4.1. Sturdy survival characteristics
The dominating water weeds in the VBL have various advantages over native species, including the ability to endure a wide variety of environmental conditions and a generalist distribution pattern (Daehler, 2003; Forrest Meekins and McCarthy, 2001, Jakobs et al., 2004). They have superior reproduction strategies as well as effective dispersion mechanisms (Reddy 1984, Li, 2014, Bajwa et al, 2016). Furthermore, advantageous survival characteristics such as quick growth, high phenotypic plasticity, long seed dormancy, a deep and extensive root system that absorbs the most nutrients, and low grazing pressure provide them with an edge when competing with native flora (Newsome and Noble, 1986; Rotherham, 1990; Richards et al., 2006; Di-Nino et al., 2007). Natural enemies and predators keep these invasive weeds under check in their native habitats, but they escape their natural foes in unfamiliar situations, allowing them to grow fast (Van Driesche and Bellows, 1996). Most water weeds employ several reproductive strategies and exhibit discontinuous germination, wherein seeds and other structures have varied dormancy mechanisms that prevent all new plants from sprouting at the same time. Eichhornia has developed strategies for storing excess nitrogen and phosphate in modified petioles (Penfound and Earle, 1948), allowing them to thrive when nutrients in their environment are scarce (Gossett and Norris 1971).
Eichhornia reproduces both vegetatively and sexually, and their seeds can last for at least 20 years, making it almost impossible to irradicate them from natural open systems using usual approaches (Tellez et al., 2008; Patel, 2012). Monochoria is considered the most productive of all aquatic macrophytes because they use all three possible states for their survival-their roots in sediments, stalk beneath the water, and their photosynthetic portions in the air, which reproduce primarily through seed, with tubers providing occasional new growth (Westlake, 1963). Even though Salvinia does not reproduce sexually, its rapid vegetative growth makes it a successful invader; in fact, it is one of the world's fastest-growing water weeds, capable of doubling its biomass in less than 10 days (Blackman, 1961; Abbasi and Nipaney, 1986). The ability of Limnocharis flava to produce a large number of seeds (1,000,000 seeds per plant) combined with favourable climatic conditions makes them a hazardous alien invasion in many parts of the world (Karthigeyan et al., 2004). Changes in hydrology, such as flooding, efficiently distribute Limnocharis seeds. The fruiting capsules break quickly in the water, and some survive intact for a few days, allowing the seed to disseminate from the parent plant. It is most likely the seed that will be distributed by mud attached to boots, cars, machinery, animals, and birds (Nishan and George, 2018a,b). Hydrilla can reproduce asexually as well as sexually though the asexual plant fragmentation is their more typical dispersal mode. Their submerged tubers pose a serious problem as they can lie dormant for several years, making it extremely difficult to irradicate from waterbodies. Hydrilla also has a good resilience to saltwater, and it can also produce allelopathic compounds that restrict the growth of co-existing species in their habitat (Kulshreshtha and Gopal, 1983).
