A discordant drainage system, also known as an antecedent drainage system, is a type of drainage pattern that does not conform to the underlying geological structure or rock layers of an area. It occurs when a river or stream continues to flow across different rock formations or structures, irrespective of their orientation or resistance to erosion. This creates a mismatch between the drainage pattern and the underlying geology. Here are some key characteristics of a discordant drainage system:
- Irregular Flow: In a discordant drainage system, the rivers or streams flow in a non-uniform manner, cutting across different rock types, fault lines, or geological structures. This results in an irregular or zigzag pattern of drainage channels.
- Independent of Rock Layers: Unlike a parallel or trellis drainage system, where the rivers flow parallel to the geological structures or rock layers, a discordant drainage system disregards the orientation of the underlying rock formations. The rivers or streams maintain their course, irrespective of the rock layers they encounter.
- Pre-existing Drainage: A discordant drainage system often develops before the uplift or deformation of the landscape. The rivers or streams were already established prior to the geological changes, and as the landform evolves, they continue to erode and maintain their original paths.
- Crosses Geological Boundaries: The rivers in a discordant drainage system can cross different types of rock formations, including hard and resistant rocks, as well as softer and more easily eroded rocks. This can lead to variations in channel depth, width, and erosional features along the course of the river.
- Geologically Controlled Features: Despite the mismatch between the drainage pattern and underlying geology, certain features of the discordant drainage system may still be influenced by geological factors. For example, waterfalls, rapids, or gorges may occur where the river encounters resistant rock formations or geological faults.
- Examples: The Nile River is a notable example of a discordant drainage system. It flows across various rock types and geological structures, disregarding the changes in the underlying geology. Other examples include the Columbia River in the United States, the Indus River in Pakistan, and the Brahmaputra River in India and Bangladesh.
Overall, a discordant drainage system showcases the resilience of rivers or streams in maintaining their course across different rock formations and geological structures. It is a result of the interaction between geological processes and the erosional power of flowing water over time.