DEFINITION OF LOAD

The total amount of material that is transported by

various mechanisms by a stream, glacier, the wind, waves, the

tides, or other transporting agent is referred to as the load.

Streams carry a variety of materials as they make their way to

the sea. These materials range from minute dissolved particles

and pollutants to giant boulders moved only during the most

massive floods. The bed load consists of the coarse particles

that move along or close to the bottom of the streambed. Particles

move more slowly than the stream, by rolling or sliding.

Saltation is the movement of a particle by short intermittent

jumps caused by the current lifting the particles. Bed load

typically constitutes 5–50 percent of the total load carried by

the stream, with a greater proportion carried during high-discharge

floods. The suspended load consists of the fine particles

suspended in the stream. This makes many streams

muddy, and it consists of silt and clay that moves at the same

velocity as the stream. The suspended load generally accounts

for 50–90 percent of the total load carried by the stream. The

dissolved load of a stream consists of dissolved chemicals,

such as bicarbonate, calcium, sulfate, chloride, sodium, magnesium,

and potassium. The dissolved load tends to be high

in streams fed by groundwater. Pollutants such as fertilizers

and pesticides from agriculture and industrial chemicals also

tend to be carried as dissolved load in streams.

There is a wide range in the sizes and amounts of material

that can be transported by a stream. The competence of a

stream is the size of particles a stream can transport under a

given set of hydraulic conditions, measured in the diameter of

the largest bed load. A stream’s capacity is the potential load it

can carry, measured in the amount (volume) of sediment passing

a given point in a set amount of time. The amount of material

carried by streams depends on a number of factors.

Climate studies show erosion rates are greatest in climates

between a true desert and grasslands. Topography affects

stream load, as rugged topography contributes more detritus,

and some rocks are more erodable. Human activity, such as

farming, deforestation, and urbanization, all strongly affect

erosion rates and stream transport. Deforestation and farming

greatly increase erosion rates and supply more sediment to

streams, increasing their loads. Urbanization has complex

effects, including decreased infiltration and decreased times

between rainfall events and floods, as discussed in detail below.

The amount of sediment load available to the stream is

also independent of the stream’s discharge, so different types

of stream channels develop in response to different amounts

of sediment load availability. If the sediment load is low,

streams tend to have simple channels, whereas braided

stream channels develop where the sediment load is greater

than the stream’s capacity to carry that load. If a large

amount of sediment is dumped into a stream, the stream will

respond by straightening, thus increasing the gradient and

stream velocity and increasing the stream’s ability to remove

the added sediment.

When streams enter lakes or reservoirs along their path

to the sea, the velocity of the stream will suddenly decrease.

This causes the sediment load of the stream or river to be

dropped as a delta on the lake bottom, and the stream

attempts in this way to fill the entire lake with sediment. The

stream is effectively attempting to regain its gradient by filling

the lake, then eroding the dam or ridge that created the lake

in the first place. When the water of the stream flows over the

dam, it does so without its sediment load and therefore has

greater erosive power and can erode the dam more effectively.

Glaciers transport enormous amounts of rock debris,

including some large boulders, gravel, sand, and fine silt in

their load. The glacier may carry this at its base, on its surface,

or internally. Glacial deposits are characteristically

poorly sorted or non-sorted, with large boulders next to fine

silt. Most of a glacier’s load is concentrated along its base

and sides, because in these places plucking and abrasion are

most effective. In contrast, wind can only transport relatively

small particles in its load, and windblown deposits tend to be

well sorted reflecting the strength of the wind. Currents along

the seashore and on the beach environment move huge

amounts of sediment in littoral drift, and in seasonal adjustments

to the beach profile.

See also GLACIER; RIVER SYSTEM; SAND DUNES.

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