Seasonal dynamics of geomorphic units and denitrification rate in a large lowland tropical river

The present investigation was carried out over a 50 km reach, the Padma River of Bangladesh, downstream of the confluence of the Ganges and Brahmaputra rivers. The study area is highly dynamic, with diverse geomorphic units, with high rates of bank erosion, and sediment abstraction. Sentinel-2 imagery (2019-2020) was processed using NDVI values to classify in-channel geomorphic units (GUs). The identified GUs were categorised as islands (human-dominated and vegetated), bars (side bar and mid bar), primary channel, secondary channels, water depressions and dry channels. Monsoonal water flow was considered an essential factor for reshaping and supplying nutrients to GUs. Based on discharge and the surface area of the identified GUs in different seasons, the dynamics of nutrient-retention-relevant geomorphic units (NRRGUs) were assessed. A field study was carried out to validate the classification of GUs and to measure the potential denitrification rate (PDR) in each type. Results of the analysis showed that changes in the surface area of GUs across seasons influence the rate of PDR significantly. Along the surface area, the number of GUs impacted PDR estimation, mainly regulated by river discharge. This type of systematic investigation of the spatial and temporal distribution of geomorphological and biogeochemical process measuring and monitoring will be useful for planning river restoration and ecosystem management programs.