Abstract:
Drylands cover approximately 41% of the global land area and support diverse fluvial systems. Identifying the geomorphic diversity of dryland rivers and their maintenance is essential for sustaining ecosystems in arid and semi-arid regions. Furthermore, amidst climate change and the anticipated expansion of dryland areas, gaining insights into this diversity is crucial for developing adaptive and effective management strategies for dryland rivers. However, dryland rivers are often generalized, with studies focusing more on their distinct characteristics than the inherent geomorphic diversity that shapes river character and behaviour. A comprehensive understanding of the occurrence, spatial distribution, and major controls on channel morphological diversity of dryland rivers is still lacking. To address this gap, we have examined the geomorphic diversity within and across two semi-arid dryland river basins in western India: the Mahi River Basin (MRB) and the West Banas River Basin (WBRB). We employed River Styles classification for geomorphic characterization, combined with hydrological analysis, total stream power and specific stream power assessment for a more comprehensive evaluation. Hydrological analysis indicates that MRB and WBRB are monsoon-dominated rivers. MRB is a perennial dryland river with high flow permanence downstream, whereas WBRB is intermittent, with discharge decreasing downstream. Geomorphic characterization shows that MRB predominantly exhibits a confined, terrace margin controlled, meandering, gravel bed River Style. Only a small section of the estuarine zone exhibits a partly confined, terrace margin controlled, fine-grained bed River Style. Terraces impose antecedent confinement on the contemporary river processes in the MRB, limiting floodplain development. On the contrary, WBRB predominantly features laterally unconfined, continuous channel, low sinuosity, gravel-to-sand bed River Style with extensive floodplain development. The midstream section shows a partly confined, terrace margin controlled, gravel bed River Style in the pediment zone. Stream power analysis showed high stream power even in the mid-to-downstream pediment zone of both basins, primarily driven by site-specific structural controls influencing current channel processes. Field investigations indicated that erosion processes, notably plucking, predominantly shape the reaches with higher stream power. The maximum specific stream power in the pediment zone is 98 W/m² and 255 W/m² in the WBRB and MRB, respectively. Geomorphic diversity within the basin is primarily shaped by geological control in the rocky uplands, while the pediment and alluvial zones reflect a combination of geological controls and Holocene climatic imprints. Although both basins are in semi-arid regions, the observed geomorphic diversity across the basin is governed by stream power distribution patterns with underlying geological controls and valley evolution at the millennial time scale. Insights from this study can enhance ground-level river management practices by incorporating the diversity of dryland rivers and contributing to the global inventory, thereby enriching our understanding of dryland river systems.