DRAINAGE PATTERNS AND TEXTURE

DRAINAGE PATTERNS

Drainage pattern is the general arrangement of channels in a drainage basin. Drainage patterns reflect the influence of such factors as initial slope, inequalities in rock hardness, structural controls, recent diastrophism, and recent geomorphic and geologic history of the drainage basin. Looking at them in the most general manner, one can classify drainage patterns into the following categories:

Dendritic or branch-like pattern- This is probably the most common drainage pattern. This is characterised by irregular branching of tributary streams in many directions and at almost any angle usually less than 90°. Dendritic patterns develop on rocks of uniform resistance and indicate a complete lack of structural control. This pattern is more likely to be found on nearly horizontal sedimentary rocks or on areas of massive igneous rocks. They may also be seen on complex metamorphosed rocks [Figure 2.1 (a)].

Trellised or lattice-like pattern- It displays a system of sub-parallel streams, usually along the strike of the rock formations or between parallel or nearly parallel topographic features recently deposited by wind or ice [Fig. 2.1 (b)].

Radial pattern- It is usually found on the flanks of domes or volcanoes and various other types of isolated conical and sub conical hills [Fig. 2.1 (c)].

Parallel drainage pattern- It is usually found in regions of pronounced slope or structural controls that lead to regular spacing of parallel or near parallel streams [Fig. 2.1 (d)].

Rectangular drainage pattern- It has the main stream and its tributaries displaying right-angled bends. This is common in areas where joints and faults intersect at right angle. The streams are thus adjusted to the underlying structure [Fig. 2.1 (e)].

Deranged drainage pattern- It indicates a complete lack of structural or bed rock control. Here the preglacial drainage has been affected by glaciation and new drainage has not had enough time to develop any significant degree of integration. It is marked by irregular stream courses that flow into and out of lakes and swamps and have only a few short tributaries [Fig. 2.1 (f)].

Centripetal pattern shown- This is encountered locally. Here the drainage lines converge into a central depression. These are found on sinkholes, craters and other basin like depressions [Fig. 2.1 (g)].

Highly violent pattern- It is characteristic of areas of complex geology [Fig. 2.1 (h)].

Drainage Texture

An important geomorphic concept about the drainage pattern is the drainage texture by which one means relative spacing of drainage lines. Drainage texture is commonly expressed as fine, medium or coarse. Climate affects the drainage texture both directly and indirectly. The amount and type of precipitation influence directly the quantity and character of runoff. In areas where the precipitation occurs primarily in the form of thunder showers, a larger percentage of rainfall will result in runoff immediately and hence, other factors remaining the same, there will be more surface drainage lines. The climate affects the drainage texture indirectly by its control on the amount and types of vegetation present which, in turn, influences the amount and rate of surface runoff. With similar conditions of lithology and geologic structure, semiarid regions have finer drainage structure than humid regions, even though major streams may be more widely spaced in semiarid than in humid regions. It is also noticed that drainage lines are more numerous over impermeable materials than over permeable areas. The initial relief also affects drainage structure; drainage lines develop in larger number upon an irregular surface than on the one that lacks conspicuous relief.

Bad-land topography promotes fine drainage structure. Impermeable clays and shales, sparse vegetation and existence of thundershowers are responsible for very fine drainage structure. Coarse drainage structure is in particular found on sand and gravel outwash plains. Gravel plains have fewer drainage lines on them than adjacent till plains underlain by relatively impermeable clay till.

Reference- RIVER MORPHOLOGY by R. J. Garde; pp. 12-14.