THE ecosystem of the Indus delta is under threat owing to a drastic fall in river flows below the Kotri barrage.
As a result, the mangrove population in tidal floodplains and fish and shrimp production have decreased, agricultural land and riverine forestry have dwindled, surface and subsurface seawater intrusion has increased, soil has become more saline, surface and groundwater bodies stand polluted and irrigation water for agriculture has significantly dropped.
This hydrological, geomorphological and environmental degradation of the delta poses a risk to the livelihood of people living alongside it. It also makes them more vulnerable to cyclones, floods and droughts.
But the extent to which the delta has been damaged and its revival have always been a burning issue for environmentalists, civil society, agriculturalists, local communities, non-governmental organisations, policymakers and government officials for the last few decades.
Before the construction of diversion hydraulic structures over the Indus river, there were 13 active creeks in the delta, which have now decreased to only two active creeks, namely Khobar and Khar. The active delta has shrunk by 92 per cent from 13,000 square kilometres in 1833 AD to only 1,000 sq km now.
This fan-shaped delta supports the seventh largest mangrove forest system of the world in vast tidal mud floodplains.
But it is shrinking rapidly owing to decreasing river flows to the delta, land subsidence, a rise in sea level, low rainfall because of climate change, and surface and subsurface seawater intrusion.
The US-Pakistan Centre for Advanced Studies in Water at the Mehran University of Engineering and Technology, Jamshoro, recently conducted a study to gauge the impact of seawater intrusion on the Indus delta, using remote sensing and geospatial tools.
The active delta has shrunk 92pc to 1,000 sq km in the last almost two centuries
The study found that mangrove swamps have decreased to only 80,000 hectares. The area under water bodies in the delta has increased from 1,600 sq km to 3,000 sq km in the last 26 years. During the Rabi season, the vegetation region shrunk 19pc from 3,100 sq km in 1990 to 2,500 sq km last year.
The analysis of coastline positions in 1972, 1979, 1990, 2000, 2010 and 2017 showed that the inland shift of coastline varied from 10 to 150 metres per year with an average of 19 metres annually. The total inland shift during the 45-year period was estimated at 1.3km.
The Indus delta shoreline is exposed to withstand the highest average sea wave energy compared to other major deltas in the world. However, small accretion was also observed at some places.
The inward shift in the coastline was 22 metres per year on the left bank of the Indus river and 16 metres on the right bank.
The shoreline of the delta on the left bank is more vulnerable to coastal erosion because of less mangrove population and more and flat tidal floodplains as compared to that of the right bank of the delta.
The total tidal floodplain area has increased from 6,000 sq km in 1972 to 6,500 sq km at present.
The impact of sub-surface seawater intrusion was estimated over an area of about 600,000 hectares, spoiling not only fertile alluvial agricultural lands of Sujawal and Thatta districts, but also having a devastating environmental and social impact on the rich ecosystem of the Indus delta.
Policymakers can take the following measures to revive the Indus delta:
• A protective levee — about
200km long and two to three metres high — should be constructed along the periphery of the tidal flood on an emergency basis to mitigate the adverse impact of surface seawater intrusion on the delta.
• At least 10 million acre feet of water should be allowed every year as an environmental flow below the Kotri barrage.
• The environmental river flow might be useful in controlling seawater intrusion only in the active delta. Therefore, for minimising surface and subsurface seawater intrusion in the entire delta, irrigation canals off-taking from the Kotri barrage should be supplied enough water so that irrigation water should reach the tail end of the delta. Field surveys and satellite images showed that irrigation channels in the delta have a significant impact in controlling seawater intrusion in areas far from the Indus river.
• If possible, relic river channels such as Ochito and Old Pinyari should be restored. These channels should be used to carry extra flood water during peak flood in the sea to shun the flood pressure on the main river and thus minimise the possibility of a breach in the levee. This will help supply freshwater to coastal communities living far from the main river.
• Mangrove plantation on the tidal floodplains along coastline should be encouraged by establishing community-based natural resource management committees. Thick mangrove forests provide defence-line against natural calamities such as extreme tides, cyclones and tsunamis; trap river silt to support accretion along the coast; provide natural breeding ground for fish, shrimps and other marine life; and provide people living along the coast with wood, fodder and livelihood.
• The government should introduce and encourage the cultivation of value-added halophytes such as Salicornia, Sea Aster, Spartina alterniflora, Suaeda, etc, in the tidal flood plains.
• The government should ban overgrazing and cutting of mangroves for wood, and the use of fine mesh nets for catching small size fish and shrimps.
• The tourism Industry, especially boat cruising, should be encouraged in the mangrove-laden creeks in the delta to improve socio-economic conditions of poor local communities.
The writer teaches at the Mehran University of Engineering and Technology, Jamshoro