4.2. Upset of the hydrography, increased stagnancy and eutrophication
A pronounced seasonality exists in the amount of freshwater reaching into the VBL, most of which occurs during the SWM when the region receives more than 70% of its annual rainfall (Fig. 2; Qasim, 2003; Jyothibabu et al., 2006; 2015; Arunpandi et al., 2021 a,b). Thus, the entire VBL has freshwater dominant hydrography during the SWM, but during the rest of the period, it has a saltwater dominance in its downstream northern and central sections (Qasim, 2003; Jyothibabu et al., 2006). Many barriers/bunds have been built in the VBL over the years to prevent salt water intrusion into the upstream interior water bodies, among which the Thannermukkom Barrage (TB) in the southern sector is a massive engineering structure (Fig. 1; Supplementary Material 3). The other two prominent barriers are at the Pathalam and Manjummel, both in the northern sector of the VBL (Fig. 1; Supplementary Material 3). It was noted across the world that the saltwater barriers have an adverse ecological impact, as they create isolated and stagnant sections of water bodies, which accumulate nutrients and other pollutants (Kumar, 2011). Over many decades, the degradation of the VBL was caused largely by the flow restrictions of these vast barrages, spillways, and numerous landfilled roads all of which, in one way or the other, eventually favoured water stagnation and eutrophication (Kannan, 1979; Gopalan et al., 1983; Balchand, 1983; WISA, 2013). Numerous non-point nutrient loadings are quite common in the VBL associated with agriculture, municipal and domestic sewages, which results in a eutrophicated water column regardless of seasons (Balachandran, et al., 2005; Jyothibabu et al., 2006; 2015; Madhu et al., 2010; Ramani et al., 2010; Martin et al., 2008; 2010; 2011; 2012), and an updated long-term trend is shown in Fig. 5a. Similarly, the concentration of the nitrate and phosphate measured from the water weed proliferation regions in the VBL is presented in Fig. 5b, which evidences the widespread eutrophication prevailing in the VBL as observed in many earlier studies (Balachandran, et al., 2005; Jyothibabu et al., 2006; 2015; Ramani et al., 2010; Martin et al., 2008; 2010; 2012). In effect, the numerous land-filled roadways that have been built up in VBL operate as coastal dams, preventing floodwaters from draining freely to the open waters and then to the SEAS. The TB in the southern sector of VBL generated a wide perennial limnohaline/limnetic region upstream. The TB notably decreased the saline nature of the VBL, especially towards the upstream areas, which is evident in the long-term salinity distribution before and after the construction of the TB (Fig. 6; Balchand, 1983). In addition, these barrages facilitated the collapse of natural ventilation and nutrient regulation by tidal flushing in the interior sectors of VBL, which had evolved over a long period (Kannan, 1979; Gopalan et al., 1983; Balchand 1983; Dinesh Kumar, 1997). The large areas of stagnant freshwater sections created by barrages favoured the proliferation of water weeds (Balchand, 1983; Unni and Nair 1995; Dinesh Kumar, 1997), where they took advantage of the eutrophic environment and posed a serious ecological threat (Balchand, 1983; Unni and Nair 1995; Menon et.al., 2000). But on the other hand, in the past, saltwater used to reach the vast areas of the VBL that are now infested with water weeds, making any water weed proliferation difficult. Restricting natural tidal ventilation and flushing enhanced nutrient and pollutant buildup, as well as siltation in the VBL (Balchand, 1983; Unni and Nair 1995; Menon et.al., 2000; WISA, 2013)
The current human-caused flow restrictions in the VBL, which make the region favourable to water weed growth, have been there for a long time, and a historical review of them is provided in Kannan (1979) and Gopalan et al (1983). In the 1930s, the Government Kerala considered mega environmental modification proposals in the Kuttanad of VBL, including (a) fast drainage of floodwater from Kuttanad into the adjacent SEAS during the Monsoon seasons, and (b) prevention of saline water intrusion into the Kuttanad during the Pre-Monsoon (March to May) to intensify paddy cultivation in the Kuttanad. After two decades of planning, these proposals were finally initiated in the 1950s, with (a) a spillway at Thottappally (30 km south of Alappuzha) to drain floodwaters from the Achancoil-Pamba river basin into the SEAS, (b) a saltwater regulator/barrage at Thanneermukkom to prevent saline water intrusion into the Kuttanad and (c) a 42 km land-filled link road between townships Alappuzha and Changanacherry (AC Road), which was built cutting across the VBL in a west (Alappuzha) and South-east-(Changanacherry) direction. The Thottappilly spillway opened in 1955, the Thannermukkom Barrier in 1974, and the AC road in the 1980s. Unfortunately, none of the above engineering structures in the VBL had the desired effect, and they all became typical ‘ecological backlashes' (Kannan, 1979; Balchand 1983; Dinesh Kumar, 1997; Sreejith, 2013) due to the outweighing negative environmental effects they impose on the environment. These include; (a) the Thottappilly spillway, which was built to prevent flooding in the upper Kuttanad, failed miserably and actually made flooding worse in those regions and also supported the proliferation of water weeds due to the stagnancy of the water (b) The Thannermukkom barrier, which prevented saline water intrusion into the paddy fields in the ‘Kayal nilangal' in the lower Kuttanad and ‘Karappadangal' in the upper Kuttanad, caused massive environmental degradation through eutrophication, a decline in fishery stocks, the spread of epidemics and the proliferation of water weeds (Kannan, 1979; Gopalan et al., 1983; Balchand, 1983; Dinesh Kumar, 1997; Sreejith, 2013) and (c) the 42 km long land-filled AC road, which was built for better transportation, failed miserably due to floodwater inundation during the peak SWM, and in recent years, a week of consistent monsoon rain has been enough to flood this road in several kilometres on various sections.
