Drainage Pattern

A drainage pattern refers to the arrangement of streams, rivers, and lakes within a particular drainage basin. These patterns are influenced primarily by topography (such as slope and vegetation) and geology (rock type and structure, including folds, fractures, and faults).

When the pattern of tributaries is determined by the underlying rock type and geological structures, the drainage is described as accordant, meaning the flow of water is directly controlled by geology. Over time, however, rivers may erode and reshape the landscape, producing patterns that no longer correspond to the present rock structure. Such patterns are called discordant or superimposed.

Note:
Rock structure includes the presence of cracks and joints, layer orientation, and rock shape. Geology influences rock resistance (for example, igneous rocks are highly resistant) and permeability. Topography refers to land height and slope. These factors collectively determine drainage patterns.

Formation of River Patterns Based on Rock Structure (Accordant Drainage)

1) Dendritic Pattern

This is the most common drainage pattern, resembling the branches of a tree. It forms in areas where the rock is homogeneous (the same type throughout), so the underlying geology does not influence the direction of streams. Because all areas weather at the same rate, there are no small landforms to redirect flow. Dendritic patterns also occur over unconsolidated material that erodes easily in all directions. Common in granite, gneiss, volcanic rocks, and unfolded sedimentary rocks.

2) Radial Pattern

This pattern develops when streams radiate outward from a central high point such as a volcanic cone or mountain peak.

3) Centripetal Pattern

The opposite of radial drainage. Streams flow inward toward a central depression or low-lying area, such as a basin or caldera

4) Trellis Pattern

Occurs in regions with folded structures, such as alternating anticlines and synclines. The main river flows along the syncline, and tributaries enter at right angles from the anticlines. This pattern can also develop where hard and soft rock layers alternate, allowing tributaries to erode softer rock and join the main stream perpendicularly.

5) Rectangular Pattern

Develops where joints and faults intersect at approximately 90 degrees, influencing stream direction. Common in limestone regions, where water follows these fractures. Rectangular and trellis patterns differ in spacing: rectangular drainage has widely spaced streams, while trellis drainage has closely spaced streams.

6) Deranged Pattern

A disorganized network of streams, lakes, and marshes, typically found in areas recently glaciated. Drainage is poorly integrated, and watercourses often lack clear outlets.

Formation of River Patterns Unrelated to Current Structure (Discordant Drainage)

1) Superimposed Pattern

Occurs when an original drainage system developed on a former geological surface remains intact even after the land is eroded down to a different underlying rock structure. The rivers retain their original courses regardless of the current rock arrangement. This usually happens when uplift increases stream gradient, giving rivers enough erosive power to cut through resistant bedrock while maintaining their previous pattern.

2) Antecedent Pattern

Develops when rivers existed before tectonic uplift or folding. As the land rises, the river cuts downward at the same rate as the uplift, preserving its original flow direction through newly formed mountains.

River Capture

River capture occurs when a dominant river with greater discharge diverts the headwaters of a smaller river, reducing the latter’s flow.

Main Processes of Capture

Headward Erosion
The dominant river erodes upstream until it intercepts another river. Favorable conditions include a steeper gradient, less resistant rock, and greater water volume. Key resulting features include the elbow of capture (point of diversion), a misfit stream (abandoned section of the smaller river), and a wind gap (dry former river channel).

Lateral Erosion
Occurs in the mature stage of river development, when lateral erosion and valley widening dominate. A larger river gradually erodes sideways into smaller parallel streams, capturing their flow. This process is called stream abstraction or natural selection.

Migration of a Divide
Happens when the watershed shifts toward a less dominant river, causing it to lose water to a more dominant neighboring stream.

Effects on Captured Area

• Decrease in discharge and flow velocity
• Reduced flooding and soil fertility
• Decline in freshwater availability