• River mining, the extraction of sand and gravel from riverbeds, is increasing in developing countries, causing detrimental effects such as altered river morphology, reduced water levels, increased turbidity, and erosion, endangering aquatic and benthic life. Bangladesh, along with the Indian subcontinent, is particularly vulnerable to these impacts due to extensive natural aggregate mining. Urgent measures are indispensable to address these adverse consequences, emphasizing the importance of further research to assess and mitigate the future effects of river mining.

Sand mining and trading is a common sector in the construction industry. Availability and easy access to riverbeds make it popular, especially in developing countries. The total value created by the sand industry was equivalent to USD 1.71 billion in 2016 in the United States (Department of Economic and Social Affairs/United Nations Statistics Division, 2018). The natural sedimentation rate somehow meets the requirement of fine aggregates having favorable physical properties as a building material (Harrison et al., 2005), (Geology & Goldman, 1964). However, mining sand from flood plains disrupts the natural flow of water and alters the sediment's accumulation and movement patterns, which, as a result, threatens the health of the aquatic environment. In fact, floodplain mining acts as a highpowered suction pump degrading soil, water quality, and the aesthetic impact of the area (Sreebha & Padmalal, 2011a). Moreover, mining indirectly changes sediment transport rates, causes the widening and deepening of river channels, and the development of river bluffs and lumps which can exacerbate the issue (Jose et al., 2014), (Sreebha & Padmalal, 2011a). Increases in turbidity, river bank erosion, land degradation, salinity intrusion, and lowered water tables are a few of the indirect consequences of uncontrolled aggregate mining from stream beds (Piyadasa, 2011).

Considering the geology, river networks, and corresponding sedimentation, Bangladesh, the largest delta in the world, is now in a blooming phase of the construction industry. Three major rivers, the Ganges, the Brahmaputra, and the Meghna, are among the most sediment-laden rivers in the world. These rivers, with their numerous tributaries and distributaries, built up the Ganges–Brahmaputra–Meghna (GBM) Basins (Figure 1), These three rivers originate and flow through Nepal, Bhutan, Tibet region of China and India before entering Bangladesh and finally discharging into the Bay of Bengal. However, with approximately 90% of the catchment area of the total river system located outside the country, Bangladesh's situation with regard to surface water is very vulnerable. Fifty-seven rivers are common in India and Bangladesh (Islam, 1992; Milliman & Meade, 1983; Sood & Mathukumalli, 2011). Every year, the cross-boundary rivers carry a large sediment load to the Bengal Basin (M. R. Islam et al., 1999).

Figure 1. Ganges–Brahmaputra–Meghna (GBM) Basins (M. R. Islam et al., 1999).

Globally, illegal sand mining is found to be very common (Rege, 2016; Young & Griffith, 2009); particularly in developing countries where government regulations are relatively weaker (Gavriletea, 2017). For instance, in India, in the discourse of sand mining, ‘sand mafia’ is a very commonly used term (Peduzzi, 2014; Rege, 2016). In Bangladesh, the print and digital media often feature news on illegal sand mining in different river basins. However, the lack of proper statistics on sand mining in Bangladesh makes it difficult to evaluate the consequences of sand mining (Bari & Haque, 2022; Rabbani & Panday, 2022). While the detrimental effects of river sand mining are well documented globally, there is a significant knowledge gap regarding the specific consequences in Indian subcontinent, particularly vulnerable due to its dependence on these transboundary rivers and the booming construction industry. This review aims to bridge this gap by analyzing existing research on sand mining in regions with similar characteristics globally and attribute similar characteristics in the domain of Indian subcontinent, informing future research directions.

The presence of sand mining has significant effects on morphological characteristics such as channel bed degradation, erosion of riverbanks, and change in the plan form of a river (Collins & Dunne, 1990; Sear & Archer, 1998). The mechanism behind the morphology alteration is related to hydrodynamic characteristics, including velocity profile, higher order moments, and turbulent kinetic energy fluxes in the pit region that affect the morphology and bedload transport of the channel (Barman et al., 2019).