With the awful frequent floods in the region in recent years and restriction of vehicle movement due to the submergence of the land filled AC road at multiple points, the Kerala government has now adopted to construct the ‘elevated highway' at several points along this road. The long, land-filled AC road over the last several decades accumulated human settlements on both sides, increasing the direct discharge of domestic sewage into the neighbouring AC canal, which was before acted as the major waterway connecting the townships of Alappuzha and Changanachery. Currently, the AC canal is the home to large meadows of water weeds, though there is the occasional physical removal of water weeds from certain sections, which is unscientific and inadequate in offering a permanent solution (Supplementary Material 4). Similarly, several other land-filled minor highways intersect the VBL's water bodies (for eg. Ambalappuzha-Thiruvalla road), exacerbating the region's stagnancy and waterlogging concerns, and all of this has aided one way or the other the fragmentation of water bodies, eutrophication and the spread of water weeds in the VBL (Gopalan et al., 1983; Balchand, 1983; Dinesh Kumar, 1997; Revichandran et al., 2012; Sreejith, 2013; Padmakumar et al., 2019). In conclusion, neglect of Kuttanad's hydrological regime is a major contributor to the region's deteriorating environmental regime, and it is clear that in the past, far more attention was paid to socio-political interests than scientific management of the hydrological regime (Kannan, 1979; Gopalan et al., 1983; Balchand, 1983; Dinesh Kumar, 1997, Sreejith, 2013; WISA, 2013).
4.3. Agriculture, reclamation, and tourism
The Kuttanad in the VBL is known for its extensive rice/paddy fields, and its landscape comprises roughly 1100 km2, of which approximately 304 km2 is below sea level (MSSRF, 2007; Sreejith, 2013). Most of the land that is now inhabited in the Kuttanad was created by reclaiming waterlogged areas over time (Kannan, 1979, Gopakumar and Takara., 2009; MSSRF, 2007; Vallikappen, 2012; Sarath Chandran and Subrata, 2018). Its landscape consists of Kayalnilangal (8100 ha), Karinilangal (6,075 ha), and Karappadangal (42,505 ha). Kayalnilangal is situated below sea level and even though the soil is acidic here, if the saline intrusion is avoided, the area can be used for paddy cultivation twice a year. Karinilangal is waterlogged, and due to the presence of high acidity, it contributes very little to rice cultivation. Karappadam is the reclaimed land, which constitutes the North Kuttanad, Middle Kuttanad, and Upper Kuttanad with a relatively fertile area that is less affected by saline water intrusion. During the SWM, floodwaters enter Kuttanad from upstream catchments, which carry a substantial sediment load that eventually spreads across the lowland. During high floods, water overflows the bunds, roads and homes, inflicting a chaotic situation in the region. Farming is the main source of income for the people of Kuttanad, and in the lowlands, paddy cultivation predominates, whereas the bunds and reclaimed land are used to plant coconut palms, pepper, bananas, and yams. The reclamation of land for habitation and the expansion of homestead cultivation has reduced the available area for floodwater storage, causing flood levels to rise. Also, the overloading of fertilisers and pesticides eutrophicates and pollutes not only the agricultural fields but the entire VBL due to flushing and seepage. The Indo-Dutch programme n the 1980s estimated a quantity of 25,000 tonnes of fertiliser and 500 t of highly toxic pesticides in the 55,000 ha of Kuttanad paddy fields annually (Prakash Pillai, 2015).
