__BASIN
NETWORKS__

Three basin properties which apply to all basins;

·
Linear properties
- one dimension

·
Areal properties
- two dimensions

·
Relief properties
- three dimensions

__Linear
Properties__

The linear parts of a river
basin are the channels themselves. Water falling within the boundaries of a
river basin eventually enters the stream channel and the stream transports the
material which the slope processes bring to the bottom of the valley. The size
of rivers and basins varies greatly, thus they are classified and ordered.

The ordering system was developed by A.N.Strahler. All the streams which flow
from a source and have no tributaries are classified as first order streams.
The confluence of two first order streams produces a second order stream, and
the confluence of two second order streams produces a third order stream etc.

The relationships between the number of streams and order is known as **Horton’s
Law of Stream Numbers**. It states that:

*‘there is a geometric ration between the number of streams of one order
and the next’
*This ratio is known as the

The bifurcation ration of large
basins is generally the average of the bifurcation rations of the stream orders
within it. Most natural stream systems have a ration of between 3 and 5.

There is also an informal relationship between stream length and order. Higher
order basins generally have longer rivers.

__Areal
Properties
__By introducing
a second dimension, the areal properties of a basin can be measured. It is possible
to delimit the area of the basin which contributes water to each stream segment.
The watershed can be traced from where the stream has its confluence with the
higher order stream along hillcrests to pass upslope of the source and return
to the junction. This line separates slopes which feed water towards the streams
from those which drain in to other streams.If the area of the 1

(i) Density

The average length of channel per unit area of the drainage basin is called the drainage density.

**Drainage
density = SL
/ SA**

where SL is the total length of streams in the basin, and SA is the total area of the whole basin.

This indicates how frequently
streams occur on the land surface. Factors affecting drainage density include
geology and density of vegetation. The vegetation density influenced drainage
density by binding the surface layer, thus preventing overland flow from concentrating
along definite lines and from eroding small rills which might become small channels.
The vegetation slows down the rate of overland flow, and stores some of the
water for short periods of time.

The effect of lithology on drainage density is marked. Permeable rocks with
a high infiltration rate reduce overland flow, and consequently drainage density
is low. Groundwater flow is important. Example; chalk areas of the South East.

Impermeable rocks with little vegetation, with heavy downpours will produce
high drainage densities. Example; Badlands of Arizona.

(ii) Shape

Shape is important. Long basins have flatter hydrographs and take longer
to achieve a throughflow of water from a rain storm. The most efficient basin
would be one in which the watershed is circular and all the water disappears
down a hole in the middle. Two measures have been devised to assess shape.

(a) Basin
circularity compares the area of the basin to the area of a circle of the same
circumference.

**Basin
circularity = 4p
SA / P ^{2}.**

where SA is the total area of the basin, and
P is the length of the basin perimeter of watershed.

The closer the number is to 1, the more like a circle the watershed is.

(b) Basin
elongation compares the longest dimension of the basin to the diameter of a
circle of the same area as the basin.

**Basin
elongation = ÖD _{l}p
¸ 4SA**

where D_{l} is the longest dimension of the
basin, and SA
is the total area of the basin. This indicated how nearly circular the area
of the basin is. The nearer 1, the greater the correspondence to a circle.

__Relief
Properties
__All the
above features have been considered to lie on a plane surface. The third dimension
introduces the concept of relief. By measuring the vertical fall from the head
of each stream segment to the point where it joins the higher order stream and
dividing the total by the number of streams of that order, it is possible to
obtain the average vertical fall. If this is plotted against the average stream
length of that order, then the average gradient is obtained.The
average gradients of streams of each order when linked together produce an average
long profile of the basin. Generally, these long profiles illustrate that the
lower order tributaries are steeper than those of the higher orders. This relationship
is well established in geomorphology.