In the southern part of India, the Periyar River is of utmost significance to the economy of Kerala. The morphology of this river is similar to other perennial rivers in that region. Prolonged sand mining in the Periyar River for the industrialization and urbanization of Kerala recorded an annual bed lowering of 0.19 m/year (Figure 2). From 1975 to 1995, approximately 50% reduction in channel depth was observed. The lowering intensity increased markedly in the last 2–3 decades (Padmalal et al., 2008). Similar to the Periyar River, mining activities also disrupted the natural flow of the river Ganga by dividing it into smaller stream sections (Kamboj & Kamboj, 2019). However, limited studies lack the proper documentation and data in this region. The dredging effect, to some extent, can represent the scenario in Jamuna River (Figure 3), including the changes in the width of the river/channel, bank line shifting, channel migration tendency, and incidence map of the river/channel in the vicinity of the study reach (around Jamuna Bridge and Sirajganj Hard Point area) (Rahman et al., 2021).

Figure 2. Riverbed lowering of Periyar river from 1980 – 2000 (Modified from Padmalal et al., 2008).

Figure 3. Effect of Dredging in Jamuna River for the year 2000 to 2018 (Rahman et al., 2021).

To illustrate the challenges faced by non-perennial rivers, we examine the Kangsabati River located in West Bengal, India. This seasonal alluvial river shares characteristics with the braided rivers of Bangladesh, and both face the additional stress of sand extraction activities. The Shields parameter, which is used to calculate the initiation of motion of sediment in a fluid flow is highly affected in this river, resulting in reduced transport capacity and initiated turbulent flow in pit sites, which gradually converted into a pool (Figure 4) (Bhattacharya et al., 2019).

Figure 4. Channel geometry from Raipur to Bikrampur between 2002 and 2016 (Modified from Bhattacharya et al., 2019).

Changes in the rate of bed-material supply result in the transformation of channel geometry. Hence, depending on the changes in flow rate and supply of sediment, river channels may be evolved into several bed-form units. An imbalanced supply of deposits and transportation capacity of the stream causes the incision (downward cutting), which is basically a natural process. However, human activities, including agriculture, dam construction, sand mining, and imposition of erosion control, can accelerate the process of incision. Lowering of base levels for the tributaries due to channel incision consequently causes the destabilization of the entire watershed landscape (Lach & Wyżga, 2002; Shields et al., 1994; Wyżga, 2007). Being one of the most harmful human activities, sand mining significantly reduces the downstream flow of sediments and speeds up the incision, which can proceed up to tributaries (Ashraf et al., 2011; Harvey & Schumm, 1987). Hence, the impacts of sand mining on river geomorphology are required to be better understood (Padmalal et al., 2014).

In active channels, excavating a mining pit can alter the stream bed profile and create a knick point with a steeper gradient which attributes increased stream power, causing bed erosion. The process of such erosion is called headcutting or knick-point migration. Head-cutting transports a considerable amount of stream bed material to be deposited in the excavated areas (Figure 5). Through the increase of the channel depth, pit excavation reduces the flow velocities causing sediment deposition at the mining site. Water leaving the pit has higher flow energy and fewer sediments compared to the carrying capacity of the stream. The bedload- free water causes erosion of its bed and banks and tries to regain sediment load partially. Considering the behavior of eroding the riverbed due to higher stream power, such type of water is referred to as ‘hungry water’ (Kondolf, 1994). The effects consequent channel incision of sand mining can proceed up to the mining prohibited areas, including the site of bridge construction and other engineering structures. A bridge built over the Bharathapuzha river near the Shornur locality in southwest India collapsed, suggesting significant evidence of the destructive effect of sand mining (Padmalal et al., 2014; Kondolf, 1994).