The VBL has a long history of indiscriminate reclamation by different sections of society, which includes farmers, agriculturists, industrialists and tourism promoters, all contributing to its considerable shrinkage and present stagnancy of water bodies conducive for water weeds proliferation. A significant share of the major environmental degradation in the Kuttanad region is the result of the government's weighted strategy to establish a rice-based economy in Kerala without considering the long-term ecological backlashes of significantly altering a very sensitive ecosystem that has evolved over thousands of years. (Gopalan et al. 1983) illustrated the specifics of the large-scale reclamation of VBL for agriculture over several decades, as well as the environmental imbalances that resulted. By offering financial support in the late 1880s, the then-Royal Government of Travancore encouraged farmers to recover the open waters of the VBL to extend their agricultural fields. In that period, land reclamation and flood management in the Kuttanad were largely carried out by private farmers and during this early stages of land use pattern changes, approximately 2,226.27 hectares of the open waters of VBL were reclaimed for agriculture. Following that, reclamation efforts were suspended by a government notification in 1903, based on the assumption that rising siltation induced by reclamation would jeopardise Cochin Harbour's existence. Large-scale reclamation resumed in 1912, resulting in an additional 5,223.15 ha reclaimed by 1931. Between 1941 and 1950, the subsequent reclamation in VBL resulted in 700 ha of QST-block and 620 ha of R-block (Gopalan et al., 1983).
In addition to the large-scale reclamation for the expanse of agriculture in the Kuttanad, widespread isolated reclamation all along the VBL in the 1900s resulted in an additional area of about 1,500 ha being developed by private owners for agriculture, cottage industry, and housing along the banks of the main channels, connecting canals, and islands (Gopalan et al., 1983). Subsequently, farmers desired to transform the vast reclaimed paddy fields in the Kuttanad region for double or triple cropping of paddy, whereas previously only one crop of paddy could be grown each year. To facilitate this desire of the farmers, the Kerala government built a spillway for flood control at Thottapally in 1955 and a saltwater barrage at Thannermukkom in 1974 to prevent the intrusion of saline water into the Kuttanad region. The tremendous ecological imbalance caused by these major hydrological changes in the VBL completely upsets the natural balance and periodic flushing of the VBL. Interestingly, over time, the subsistence farming practised in Kuttanad before the 1990s had been largely replaced by large-scale commercial harvesters through increased mechanization, which helped them to reduce the labour involved in paddy cultivation, thereby obtaining an increased profit from paddy culture (Kannan, 1999).
The recent boom in tourism activities in the VBL is another factor degrading water quality and encouraging water weed proliferation. The scenic beauty of Kerala's backwaters, particularly of the Kuttanad, is popular the world over, making tourism a booming industry in the region. Tourism is primarily promoted in the region through hundreds of houseboats plying and countless tourist resorts established along the waterfront on the banks of VBL, especially in the Kuttanad region. The Kuttanad houseboat tourism began in the early 1990s, and the industry that began with manually propelled boats (powered by oars) and only one room now offers ineffable luxuries. The industry supports over 8,000 permanent jobs, excluding those associated with houseboat tourism (Michael, 2017). Nearly 1,500 houseboats are cruising Kerala's backwaters, but only 638 have legal operating licences and according to a recent study, the maximum number of houseboats that can cruise on VBL is 328 and any addition to this could endanger the wetland ecosystem (Abdulla et al., 2014).
