Drainage system
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A drainage system is the pattern formed by the streams, rivers, and lakes in a particular watershed. They are governed by the topography of the land, whether a particular region is dominated by hard or soft rocks, and the gradient of the land.
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[edit] Types of drainage system
Drainage systems fall into one of several categories, depending on the topography and geology of the land:
[edit] Dendritic drainage system
Dendritic drainage systems are the most common form of drainage system. The term dendritic comes from the latin word "dendron", meaning tree, due to the resemblance of the system to a tree. In a dendritic system there is one main river (like the trunk of a tree), which is joined and formed by many smaller tributary rivers. Dedritic systems form in V-shaped valleys; as a result, the rock types must be impervious and non-porous.
[edit] Parallel drainage system
A parallel drainage system is a pattern of rivers caused by steep slopes with some relief. Because of the steep slopes, the streams are swift and straight, with very few tributaries, and all flow in the same direction. This system forms on uniformly sloping surfaces, for example, rivers flowing southeast from the Aberdare Mountains in Kenya.
[edit] Trellis or Rectangular drainage system
Trellis systems form in areas of alternating geology, particularly chalk and clay. The main river (the consequent) flows straight down hill. Subsequent streams develop perpendicular to the consequent along softer rock and erode it away, forming vales. The consequent river then cuts through the escarments of harder rock. Obsequent streams flow down the dip slope of the escarpments to join the subsequent streams. The River Ain with its tributaries is an example, whilst in Southern England the North Downs and the South Downs have dry valleys in a trellised pattern due to erosion after the ice age.
[edit] Annular drainage system
Annular drainage systems are arranged in ring-shaped patterns around a central basin.
[edit] Radial drainage system
Rivers radiate outwards from a central point, e.g. a volcanic cone or from a mountain range batholith
[edit] Deranged drainage system
A deranged drainage system is a drainage system in watersheds where there is no coherent pattern to the rivers and lakes. It happens in areas where there has been much geological disruption. The classic example is the Canadian Shield. During the last ice age, the topsoil was scraped off, leaving mostly bare rock. The melting of the glaciers left land with many irregularities of elevation, and a great deal of water to collect in the low points, explaining the large number of lakes which are found in Canada. The watersheds are young and are still "sorting themselves out". Eventually the system will stabilize.