Figure 5. Incision due to instream sand mining. (a) knick point creation; (b) The knick point migration and erosion of bed downstream for hungry water (Padmalal et al., 2014).

Considering alluvial rivers, channel narrowing was exhibited as a response to river mining downstream in Italian rivers. In Piave and Sesiarivers, the narrowing process was so intense that it altered the initial morphological conditions of the rivers from Braided to Transitional (Surian & Rinaldi, 2003). In recent decades in Italy, the alluvial channels have experienced notable configuration adjustments, including narrowing up to 80% and incisions up to 8 m-10 m. (Surian et al., 2011). From the perspective of stream volume and flow, similar to major rivers in Bangladesh, The Pearl River, the second largest amount of water in China, over the last decades, experienced significant down-cutting at Makaou hydrological station (Figure 6) (Surian et al., 2011). According to several studies, down-cut records obtained from different countries of different continents are enlisted in Table 1 and shown in Figure 7.

Table 1 . Sediment mining effects on alluvial river morphology considering channel incision.

River	Country	Year	Deepest down cut	References
Jarama	Spain	1960-1990s	4 m	(Rinaldi et al., 2005)
Po River	Italy	1880-1990s	1-6 m	(Rinaldi et al., 2005)
Manawatu River	New Zealand	1967-1976	0.25 m	(Rinaldi et al., 2005)
Wisłoka River	Poland	1950s-1990s	2.6 m	(Rinaldi et al., 2005)
Gaoyao, Pearl	China	1995	6.3 m	(Lu et al., 2007)
Makou, Pearl	China	1998	7.1 m	(Petit et al., 1996)
Sanshui, Pearl	China	2003	5.8 m	(Rinaldi et al., 2005)
Rhone River	France	1847-1952	4.5 m	(Petit et al., 1996)

Figure 6. Cross-section channel profiles in different years between 1990 and 2003 at Sanshui station (Modified from Surian et al., 2011).

Figure 7. Alluvial rivers with down-cut records across different countries of different continents.

Sand mining significantly alters channel geometry and causes incision, destabilizing watershed landscapes and impacting tributaries. This phenomenon, observed globally, has serious implications for the Indian subcontinent's dynamic river systems, such as in Bangladesh and India. For example, the collapse of a bridge over the Bharathapuzha River in India highlights the urgent need for stringent monitoring, regulation, and community engagement to manage sand mining. Comparative studies from Italy and China reveal similar issues, providing valuable insights for mitigating these effects locally. Addressing sand mining impacts requires a multifaceted approach, including adaptive infrastructure design and leveraging global research to protect riverine ecosystems and dependent communities in the Indian subcontinent.

Geomorphological changes, including alteration of bed level in rivers are also accompanied by lateral instability, which triggers bank erosion and channel migration (Rinaldi et al., 2005). In Central Italy, Southern Poland, and China, it is emphasized that instream sand mining is a prominent cause of incidents such as rapid channel incision and subsequent river bank failure (Rinaldi et al., 2005; Lu et al., 2007; Rinaldi, 2003) Along with other indirect ecological effects, this channel instability may also cause significant riparian vegetation loss and direct habitat loss for species (Newell et al., 1999; Sandecki, 1989). Notable alteration of bed-level in Arno River, Italy, was observed over the last 50 years (Figure 8). The mechanisms behind the loss of vegetation and the consequent effect on aquatic life related to the incision-induced bank failure and streambed shallowing (Figure 9), which cause the loss of hectares of fertile land, valuable timber resources, and wildlife habitats in riparian areas. Mining-induced bed degradation and other channel changes may not develop for several years until significant channel-adjustment flows occur, and adjustments may continue long after extraction has ended (Abdulazeez, 2016). Similarly, in the subcontinent, such instability can result in the loss of fertile land, timber resources, and wildlife habitats. The effects of mininginduced bed degradation and channel adjustments may persist long after extraction has ceased, necessitating proactive management and mitigation strategies.

Figure 8. Alteration of bed-level along the Arno River. (A) Location of dams, alluvial reaches, and Arno River system mining sites before 2000. (B) Lowering of bed level of the Arno River after 1845 (Rinaldi et al., 2005).

Figure 9. Diagram of channel cross sections showing (1) a typical sand-gravel bar about the low-flow channel, riparian zone, and water table, and (2) the wide shallow channel that results from unrestricted mining characterized by bank erosion, braided flow, sedimentation, and increased water temperatures (Ayyam et al., 2019).

Entirely or partially to a large extent, removing the plant and soil profile threatens biodiversity and aquatic (Newell et al., 1999; Sandecki, 1989). Moreover, sand mining and habitat destruction decrease the river's fish population (Arun, 1999; Kurup et al., 2004). Water turbidity resulting from mining deprives aquatic life of oxygen and photosynthesis (Supriharyono, 2004). High turbidity inhibits aquatic lifés feeding, breathing, and reproducing ability. Indiscriminate sand and river mining stir up clouds of fine organic and inorganic particles in the underlying waters, which are later transported by the river flows and collected in mining-free zones (Sreebha & Padmalal, 2011b). Benthic creatures and beneficial food-providing insect larvae, including mayflies, dragonflies, caddisfly, and other Diptera, also get reduced for the anthropogenic activities in mining spots. India's Achankovil river is one of the notable examples of such types of impacts (Sunilkumar, 2002). Heavy equipment for sand extraction affects soil compaction and infiltration, which can be detrimental to flora and fauna (Sreebha & Padmalal, 2011a), (Hill & Kleynhans, 1999). They generate noise pollution, disturb nesting/breeding, and reduce animal diversity (Byrnes & Hiland, 1995). The riparian vegetation that serves as a resting and breeding area for migratory birds is destroyed by indiscriminate sand mining from rivers (Sreebha & Padmalal, 2011b). Riverbank slumping, channel incision, and dropping water table harm riparian plants and fauna (Sreebha & Padmalal, 2011b). Loss of instream vegetation causes spawning abnormalities in photophilic and psammophilous fishes and impacts protein and energy flow to fish-feeding aquatic and terrestrial organisms (Cowx & Welcomme, 1998). Macroinvertebrates collapse was also observed in a study due to habitat deterioration in sand mining sites (Zou et al., 2019). Furthermore, excessively piled and discarded mining waste, organic matter, and oil leaks from machinery and vehicles used for excavation contribute to a rise in shortterm turbidity (Ashraf et al., 2011 ;Wu et al., 2007; Cao et al., 2017).

While sand mining significantly disrupts aquatic ecosystems, posing severe threats to biodiversity in the Indian subcontinent applying the global insights can help mitigate the destruction of riparian vegetation, prevent riverbank erosion, and maintain healthy aquatic ecosystems, ensuring the sustainability of these vital resources.

The natural balance between the freshwater and the saline water is disturbed by the rampant illegal sand mining and also the excess groundwater withdrawals. Sand mining lowers the groundwater levels, reduces freshwater flow into the coastal aquifers, and ultimately causes saline water intrusion (Damodaran & Balakrishnan, 2018). The incision in minor river valleys with instream mining may diminish alluvial aquifer storage by 1% - 6%, according to Lake County, California's Planning Department (County, 1992). Sand mining causes vast, deep excavations in the riverbed, lowering the groundwater table and leaving wells on the embankments (Matlock, 1965; Herrling, 1982). Wells near sand mines are noticed in the gradual lowering of the water table in India's lowlands and midlands. More than 60% of Manimala river's floodplain wells are severely water-stressed due to sand mining (Sreebha & Padmalal, 2011b). Moreover, in Sri Lanka, sand extraction tripled in the Nilwala River, causing the lower river reaches to move inland by 50 kilometers in the last two decades (Piyadasa, 2011).