A sewage treatment plant for houseboats plying in Kuttanad was established only in 2013, and before that, houseboats used to discharge sewage directly into open waters of VBL with no treatment (Michael, 2017). The district tourism promotion council regulates the use of the treatment facility, which is only available to houseboats with a valid licence. Other houseboats continue to pollute the VBL by discarding organic and inorganic trash directly into the water. Every day, the Kerala houseboat tourism industry is expected to dump 4.25 tonnes of garbage into the VBL and inorganic waste accounts for 1.2 tonnes of total waste deposited each day (Michael, 2017). The expansion of backwater tourism has resulted in encroachment on open waters of the VBL (MSSRF, 2007; Roopa and Vijayan, 2017). The VBL is also being encroached upon for the construction of homestays, tourist resorts, and other commercial structures to attract tourists. The encroachments are intended to maximise the waterfront for tourist resorts, which is one of Kuttanad's main ecological concerns in recent times. The invasion of the lake causes the water bodies in the wetland to shrink, exacerbating the problem of solid waste discharge into the water (MSSRF, 2007; Roopa and Vijayan, 2017). In short, while paddy agriculture was primarily responsible for the reclamation of VBL in the Kuttanad region until the 1980s, the present fall in the open waters of VBL is due to the development of tourism and its necessary amenities (MSSRF, 2007; Roopa and Vijayan, 2017). A recent study found that the reclamation of the open waters of the VBL is still a severe problem, as it observed that the water body area of 195.95 km2 in 1990 declined to 140.84 km2 by 2014 (Raju and Manasi, 2019), which is a matter of concern to immediately tackle to conserve the ecology of the VBL. Due to human intervention, the already shrunk, fragmented and stagnated water bodies of VBL are so conducive to the rapid proliferation of water weeds.
4.4. Industries, urbanization and human settlements
Several industries, including chemical, petroleum, cement/ores, paper, coir, and distillery/food drinks, line the banks of the VBL (Fig. 1). The VBL's centre and northern sections, as well as the banks of the River Periyar, are semi-urbanized areas dominated by the chemical, petroleum, cement, ores, and paper industries. There is a dense concentration of large industries on the banks of the Periyar River in the Udyogamandal area, 10 kilometres north of Kochi seaport, which is estimated to discharge more than 260 million gallons of untreated wastewater into the Periyar River every day (Priju and Narayana, 2007). Previous research indicates considerable nutrient enrichment as well as increased phytoplankton biomass production in the River Periyar as a result of these industrial activities, rising urbanisation, and human density (Gopalan et al., 1983; Joy et al., 1990). During India's pre-independence time (1940s), factories were primarily developed along river banks with little/no regard for the complexities of VBL hydrodynamics and its possible implications for the sinking and redistribution of chemical and effluent discharges. Due to a lack of technology and prohibitively expensive sewage treatment costs, effluents were finally released straight into the VBL's northern sections (Qasim, 2003; Jyothibabu et al., 2006). Ambalamugal has witnessed many fish kill events in Chitrapuzha since 1973 as a result of the harmful effects of industrial pollutants in waste discharge into open waterways. The coir and beverage industries are also located in the southern sector of the VBL, and their effluents and wastes are directly discharged into the open waters of the VBL, which has various socioeconomic implications (Gopalan et al., 1983; Dinesh Kumar, 1997; Padmakumar et al., 2002, 2019).
