Channel avulsion plays a critical role in shaping alluvial river systems, yet its morphometric and tectonic consequences remain underexplored in transboundary basins. This study assesses the morphometric response to a historical avulsion event in the lower Mahananda River Basin, spanning Nepal, India, and Bangladesh. Using georeferenced historical maps (1928, 1974), multi-resolution DEMs (SRTM and ALOS PALSAR), and hydrological modeling in ArcGIS Pro, we delineated three watershed units: the Old Mahananda Basin (OMB), the Current Mahananda Watershed (CMW), and the Abandoned Mahananda Watershed (AMW). A detailed morphometric comparison across linear, areal, relief, and tectonic indices reveals significant divergence post-avulsion. AMW exhibits an approximately 45% reduction in total stream length, lower drainage density (0.34 km/km²), and diminished relief energy, indicating hydromorphological degradation. In contrast, CMW maintains high fluvial activity, with a greater dissection index and slope variability. Tectonic asymmetry indicators—Asymmetry Factor (AF) and Transverse Topographic Symmetry Factor (TTSF)—suggest lateral tilting toward the east, likely influencing the channel’s relocation. Statistical analyses—including Principal Component Analysis (PCA), hierarchical clustering, and correlation heatmaps—identified drainage density, stream frequency, and hypsometric integral as key drivers of post-avulsion differentiation. Custom visualizations further confirmed distinct morphometric regimes across the three sub-watersheds. The integrated approach—merging GIS-based delineation with Python-assisted statistical modeling—offers a replicable framework for assessing avulsion-driven watershed transformation in tectonically sensitive alluvial plains. These findings contribute to a deeper understanding of geomorphic divergence and provide actionable insights for transboundary water resource planning and climate-adaptive land management.

A catastrophic Glacial Lake Outburst Flood (GLOF) occurred from the South Lhonak Glacial Lake (SLGL) in the Sikkim Himalaya between October 3 and 4, 2023, highlighting the urgent need to understand its triggering mechanisms. Given the inaccessibility of the lake region, we employed multiple state-of-the-art remote sensing approaches, including optical image interpretation, interferometric coherence and geospatial analysis, to examine all plausible triggering mechanisms while systematically eliminating non-contributing factors. Our primary objective was to evaluate the potential triggers, with particular focus on a landslide originating from the left lateral moraine, which appears to have been the direct cause of the GLOF. The estimated landslide volume was similar to 13.1 +/- 2.7 x 10(6) m(3). Subsequent analysis aimed to determine the underlying cause of this landslide. Results indicate that increased inflow-comprising both surface runoff and subsurface seepage, into the left lateral moraine significantly elevated slope saturation, thereby compromising stability and triggering the failure. Other possible contributors were also investigated but found to have negligible or no influence on the event. These included hydrostatic pressure from the adjacent North Lhonak Glacier, a potential outburst from the North Lhonak Glacial Lake (NLGL), blockage of the SLGL outlet by landslide, seismic activity, and precipitation events.

In 2000, the Government of India announced plans of eight hydro-power projects along the Nagavali river, one of the East-Indian River flows through Odisha and Andhra Pradesh before entering the Bay of Bengal. Of these, three projects have been implemented which is significantly altering the river's dynamics. This study evaluates anthropogenic impacts on fluvial processes of these sites, with particular emphasis on river avulsion, channel incision, and reservoir-induced geomorphic change. For detecting such changes, Google Earth Images of different time periods have been used and hydrological, meteorological, geologic-geomorphic, and lithological data were collected and analysed from different Govt. sources as well as from the detailed field study. At the Hathipahar Waterfall site, dam construction triggered a rapid avulsion, resulting in a westward channel shift of approximately 550 m, channel incision of similar to 45 m, and land loss of about 0.54 km(2) during a single flash-flood event, indicating severe geomorphic instability. At the Thotapalli Regulator, post-construction flood events caused substantial upstream channel widening (from similar to 500 m to similar to 1200 m) and reservoir expansion from 1.39 km(2) to 9.91 km(2) within nine years, reflecting strong backwater effects and sediment redistribution.

