Drainage Pattern

A drainage pattern is the arrangement formed by streams, rivers, and lakes within a particular drainage basin. These patterns are influenced by factors such as topography (slope, elevation, vegetation) and geology (rock type, rock structure, folds, faults, joints, fractures).

When tributary patterns depend on the underlying rock type and structures, the drainage is termed accordant, meaning the geology directly controls the flow of water. Over time, rivers may modify the landscape and develop a pattern that does not match the current rock structure. This is called a discordant or superimposed drainage pattern.

Key Notes: Rock structure includes joints, cracks, orientation of layers, and the shape of rocks. Geology affects rock resistance and permeability (for example, igneous rocks are highly resistant). Topography relates to land height and slope. All these factors together influence drainage patterns.

Accordant Drainage Patterns (Structure-Related)

1) Dendritic Pattern

This is the most common drainage pattern. It forms in areas with homogeneous rock (same rock type) where subsurface geology does not control stream flow. All areas weather equally, so rivers branch like the limbs of a tree without directional control from landforms. It is common on granite, gneiss, volcanic rocks, and unfolded sedimentary rocks.

2) Radial Pattern

This pattern develops when rivers flow outward from a central high point, such as a volcano or a mountain summit. Streams radiate in all directions from the central peak, for example, Nevis Peak.

3) Centripetal Pattern

This pattern is the opposite of radial. Rivers flow toward a central low-lying area, such as a caldera or basin.

4) Trellis Pattern

This drainage pattern forms in areas with folded topography, such as alternating anticlines and synclines. The main river flows along the syncline, while tributaries join it at right angles from the anticlines. It can also develop where rocks alternate between hard and soft layers; the tributaries erode the softer rocks and join the main river at right angles.

5) Rectangular Pattern

This pattern occurs in areas where joints and faults intersect at 90-degree angles, guiding river flow. It is common in limestone regions, where streams follow these fractures. The difference between rectangular and trellis patterns is spacing: rectangular patterns have widely spaced streams, while trellis patterns have narrowly spaced streams.

6) Deranged Pattern

This is an irregular and uncoordinated drainage pattern typical of regions recently covered by ice sheets. It features numerous lakes, marshes, and disconnected streams, indicating a youthful drainage stage.

Discordant Drainage Patterns (Unrelated to Current Structure)

1) Superimposed Pattern

This occurs when an original drainage pattern develops on a former rock layer, then erodes down to the underlying rock without changing its course. The drainage retains its original pattern even though the geology has changed. It commonly happens where rivers cut through resistant bedrock after uplift.

2) Antecedent Drainage

This pattern forms when rivers existed before uplift or folding and maintained their course by eroding downward as the land rose. For example, a river may cut through rising mountains but keep its original direction.

River Capture (Stream Piracy)

River capture occurs when a dominant river with a larger discharge captures the headwaters of a smaller stream, diverting its flow.

Methods of River Capture

Headward Erosion The dominant river erodes upstream and intercepts another river. Conditions that favor this include a steeper gradient, less resistant rock, and greater water volume. Key features after capture include the elbow of capture (where capture occurred), the misfit stream (abandoned section of smaller river), and the wind gap (dry valley left behind).

Lateral Erosion This occurs during the mature stage of a river when lateral erosion and valley widening become more active than vertical erosion. The larger river erodes sideways and captures smaller streams. This process is known as stream abstraction or natural selection.

Migration of a Divide This happens when a watershed or divide shifts toward the weaker river, reducing its basin and causing capture by the dominant stream.

Effects on Captured Area

• Reduction in discharge and velocity

• Less flooding, which lowers soil fertility

• Decline in freshwater supply