to move a little bit forward before falling back to the bottom. The largest
sediments (rocks or boulders) move by rolling, or
traction,
along the channel bottom.
Dissolved load
Sometimes when water moves sediments, the particles break down into their basic ele-
mental ions. (I describe elements and ions in Chapter 5.) When this breakdown occurs,
the particles are not visible in the streamflow. These ions compose the
dissolved load
of
the stream.