The Mayurakshi River in eastern India has been significantly altered by natural and anthropogenic disturbances since the early 20th century. Five distinctive phases of channel adjustment were identified from historical topographical maps and more recent satellite images spanning the periods 1922-1969, 1969-1990, 1990-2001, 2001-2010 and 2010-2023. Six parameters like the Active Channel Width Variation Index (Delta W%), Erosion/ Deposition Index (ED), Active Channel Stability Index (ACSI), Net Active Channel Change (NACC), Active Channel Formation Rate (ACFR), and Active Channel Abandonment Rate (ACAR) were used to measure these channel adjustments. Additionally, the active channel width, active bar area, and vegetated stable bars were mapped to predict channel morphological activity by computing the Geomorphic Status (GS) Index. Results reveal that construction of the Massanjore Dam and Tilpara Barrage during the 1950s and 1960s altered the Mayurakshi River's single-thread sinuous channel planform into a braiding-wandering channel. Several floods during 1990-2001 triggered changes in the channel width (Delta W%: 11-57%), erosion and deposition extents (ED: 2-84; ACFR: 2.53-12.87; NACC: 1.86-12.72), and increased the channel geomorphic status (GS: 4.22-4.66). Thereafter, from 2001 to 2023, the Mayurakshi River went through multiple phases of channel degradation (GS: 2.99-4.05) and narrowing (Delta W%: -22.4 - +5.8, ED: 1.08-0.02, NACC: 0.17 - -8.7) due to stabilization of the marginal bar area (ACSI: 89-99%; ACAR: 3.18-9.00). Principal component and cluster analyses denote ambient stream hydraulics, cross-sectional area, instream sand mining, road density, and agricultural concentration as crucial factors that govern such channel adjustment.

Accelerating global warming has led to the retreat of glaciers and the concurrent expansion of glacial lakes in the Himalayas, increasing the risk of glacial lake outburst floods (GLOFs) with potential to destroy infrastructure and lives in the downstream river valley. In the Sikkim Himalayas, the potentially hazardous Gurudongmar lake complex (GLC) consists of four lakes containing approximately 148 x 106 m3 of water with an enlargement rate of 74 +/- 3 %. Therefore, evaluating possible GLOF hazards from GLC under the current scenario is very important. We have presented in this paper an integrated and robust GLOF model that combines the physical moraine- dam breach using TELEMAC 2D and SISYPHE with 1-D inundation modelling using HEC-RAS for the multi-lake complex to estimate the peak flood (m3/s), flood depth (m), and flow velocity (m/sec) and finally assess the downstream inundation exposure. The present study also incorporates different breaching scenarios calculated based on remote sensing and field survey data of surrounding moraine-dam parameters of the GLC. In the largest- cases of GLOFs, with an 80 % overtopping dam breach, the flood peak would reach 5833.7, 7684.0, and 8882.0 m3/s for scenarios 1C, 4C, and 5C, respectively. These floods would release a total volume of 39.9, 48.5, and 59.4 x 106 m3 of water for the three scenarios, respectively. The downstream flood exposure assessment is based on different land use types and flood heights observed across nine scenarios (labelled 1A-5C) for each settlement using a 15 x 15 m fishnet method. The results for all 19 settlement sites along the main Tista River channel show a logical trend of increasing potential exposures. However, when the water volumes from GL-2 and GL-1 or GL-3 and GL-1 are combined, the risk of a potential flood rises. An increase in the potential flood depth directly results in higher exposure to infrastructure. The GLOF exposure levels for each scenario at 19 settlement sites differed. However, the Thangu Valley and Chungthang town have the highest inundation exposure due to their size and potential losses. This research article contributes to developing moraine-dam erosional breach modelling for multi-glacial lake outburst scenarios and downstream flood exposure assessment in the Hindu Kush-Karakoram- Himalayan region.

