Tectonic, Litho-Structural, and Anthropogenic Controls on River Profile Characteristics in the Brahmani River Basin, Eastern India

The Brahmani River Basin (BRB), located within the Eastern Ghats and encompassing the Chhotanagpur Granite Gneissic Complex and Singhbhum Craton, represents a morphodynamically active system governed by structural, climatic, and anthropogenic interactions. Spanning approximately 40,000 km², the basin displays pronounced physiographic contrasts, transitioning from steep, dissected highlands in its upper reaches to low-gradient alluvial and deltaic plains along the coastal outlet. Quantitative morpho-tectonic analyses using stream-length gradient (SL), normalized channel steepness (Ksn), and χ-integral metrics revealed systematic variations that indicate active drainage rearrangement and ongoing surface uplift.

Higher χ values and steep Ksn indices within the Lawa, Sankh, South Koel, and Karo sub-basins signify tectonically rejuvenated terrains, whereas low values in the lower Brahmani and Tikera systems denote mature, equilibrium conditions. Notably, abrupt increases in Ksn and SL values immediately downstream of Rengali Dam correspond to both reservoir-induced base-level perturbations and reactivation of structural lineaments, including the Kerajang Shear Zone (550–500 Ma) and the Barakot Fault (950–700 Ma). Spatial χ-gradient reversals identify active drainage divide migration, toward the northwest in the Lawa Basin, northeast in the Sankh and Koel systems, and southeast within the South Karo Basin. The alignment of these divide shifts with major fault systems (e.g., NOBF, Akul Fault, Malaygiri Lineament) implies tectonic rejuvenation possibly linked to Himalayan compressional stresses. A total of 154 knickpoints were identified, 60% of which are structurally controlled, with 34% lithologically derived, underscoring the interplay between tectonic inheritance and rock erodibility. Basin-wide Ksn values (1.54–29.40 m⁰·⁹) confirm heterogeneity in uplift and incision dynamics, supporting relatively active tectono-geomorphic evolution across the region.

Hydromorphic assessment using the Revised Universal Soil Loss Equation (RUSLE) indicated intensified anthropogenic erosion. The mean soil erosion rate between 2015 and 2025 was estimated at 12.6 t ha⁻¹ yr⁻¹, with nearly 9% of the basin undergoing severe erosion (>40 t ha⁻¹ yr⁻¹), concentrated in structurally deformed uplands. Stable alluvial plains recorded significantly lower values (~3.2 t ha⁻¹ yr⁻¹). Under future climatic scenarios (RCP8.5), basin-wide soil erosion is projected to increase by approximately 16% between 2050 and 2070, driven by enhanced monsoonal erosivity and rapid cropland and built-up expansion.

Sediment connectivity analysis further revealed spatial variability in sediment transfer efficiency. The IC_channel index ranged from –6.97 to 3.03, highlighting steep, narrow valleys in the plateau margins as transport-active corridors. Conversely, low IC_outlet values (–8.59 to 4.25) along low-slope alluvial belts indicate sediment storage zones, reinforced by reservoir-induced trapping. A clear scale-dependency in the Sediment Delivery Ratio (SDR) was observed, with smaller sub-watersheds (>0.40) showing higher transfer efficiency compared to larger basins. Among the 280 delineated sub-watersheds, SW46 recorded the maximum SDR (≈0.47), whereas SW261 represented the lowest (~0.23).

Collectively, these results establish that sediment transfer within the Brahmani Basin is dominantly controlled by tectonic reactivation, topography, and anthropogenic pressures. Steep, structurally active uplands act as primary sediment sources, while downstream plains function as depositional sinks regulated by geomorphic and human interventions, providing a comprehensive understanding of erosion–connectivity coupling under evolving climatic and structural regimes.