Geology Reference
In-Depth Information
Unlike suspended or bed loads, the dissolved load is not determined by the speed of
flowing water. The content of the dissolved load is determined by chemical factors such
as water temperature, sediment composition, and acidity of the water.
Measuring what is transported
The maximum number of sediment particles that a stream can transport is called its
ca-
pacity.
The capacity of a stream is determined by how much water is being transported:
More water means more space for more sediments and, therefore, a higher capacity. Ca-
pacity is a measure of the volume of sediment that passes a set point in a certain
amount of time.
A stream's capacity is limited by the volume of water (the discharge) — not
by the speed of flowing water (its velocity).
The
competence
of a stream is a measure of the largest particle it can carry in its flow.
Because fast-moving water can carry larger particles than slow-moving water, the com-
petence of a stream is directly related to its speed, or velocity. The competence of a
single stream will vary with temperature, precipitation, and seasonal changes.
In fact, a stream's competence is its velocity squared, so even a small increase in velo-
city creates a large increase in competence. This fact explains why at times of flooding a
stream can go from carrying sand-sized particles to carrying trees or boulders.
Eroding a Stream Channel to Base Level
As streams flow toward the sea, they erode rocks and sediment. Three common pro-
cesses erode a stream channel:
Abrasion:
Particles carried in the stream (usually the size of sand grains or smal-
ler) scrape the bottom of the channel bed, scouring it.
Hydraulic lifting:
The intense pressure of fast-moving water removes sediments
from the stream bed.
Dissolution:
The stream flows over a rock such as limestone (see Chapter 7), and
the rock material dissolves into the water.