The study monitors chronological changes (1922-36 to 2023) in channel morphological status of the Mayurakshi River in Eastern India using the index of Geomorphic Status (GS). It encompasses Changes in Sedimentary Units (SU), Changes in Sediment Availability (SA), Changes in Bar Stability (BS) and Changes in Channel Flow (CF). The Index of Geomorphic Status (GS) reveals a sharp morphological degradation of the Mayurakshi River in the recent past. Therefore, a detailed checklist of channel width, bed-level, configuration dynamics and channel connectivity was prepared to identify medium- and short-term Channel Adjustment (CA). The study reveals that instream sand mining and structural interventions coupled with low discharge are the key factors hindering channel morphological recovery. Finally, a new Channel Recovery Potential Index (CRP) has been proposed by considering short-term channel adjustment, sediment connectivity, geomorphological setup and anthropogenic stresses. Preferred alternatives and optional groups assessed from the AHP-TOPSIS and AHP-VIKOR models were used to quantify the channel Recovery Potential. Outcome of the study reveals that reach 1 has more chance of channel recovery (TOPSIS: 0.77 and VIKOR: 0.00) compared to other sectors.

This study compares the hydromorphological quality of two sub-Himalayan Rivers, the Mahananda–Balason system (MBS) and the Chel–Neora system (CNS), to analyse the role of human intervention in channel hydromorphology. An indicator-based assessment, developed by Mitra et al. (Adv Space Res 71:1397–1417, 2022), is adopted here to obtain the hydromorphological quality. A hypothesis test using a two-tailed t-test and principal component analysis (PCA) is performed to identify the reaches with similar hydromorphological conditions along with aspects that needed restoration. Results show that the hydromorphological condition is highly influenced by the intensity of human alterations, and their interrelations are strongly proportionate. While the hydromorphological quality of unconfined MBS is highly degraded, that of unconfined CNS and the confined reaches of both systems have remained almost at their least disturbed condition (LAC). Intensive human interventions on MBS have impacted the groundwater table, riverine sediments, and fish habitat. Now, to maintain sustainability regarding riverine resources of CNS, close and regular monitoring must be initiated, and this methodology can serve that purpose. Being dependent on monitoring the ongoing interventions and the channel's physical forms, assessment protocols like this can serve the purpose of rapid hydromorphological assessment for the river systems of data-scarce regions.

The Sikkim Himalaya, similar to other mountain regions, has lost considerable ice cover over the years owing to the changing climatic factors leading to enlargement of glacier-fed lakes, and thus posing a potential threat to downstream communities in the mountain and Tarai (foothills) region in case of breach anytime in the future. The Chhombo Chhu Watershed (CCW) of the Tista Basin in the Sikkim Himalaya, located between the Greater Himalayan Range and the Tethyan Sedimentary Sequence, is the storehouse of number of glacial lakes with large areas and volumes. In this study, we mapped the glacial lakes' changes between 1975–2018 and assessed their dynamics based on manual analysis of optical satellite images using KeyHole-9 Hexagon (∼4 m), Landsat Series (∼15-30 m), and Sentinel 2A-MSI (∼10-20 m) imagery and verified during field surveys. The results show that the number of lakes has increased from 62 to 98, and its total area expanded significantly by 34.6 ± 5.4%, i.e., from 8.5 ± 0.2 km2 in 1975 to 11.4 ± 0.6 km2 by 2018, at an expansion rate of 0.8 ± 0.1% a–1. Lake outburst susceptibility result reveals that a total of twenty-seven potentially dangerous glacial lakes exist in the watershed; 5 have a status of ‘high’ outburst probability, 17 ‘medium’ and 5 ‘low’. The majority of the proglacial lakes in the watershed have significantly enlarged due to the faster melting and calving processes as a result of accelerating increasing long term average annual trend of temperature (+0.283° Ca–1; 95% confidence level) and homogeneous or slightly declining precipitation.

