# Drainage basins

## Drainage basins

The drainage basin - as stated earlier - is an example of an open system, and many of the terms above are central to it. Below is a flow diagram of the drainage basin system:

All rivers receive a water supply and the area of land this comes from is known as a drainage basin. The boundaries of the basin are known as the watershed and will usually be marked by areas of higher land.

Drainage basins have many different characteristics that influence how quickly or slowly the main river within them responds to a period of intense rainfall, these are outlined in more detail in the section relating to storm hydrographs.

#### Drainage patterns

Land is drained by rivers in a variety of ways these are exhibited as drainage patterns.

This is shown in the diagram below:

#### Drainage density

This relates to the number of streams in a particular drainage basin and can be measured by dividing total length of all streams in a basin (L) by its area (A). As a rule, the higher the drainage density (D) the more quickly water drains to a river.

D = total L/A

Characteristics of high and low-density drainage basins:

 High density (+2km per km2) Impermeable land surface, steep slopes, limited vegetation cover, limited rainfall, gentle slopes, large channel frequency (tributaries). Low density (-2km per km) Permeable rock, for example, chalk, much vegetation cover, limited rainfall, gentle sloes, lower channel frequency.

#### Stream ordering

As with drainage density, this allows for quantitative study of drainage basins and therefore comparison between different basins. A.N. Strahler, who states that there is a relationship between stream order and number, developed stream ordering. The basic idea is shown in the diagram below:

First order streams: original, single source tributaries.

Second-order streams: the joining of two first order streams.

Third order streams: the merging of two-second order streams.

Streams of different order may join together for example a second and third order stream. The order given is that of the highest order stream. An entire drainage basin is named after the highest order stream found within it, for example, a fourth order drainage basin.

Relationships are also found between:

• stream order and the number of stream in a drainage basin (negative correlation).
• stream length and stream order (positive correlation).
• area of drainage basin and stream order (positive correlation).

It is assumed that the results are plotted on semi-log paper. The ideal relationships found between stream order and number of streams is shown in the graphs below: