Stream erosion is when water
flowing through a channel has the ability to transport sediment to it from
hill slopes and erode its banks and bed to produce sediment to transport. Sediment
is the varied materials, organic and inorganic that can be carried away by
water, wind or ice. The volume of flow normally depends on the amount of
sediment being taken down stream. Eighty-five to ninety percent of the total
sediment transported to sea has been due to rivers. Seven percent is
transported by glaciers, whereas only one to two percent by groundwater and
ocean waves, and less than one percent by both wind and volcanoes. River networks
are multiple branching systems, beginning with tiny rivulets flowing downhill
during rainstorms that join into rills and gullies and eventually into creeks
and streams that continue to flow even when the rain has stopped. These are fed by water that has soaked into
the soil and that then slowly reemerges from stream banks to maintain the base
flow of the stream. Rivers primarily create the most erosion . They acquire soil and weathered
rock debris from hill slopes and valley walls, ranging in grain size from fine
mud, sand, and gravel to huge boulders. Most erosion by rivers is accomplished
during the brief intervals of high discharge and flooding. Rivers flow is not
only faster but also deeper and wider, and it is also much muddier. Due to the increase in turbulence in a deep fast
flowing stream, the river can carry up to one hundred to one thousand, times as
much water. That water carries up to one thousand times as much mud than at low
flow; this means more sediment is being moved.
There are three major modes of
sediment transport. The first mode is solution load; this is when dissolved
material is carried by a river. This often happens in areas where the geology
is limestone and is dissolved by slightly acidic water. Generally this is
higher where much of the flow is derived from groundwater pathways, which allow
water to stay in contact with rock for long periods. In the solution load, the sediment
will not settle to the bottom of a waterway during a low or no flow period. These
particles remain in permanent suspension as they are small enough to bounce off
water and stay afloat.
The next mode of sediment
transportation is suspended sediment/ suspended load. This is sediment that can
be found in nearly any body of water, carried along by the water flow. There is
an overlap between these two. Suspended sediment are any particles found in the
water column, whether the water is flowing or not. The suspended load is the
amount of sediment carried downstream within the water column by the water flow.
Suspended loads require moving water, as the water flow creates small upward
currents that keep the particles above the bed.
The size of particles that can be carried as suspended load is dependent
on the flow rate. Larger particles are more likely to fall thought the upward
currents to the bottom, unless the flow rate increases.
Bed load is the last mode of
sediment transportation. Bed load is the portion of sediment transport that
rolls, slides or bounces along the bottom of a waterway. “Bed load is
transported in two ways, traction, which is a scooting and rolling of particles
along the bed, and saltation, a bouncing like movement.” This sediment is not
truly suspended, as it sustains intermittent contact with the streambed, and
the movement is neither uniform nor continuous. Bed load occurs when the force
of the water flow is strong enough to overcome the weight and cohesion of the
sediment. While the particles are pushed along, they typically do not move as
fast as the water around them, as the flow rate is not great enough to fully
suspend them. Bed load transport can occur during low flows with smaller
particles or at high flows resulting in larger particles. Around 5-20% of total
sediment transport is a result of bed load.
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