The drainage pattern of Peninsular India is a classic example of how geological structure and topography profoundly influence the course and characteristics of rivers. The Peninsular Plateau is an ancient, stable landmass, primarily composed of hard crystalline igneous and metamorphic rocks, which has undergone various tectonic events over geological time. This ancient history, coupled with its present-day relief, dictates the unique drainage patterns.

Influence of Geological Structure:

1. Ancient Crystalline Rocks: The bedrock of the Peninsular Plateau consists of hard, resistant rocks like granites, gneisses, and basalts (Deccan Traps). Rivers flowing over these rocks tend to have relatively straight courses, often following lines of weakness, faults, or joints. The hardness of the rocks also leads to incised valleys and gorges, as erosion is slow.
2. Faulting and Rifting: Some of the most distinctive features of Peninsular drainage are the westward-flowing Narmada and Tapi rivers. Their courses are a direct result of faulting and the formation of rift valleys. These rivers flow through grabens (down-faulted blocks), demonstrating strong structural control over their direction, contrary to the general eastward slope of the plateau.
3. Tilting of the Peninsular Block: The overall eastward slope of the Peninsular Plateau (except for the rift valleys) is a result of a gentle tilt of the entire block towards the east. This tilt, which occurred during the Himalayan uplift, is the primary reason why most major Peninsular rivers like the Godavari, Krishna, Kaveri, and Mahanadi flow eastward into the Bay of Bengal.
4. Deccan Traps: The vast basaltic lava flows of the Deccan Traps have created a distinct landscape. Rivers originating from the central highlands of the Deccan Plateau often exhibit a radial drainage pattern, flowing outwards in different directions from a central dome or uplifted area.
5. Escarpments: The Western Ghats, a prominent geological feature, act as a major water divide. The steep western face of the Ghats leads to short, swift, and often seasonal rivers flowing westward into the Arabian Sea, while the gentler eastern slope gives rise to longer, perennial rivers flowing eastward.

Influence of Topography:

1. Plateau Topography: The undulating plateau surface, with its varying elevations, hills, and valleys, guides the flow of rivers. Rivers generally follow the path of least resistance, conforming to the general slope of the land.
2. Western Ghats as a Water Divide: The continuous chain of the Western Ghats forms a significant physiographic barrier, forcing the major rivers to flow eastward. This creates a stark contrast between the short, high-gradient rivers on the western side and the long, low-gradient rivers on the eastern side.
3. Eastern Ghats: The discontinuous and lower Eastern Ghats allow the major eastward-flowing rivers to cut through them to reach the Bay of Bengal, forming broad deltas at their mouths.
4. River Valley Development: Over geological time, rivers have carved out their valleys, often exploiting pre-existing structural weaknesses. The maturity of these valleys, characterized by broad floodplains and deltas in the lower reaches, reflects the long erosional history of the Peninsular rivers.
5. Drainage Patterns: The combined effect of geology and topography results in various drainage patterns:
   • Dendritic Pattern: Most common, where rivers and their tributaries resemble the branches of a tree, following the general slope of the land (e.g., Godavari, Krishna).
   • Radial Pattern: Rivers flowing outwards from a central high point, such as the Amarkantak plateau (source of Narmada, Son, Mahanadi).
   • Trellis Pattern: Less common but can be observed in areas with folded or faulted structures where tributaries join the main river at right angles, following valleys and ridges.

In essence, the Peninsular Indian drainage system is a testament to the region's ancient geological stability, characterized by a dominant eastward flow due to the plateau's tilt, with notable exceptions like the Narmada and Tapi, whose courses are structurally controlled by rift valleys. The topography, particularly the Western Ghats, further accentuates these patterns, creating a distinct hydrological divide.