A change in the siltation pattern in Mandovi river due to combined factors of decreasing rainfall and construction of dams upstream has raised salinity levels and the sedimentation rate in its mid-estuarine zone.
A study on changing sedimentary depositional conditions within estuaries along central west coast of India by Prof G N Nayak of Goa University's marine sciences department and research students has found that a number of 'bandharas' built for water-harvesting in River Mandovi not only obstruct the flow of fresh water but also hold the sediment within.
"In normal conditions, there was a proper mixing of saline and fresh water during tidal fluctuations and this used to facilitate the growth of mangroves and different organisms in the mangrove ecosystem," Nayak said.
The sea level rise has also enhanced the increase in salinity levels in the mid-stretches of rivers along the west coast, especially Mandovi.
The depositional environment and rate of siltation has been gradually altered by the changes in the rainfall pattern from the higher averages in 1950s and 1960s to a lower trend after 1970s. "The average rainfall in some areas along the west coast was much higher in 1950s and 1960s, and the reduction means the quantum of fresh water flowing toward the sea has gone down in recent years," Nayak explained.
The decrease in fresh water flow creates a low energy environment in the river's mid-estuarine zone, triggering a rise in sea water level into the river course. "This facilitates deposition of finer sediments from the sea into the river, especially in case of Mandovi where the deposition is more in its middle stretches," Nayak said.
Many shallow coastal mud flats or sand flats have developed in the mid-stretches of the river between Ribandar and Old Goa. The higher rate of sedimentation in some of these areas are recorded since 1980 and 1987 (Mandovi), 1982 (Gokarna) and more recently in 1996 (Malvan).
According to the fourth assessment report of the Inter-Governmental Panel on Climate Change, the global average sea level rose at an average rate of 1.8 (1.3 to 2.3) mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, i.e. about 3.1 (2.4 to 3.8) mm per year. "With the increase of sea level rise and also reduction in fresh water flow due to construction of dams, salinity is bound to increase in the mid-estuaries, causing direct effect on mangroves and mango ecosystem," Nayak said.
A water resources department engineer said that the bandharas are not closed throughout the year. "They are left open during the monsoon. The 'bandharas' are constructed in upstream stretches and the last 'bandhara' is constructed at the point of tidal influence, like Ganjem, Usgao," the official explained.
The GU researchers have pointed to evidence gained from comparing the grain size and the age of sediment obtained from the mid-stretches and also after compounding the rate of siltation. "We have also analyzed the metal content in the sediment," he added.
The finer sediments are also associated with higher organic carbon. The finding of carbon pool (around 9% organic carbon) in Mandovi mid-stretches is an indicator of the changing environment. The finer sediment contained higher metal content, iron, manganese, copper, zinc.
Among them, the manganese and cobalt are more available for the organisms, which live within sediments.
" The mussels and bivalves (shelf fish) which are filter feeders take the metals and the contents get concentrated within their cells," Nayak explained. [TOI]