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Drainage Basin Characteristics

Drainage basin densityThis falls under the quantitative analysis of drainage basins. Density is the

km of channel per sq. km of basin or the amount of stream channel length per

unit area. It is calculated by measuring the total length of all the streams withinthe basin (L) and dividing by the area of the whole basin (A).

Drainage Density = L / A* units = km per km2

It is influenced by

Geology and soils: on very permeable rocks/soils drainage densities will

be low, but increases on highly permeable surfaces.

Land use: drainage density is much greater in areas of little vegetationcover. If the area is afforested, density decreases.

Time: as river patterns develop over a period of time, the number of

tributaries will decrease as will the drainage density.

Precipitation: densities as usually highest in areas where rainfall totals

and intensity are also high.

Relief: Density is usually greater on steeper slopes than on more gentle

slopes.

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Drainage Patterns

There are various ways of classifying: the most common is by the shape ofthe river networks or based on accordancy which is the flow of channels in abasin in relation to geology and topography. Opposite to accordancy is

discordancy which are patterns which pay little attention to geology ortopography. The third method is morphometric which deals with stream orderand plan. There are many different types of drainage patterns, but for the CAPEsyllabus we will look at only a few.

Dendritic

A number of streams flow directly down the slope of the original landsurface. They are known as Consequent streams (they are a consequence ofthe slope).

In a basin of similar geology, each consequent stream will receive a numberof tributaries flowing into it at an oblique angle. These are in turn fed by smallerstreams that flow into them. The resulting pattern is described as dendritic fromthe root word Dendron, Greek for tree like.

Examples include; the Caroni River Basin, the Mississippi-Missouri river andthe Shannon River in England. These river basins are referred to as beingaccordant in structure in that they follow geological or topographical patterns.The network grows by headward erosion and more channels are initiated andlengthened to make the network larger and more complex.

This network occurs on uniform rock, which lack irregularities such as a faultor a fold. But many networks are beyond geological control. If the basin is

elongated with a master stream (trunk) and the tributaries resemble a feather;then this pattern is termed Pinnate.

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Radial

This pattern is associated with domed structures. The consequent streamsflow downwards from the dome centre radiating outwards. Examples include; theGrand Etng in Grenada and rivers that flow in the English Lake District.

Where a circular arrangement of streams can be seen, the pattern isdescribed as ANNULAR. With an annular pattern, the streams follow nearlycircular or concentric parts along belts of weak rock that surround a dissecteddome, where erosion has exposed successive belts of rocks of varying degreesof erodability.

Parallel or Sub-Parallel

This occurs on sloping surfaces underlain by homogenous rock. Forexample, parallel rills, gullies or channels are often seen on freshly exposedhighway cuts or excavation sites.

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Rectangular

Here the channel system is marked by perpendicular bends. Thesegenerally result from the presence of joints and cracks in pervious rocks orfoliation in metamorphic rocks. Such structures with their crosscutting patterns,

have guided the directions of valleys. It is accordant.

Trellised

Here there is a rectangular arrangement of channels in which principaltributary streams are parallel and very long. It is common in areas where theoutcropping edges of folded sedimentary rocks, both weak and resistant, formlong nearly parallel beds.

Centripetal

In this arrangement, the streams converge towards a central depression

such as a volcanic crater/ caldera, a structured basin, a braided dome or a basincreated by the dissolution of carbonate rock.

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Discordant drainage patterns

These streams show complete lack of adjustment to underlying structureand lithologic control. Discordant patterns show no systematic relationship to thelocal rocks and their structures. Two main types of these patterns exist, the

Derangedpattern and the Super-Imposed drainage pattern.