Landform evolution in the siwaliks of the gabharu drainage basin, Northeast India: A hypsometric approach

Hypsometric analysis provides important information regarding the process and stage of landform evolution. Nine fourth-order sub-basins occurring in the Siwaliks have been analysed based on hypsometric parameters. Of these, four sub-basins lie in the Middle and Lower Siwalik Groups (MLSG) whereas, three occur in the Upper Siwalik Group (USG). Two sub-basins include all the Siwalik units. The study has shown that, in terms of both stage and processes involved in landform development, there are significant differences between the sub-basins occurring in the MLSG and those located in the USG. The sub-basins occurring in the MLSG have attained equilibrium stage of landform development under the influence of dominant diffusive process while, those located in the USG have attained the monadnock stage where fluvial action has been found to be dominant. Under uniform climatic conditions in the study area, these differences in stage and process of landform evolution may be attributed to lithological contrast between the Upper Siwalik Group and the Middle and Lower Siwalik Groups considered collectively. While the USG is dominantly a conglomeratic sequence with soft sandstone and clays, the MLSG are mainly constituted of hard sandstones and are, therefore, more resistant.