Urbanization and rapid human settlements in townships and cities are natural outcomes of improved transportation and living conditions, and the same is true along the banks of the VBL, where there has been a significant increase in human settlements in recent decades (MSSRF, 2007; WISA, 2013 Sreejith, 2013; Raju and Manasi, 2019). Similarly, human settlements have increased significantly along the sides of highways and bunds built, as well as along the banks of the VBL during the past several decades. As a natural outcome, more and more crisscrossing roads have been built, causing further fragmentation and stagnation of the water bodies (MSSRF, 2007; Gopakumar and Takara., 2009). Resulting from all of this, the highly fragile and vulnerable VBL is subjected to environmental stress and increased nutrient loading (Gopalan et al., 1983; Joy et al., 1990). Agricultural runoff and sewage from Alappuzha and other cities also enter the VBL, and it is estimated that the Kochi City alone produces 2,550 million litres of sewage each day, which flows untreated into the VBL. During the summer, the total dissolved solid content of water reaches 53,750 mg/l but drops to 160 mg/l during the wet season. It is observed that Kochi's present sewage treatment plants barely process water from 1% of the population (Nivya and Pieus, 2016). Even though the majority of individuals utilise a septic tank sewage system, a large number of toilets near the VBL cause direct faecal pollution. Faecal coliform levels of up to 2500 MPN/100 ml have been reported. Kochi Corporation uses the Padiyathupalam, Kalvathi, Rameswaram, Pulimuttu, and Thevara canals to discharge municipal waste containing high levels of particulate organic materials into the estuary (WISA, 2013). Phosphates, sulphides, ammonia, fluorides, heavy metals (mercury, chromium, lead, copper, zinc, and pesticides) are all present in dangerously high concentrations in these discharges (DDT, BHC, and so on) (MSSRF, 2007). Livestock farming (cattle, ducks) is a frequent practice in the increasing human settlements, particularly in the Kuttanad region, and the faecal waste discharged into the open waters of the VBL is another direct source of nutrient inputs into the system. Furthermore, as previously stated, the long, land-filled Alappuzha-Changanachery and Thakazhy-Thiruvalla roads have resulted in the accumulation of human settlements on both sides of the road, increasing the direct discharge of domestic sewage into the already fragmented and stagnant water bodies (Padmakumar et al., 2002; MSSRF, 2007; John et al., 2009).
4.5. Topography, siltation and floods
The terrain of Kerala has highland, midland, and coastal zone in an east-west direction and the Western Ghats Mountain ranges are the highland that runs parallel to India's southwest coast. Six rivers that originate from the Western Ghats flow swiftly through the midland, discharging a massive amount of freshwater and sediment into VBL, especially during the SWM. This heavy inflow of freshwater and sediment remains in the bowl-like VBL for some days before being flushed out into the adjacent SEAS. Due to the narrow sea inlets and the microtidal nature of the VBL, flushing is a relatively slow process, and as a result, heavy siltation is a severe problem in the VBL (Qasim, 1974; Gopalan et al., 1983; Balchand, 1983; Dinesh Kumar, 1997; Gopakumar and Takara., 2009; Karnan et al., 2018), the severity of which is evident in the continuous maintenance dredging operations required for the Cochin Port's ship channel, where it is estimated that a quantity of silt of 10 x 106 m3 is being removed every year (Balachandran et al., 2005; Rasheed, 1997). The total annual sediment yield from all rivers draining into the VBL is estimated to be 32 million tonnes/year, and as a result, the mean depth of the VBL has reduced from 6.7 m to 4.4 m over the last 8 decades (Padmakumar 2002; Gopakumar and Takara., 2009; Ramani et al., 2010). The VBL depth has reduced over time owing to excessive sedimentation, as evidenced by a considerable decline in its water carrying/holding capacity from 2.4 km3 in 1960 to 0.6 km3 in 2000. (Padmakumar 2002, 2019; Ramani et al., 2010). Reclamation and flow restriction through barrages and crisscrossing roads have had their share of augmenting siltation in the VBL in recent decades, and a recent study showed that if the siltation continues at its present rate, the VBL will disappear in 50 years, transforming into extensive swamps (Padmakumar 2002, 2019).