Saline water intrusion in the Vietnamese Mekong Delta (VMD) is primarily driven by three human activities: riverbed incision (caused by both riverbed mining and dam construction), sea level rise, and land subsidence corroborated by experimental studies (Loc et al., 2021a). According to an investigation of hourly-to-daily hydrological data series from 11-gauge stations across the (VMD), riverbed mining can incise the channel by up to 15 cm/year and aggravate saline intrusion (Loc et al., 2021b). Eslami et al. (2019) predicted that a 2 m riverbed incision in the coastal channel might increase salinity intrusion length by 5 km and salt concentration by 1.5 Practical Salinity Unit (PSU) 20 km from the sea (Eslami et al., 2019). Based on the experimental investigation in Periyar river, the freshwater and saline water interface is demarcated (Figure 10) (Damodaran & Balakrishnan, 2018).

Figure 10. Fresh–saline groundwater interface of the study area (Damodaran & Balakrishnan, 2018).

Understanding the relationship between sand mining and the lowering of groundwater tables, as well as the intrusion of saline water, is crucial for addressing environmental and socio-economic challenges in the Indian subcontinent. By understanding and addressing these dynamics, policymakers and stakeholders in the Indian subcontinent can develop strategies to mitigate the adverse impacts of sand mining on groundwater resources and coastal ecosystems, safeguarding both environmental integrity and community well-being.

The aim of this research is to review the multidisciplinary effects of sand mining in different regions of the world and especially in the Indian subcontinent. Based on the review, the following presumptions can be drawn:

1. In recent decades, indiscriminate sand extraction from river beds has escalated to maintain the same rate of progress in the construction industry. However, in developing countries, there are no established guidelines for harvesting river sand (Alvarado-Villalon et al., 2003).

2. Excessive sand mining seeds morphological changes, including channel incisions and narrowing as well as falling groundwater levels, remarkably disrupted river flow, which leads to instability in the surrounding ecosystem and the communities depending on it.

3. Knick point migration directly related to sand mining is frequently observed in most of the significant rivers in the Indian subcontinent. Cross-sectional properties of the rivers are severely affected in the long run.

4. As a secondary effect of sand mining, disruption in standard aquatic biological processes due to increased turbidity, BOD, and the release of toxic compounds into the water, makes it harder for pelagic and benthic organisms and plants to survive.

5. Exposure to radioactive materials from extraction sites impose serious health concern depending on the availability of such substances, and aquatic life is not out of such danger.

6. In situations where local development necessitates the collection of aggregates, the challenge lies in balancing construction demands with environmental sustainability. To address this issue, one viable alternative as suggested by literature is the reuse of industrial by-products as substitutes for river sand in concrete production, specifically as aggregates. This approach not only mitigates the environmental impact associated with extensive river sand mining but also provides a sustainable solution to managing industrial waste, thereby reducing land pollution. By adopting this method, construction projects can maintain a steady supply of aggregates while contributing to a more sustainable and environmentally conscious industry(Santhosh et al., 2021).

Though the existing studies focused on the effects of sand mining from different perspectives, still in the reviewed literature, there are research gaps regarding the prolonged effect of sand mining on marine life, under-ground water table, modeling for the prediction of geomorphological changes due to mining solely and assessing the effect of sand mining on the infrastructures. Moreover, in the Indian subcontinent, the lack of enough databases for most of the rivers was a fundamental problem for monitoring the impacts of aggregate extraction. Future research should emphasize the potential gaps to augment the sand mining field and its effects.

This manuscript titled “Ecological and Geomorphic Fallout of Escalating River Mining Activities: A Review” is a review paper that provides an examination of the multiple effects of river sand extraction from a worldwide viewpoint. One key point to stress is the clear and accessible nature of the data used in this review. All the material and datasets utilized in the analysis are sourced from open-access repositories and publications, assuring their availability to the entire scientific community and interested readers. Additionally, the data utilized in this study may be publicly accessed online, further encouraging transparency, reproducibility, and research on the subject.

No funding was obtained for this study.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.