Discharge is the volume of flow of water per unit time, often measured in cubic metres per second (or cumecs).
In general, discharge increases with distance from the source to the mouth of a river. The extra water enters the river from tributary streams, surface runoff, throughflow and baseflow.
Sometimes there is not a steady increase in discharge with distance from the source.
The velocity of a river refers to the rate of water movement, often measured on metres per second. Mean flow velocity increases slightly with distance from the source. Although velocity appears to be higher in mountain streams (where there is a lot of white water) than in the lowlands, appearances are deceptive! Much of the water in the upper course stream, particularly close to the bed and banks, is almost stationary.
River velocity is determined by the efficiency of the river in overcoming friction with the bed and banks. Approximately 95% of a river's energy is lost to friction. Velocity increases as a river becomes more efficient in its lower course. Why?
Hydraulic radius = cross sectional area / wetted perimeter.
Hydraulic radius is a measure of the efficiency of the the river channel. The higher the hydraulic radius, the more efficient the river channel is.
The more efficient the river is, the more energy the water will have to
The gradient of the river bed is the ratio between vertical fall over horizontal distance. It is drawn as a graph of distance from source (x-axis) against height above sea level (y-axis). Bed gradient decreases gradually with distance from the source, shown as a downward concave curve.
Changes in gradient are related to changes in discharge. Discharge is higher in the lower course. Since gradient decreases as discharge increases, a river can transport the same quantity and size of sediment load in the gentler lower course as it can in the steeper upper course.
In practice, the long profile is not always a smooth downward curve.
Load is the total mass of material transported by a river. The way in which material is moved depends on its size. There are downstream changes in the amount and the mean particle size of load.
|Type of load||Type of particles||Diameter of particles||How they are transported|
|Bed load||Sand, pebbles||Over 0.1mm||Saltation and traction|
|Suspended load||Clay and silt||0.001 - 0.1 mm||Suspension|
|Dissolved load||Soluble material||-||Solution|
The competence of flowing water is the maximum size of particle that the river can transport. The capacity of a river is not the amount of water it contains (this is the discharge) but the maximum amount of load that the river can transport.
Therefore, a river's competence increases as the water velocity increases. At low velocity only lay and silt can be transported. As the velocity increases, larger particles such as sand and pebbles, can be transported. However, the relationship between velocity and the size of particles transported is not a simple positive correlation.
The mean particle size decreases with distance downstream. This is not because the competence of the river has decreased. Instead smaller particles have become a relatively more important component of the load. Why?
There are two reasons why a downstream decrease in particles does not always occur.
Many geographers have tried to classify the different streams in a drainage basin. Stream ordering can be useful for fieldwork if you want to compare sections of different streams in the same drainage basin. The most popular way of doing this is Strahler's method. A stream segment with no tributaries is designated a first-order segment. A second-order segment is formed by the joining of two first-order segments, a third-order segment by the joining of two second-order segments, and so on.
There is no increase in order when a segments of one order is joined by one of another order (such as a first-order and a second-order stream). Some researchers have suggested that as order increases by one, discharge is approximately double.
There are many possible questions that you could investigate
Before you start the investigation in full you must visit your chosen river and carry out a full pilot study. In a pilot study you check that the river is safe for fieldwork and that you can get access where you need to sample.
Investigations into channel variables are best carried out in the upper course of streams and rivers, within a few kilometres of their source. As you need to be able to measure the width and depth of the channel, make sure that water in the stream comes to no higher than you knees at the deepest point. Fast-moving water deeper than this can knock you over. It is possible to drown in 10cm of water.
If you are focussing on one stream (e.g. measuring a range of river variables with distance downstream), choose at least 3km of length. If you are carrying out work on several streams in a drainage basin (e.g. comparing first-order and second-order streams), choose at least 2 square km of the drainage basin.
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