Floods are undeniably an effective way of dispersing waterweed into new areas. When floods occur, the tremendous velocity and erosive force of the water flow increases sediment transport rates and causes severe damage to aquatic plants, creating an open conducive niche for ecologically advantageous plants to thrive (Elton 1958; Friedman et al., 1996; Donaldson, 1997). Flood flow will also transport seeds and plant parts to new locations transforming previously weed-free areas into infested zones (Donaldson, 1997). Thus, flooding causes a disturbance in the existing plant community and this, along with excess nutrients brought in by flood water, will favour the extensive growth of noxious water weeds that can withstand these disturbances (Pysek and Prach 1994., Donaldson, 1997). During the flooding season, rivers drain enormous quantities of freshwater into the VBL, flooding the low-lying regions like Kuttanad. This helps water weeds disperse their propagules and seeds to adjoining canals, streams, ponds, and paddy fields, establishing their healthy populations there. In recent decades, floods have been very frequent in the VBL, which submerge large geographical areas in the VBL. The extent of flooding in the VBL is evident in Fig. 7, which shows that the entire landmasses in the VBL got inundated and appeared as a single open waterbody during the flood in 2018. Exotic weeds have the advantage of exploiting these opportunity windows, empty niches, and the ability to fluctuate resources, making them successful invaders under extreme environmental conditions such as a flood (Fleming and Dibble, 2015).
The increase in depressions and cyclones in the seas around India has a remote impact on the rainfall over the catchment regions of VBL, causing severe floods in the region, and there is a general belief that these changes are linked to the region's long-term climate change scenario (Mishra et al. 2018a; Sudheer et al. 2019). Deshpande et al. (2021) observed a rise in the intensity, frequency, and length of cyclonic storms and extremely severe cyclonic storms over the Arabian Sea in recent decades. During the recent period (2001–2019), the frequency of cyclonic storms in the Arabian Sea increased by 52%. More importantly, it was noted that the last few severe floods in Kerala in the years 2018 and 2019 was triggered by a deep depression formed in the northern Bay of Bengal during the late SWM, in combination with high air moisture content and the orographic effect of the Western Ghats Mountains. Vijayakumar et al., (2021) showed that the flood of 2019 in Kerala was the result of a mesoscale cloudburst event, a highly uncommon and never previously documented phenomenon in the Kerala region. The study suggests that if 2019 is a sign of how global warming will continue to influence this region, changes in cloud structure, as well as the frequency and nature of severe rainfall events, might represent a danger to the Western Ghats ecosystems (Vijayakumar et al., 2021). Furthermore, several dams/reservoirs are situated across the Western Ghats, which provide water for agriculture and hydroelectric power generation (Ramasamy et al., 2019) and it is interesting to see how their opening during the extreme rainfall events aggravates the flood situation in the VBL (Mishra et al., 2018a; Sudheer et al., 2019). For example, the flood of 2018 was caused by two periods of heavy rain in two weeks; the first of these two caused flooding along the banks of certain rivers, and water was released from just a few dams since the rain fell mostly over their catchment regions. Following the initial round of heavy rain, most of the reservoirs in the state were nearing capacity, and most of the land in the region had become water-saturated. As a result, when the second event began a few days later, officials were forced to open the shutters of virtually all of Kerala's main dams. The combination of this intense rainfall and the opening of the dam shutters caused catastrophic flooding in Kerala (Mishra et al., 2018b; CWC, 2018). The latest catastrophic event in this series has just happened in mid-October 2021, which was the result of two depressions forming simultaneously in the Arabian Sea and the Bay of Bengal, causing torrential rainfall and flooding in VBL (Fig. 7). Given the alarming future climate change scenario and the rapidly diminishing water holding capacity of the VBL owing to enhanced siltation (Gopalan et al., 1983; Dinesh Kumar, 1997; Gopakumar and Takara., 2009; Padmakumar 2002, 2019), we anticipate many more severe and frequent floods in the future, which may favour the further dispersal of water weeds and even a change in their current pattern of infestation.