An evaluation of the hydromorphological condition of the Mahananda-Balason River system of the sub-Himalayan foothills, West Bengal, India was attempted using a multiparameter-based Hydromorphological Quality Index (HQI). After segmenting these rivers based on continuity, bed material and channel planform, a total of 18 indicators, divided into 3 subgroups Continuity (C), Planform (P), and Floodplain Morphology (FM), were quantified reach-wise and scaled upon the level of alteration (1 for highly altered and 5 for no alteration). The derived overall HQI (3.6) exhibited a moderate hydromorphological quality of the system, however, significant differences between the HQIs of confined and unconfined reaches were witnessed. Students 't-test and Multiple Correspondence Analysis both portrayed vast dissimilarities among the confined and unconfined reaches and the clustering was depending on their confinement. The deviations measured from the system's mean and least altered conditions portrayed that the confined reaches with lesser human interventions were in comparatively more pristine hydromorphological conditions. Conversely, unconfined reaches showed moderate to very poor hydromorphological conditions chiefly due to intense human-induced alterations regarding urbanization, embanking and sediment extraction. Restorations on these aspects should initiate with immediate effect to avoid a shortage of riverine resources such as fluvial sediment, fish and groundwater. Overall, this methodology was found suitable for continuous monitoring of the river sys-tems along with the precise identification of areas and aspects to be restored for upgrading the hydromorphological quality. More testing of this methodology would eventually help in validating the hydromorphological quality assessment protocol for Indian rivers.(c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

The ongoing retreat of glaciers in the Sikkim Himalaya as a result of climate change (Racoviteanu et al., 2015; Chowdhury et al., 2021) has far-reaching implications for the development and expansion of glacial lakes. Gurudongmar lake complex (GLC) represents a typical paternoster lake system, resembling a series of glacial lakes connected by single or braided streams with a surface or subsurface drainage system. A detailed study of the GLC evolution and outburst susceptibility assessment is required. Glacial lake volume estimation and lake outburst susceptibility assessment were carried out to reveal different characteristics for all four lakes (GL-1, GL-2, GL-3, and GL-4) from the lake complex. Each of these lakes has a moderate to very high potential to outburst. As the dam of GL-1 provides no retention capacity, there is a very high potential of a combined effect with the sudden failure of the moraine-dams of GL-2 or GL-3 located upstream. Temporal analysis of GLC using optical remote sensing data and in-field investigations revealed a rapidly increasing total lake area by ~74 ± 3%, with an expansion rate of +0.03 ± 0.002 km2 a–1 between 1962 and 2018 due to climate change and ongoing glacier retreat. The overall lake area expansion rates are dependent on climate-driven factors, and constantly increasing average air temperature is responsible for the enlargement of the lake areas. Simultaneously, changes in GLC expansion velocity are driven by changes in the total amount of precipitation. The deficit in precipitation probably triggered the initial higher rate from 1962 to 1988 during the winter and spring seasons. The post-1990s positive anomaly in precipitation might have reduced the rate of the glacial lake area expansion considerably.

Debris flow is one of the most dangerous natural processes in mountain regions and it occur in a wide variety of environments throughout the world. In the Italian Alps, some tens of thousands of damaging debris flow and, in general, torrential floods associated to intense sediment transport in secondary catchments have been documented in the last 300 years. These have caused socio-economic damage, damage to anthropogenic structures or infrastructures and in many cases casualties. Often, in the same basins, the occurrence of debris-flow processes recurs many years later. Prediction can often be spatial and based on the magnitude of the largest known process, while the temporal forecast is the most uncertain. It is also possible to increase the resilience of the population and of the territory. The present study aims at investigating different levels of debris-flow hazard in urban areas on Alpine alluvial fans and proposes a strategy for debris-flow prevention based on historical research and on a simplified analytical approach, methods that also involve relatively low costs. For such analysis, Ischiator stream catchment (ca. 20 km2) and its alluvial fan (NW Italy) were selected. This area was partly affected by historical torrential flood associated to intense sediment transport and debris-flow processes. Present-day instability conditions along the slope and the stream network were detected and synthesized through surveys and aerial photo interpretation integrated by satellite images (period 1954–2021). An estimation of the potential amount of moving detritus, referred to as debris flow, was carried out regarding the June 1957 debris-flow event, based on the predictive models. The individual hazard index value was estimated based on different methods. The results indicate that 56% of the area is exposed to flood associated to intense sediment transport hazard, which fluctuates from high to very high levels; such results are supported by debris-flow historical records. Since today almost half of the settlement (Bagni di Vinadio) is located on potentially risk-exposed areas, the urban evolution policy adopted after the 1957 event failed to manage the risk connection to debris-flow activity.