Gutting the innards: how riverbed sand mining is altering channel morphology, changing ecological conditions and creating new anthropogeomorphic landforms along eastern India’s rivers

The burgeoning demand for construction materials has led to marked increase in riverbed sand mining in eastern India, particularly through mechanized means. Such mining occurs within the river channel (via dredging) and especially over the exposed sand bars during the low-flow period. This has created a new class of anthropogeomorphic landforms that have seldom been documented before. Such ‘montanogenic’ features were mapped along several rivers of the region from historical and present-day Google Earth images during the period 2016–2024 and classified as sand hollows, sand pools, sand tracks/roads and sand mounds. The identified landforms primarily vary based on the river width, and history and regularity of sand mining in the examined river reaches. Their spatial distribution, hotspots of occurrence, patterns and association were also ascertained. Such multi-temporal analysis highlights the marked growth of sand pools due to continual extraction and the prevalence of more extensive mining regimes in recent times. For example, along the Subarnarekha River, sand pools increased in number by almost five times between 2020 to 2022, while their total areal coverage rose from 7650 m² to 22477 m², almost tripling in extent. The seasonal growth of sand roads reveals the progress of the mining season, which typically occurs from late-October to early-June, encompassing the post-monsoon to summer period. High-resolution DEMs from Cartosat-1 stereo images and repeat UAV surveys were used to discern volumetric changes in mined river reaches using the DEM-of-Difference (DoD) approach and assess sediment replenishment and extraction. Generation of Relative Elevation Models (REMs) further revealed the channel morphological alterations induced by mining, while hydraulic simulations (HEC-RAS 1-D) highlighted the alterations in instream flow patterns and velocities within mining sites, in comparison to that in unaffected river reaches. The cumulative effect of such riverbed mining activity is to markedly alter the channel planform morphology. Parameters like Degraded to Active Channel Ratio (DACR), Channel Belt Width (CBW), and Erosion-Deposition Index (E-D Index) were enumerated reach-wise for the examined rivers using high-resolution Resourcesat series LISS-IV MX images to highlight the above channel changes, along with measurement of river centerline and bankline shifts induced by mining. Such channel morphological alterations also change the river’s ecological character, particularly during the peak sand mining period in January–February, when mined river stretches reported high total suspended sediment (TSS) values ranging from 150-350 mg/L, notably in excess of specified standards by India’s Central Pollution Control Board. The extensively mined Damodar River was the most affected in this regard. In contrast, the unaffected reaches of these rivers reported far lower TSS values around 50 mg/L. Sand pools had even higher turbidity levels, regularly reporting TSS values higher than 500 mg/L, denoting hostile conditions for fishes and macroinvertebrate species, and their drying out and disconnection from the river results in the death of trapped fishes. Continued sand mining also exposes coarser substrates on the riverbed, due to removal of finer sand deposits, thereby potentially altering instream habitat conditions. Repeated monitoring of such riverbed mining is thus essential for framing reach restoration guidelines.