Rivers, Habitats and Biodiversity: Foundations for Sustainable River Management
Rivers are not just flowing waters but a longitudinal continuum of WEBS (water, energy, biodiversity, and sediment). They are complex ecosystems where thousands of organisms interacting with instream water, mineral habitats, and associated riparian zones survive, breed, and thrive. In any landscape, rivers are bioenergy hotspots whose geomorphology, hydrology, and water quality in the upstream reaches govern carbon concentration and nutrients in the downstream reaches. Flood pulses periodically determine inundation and exchange of nutrients, while interactions between water and organisms maintain the health of a river.
The assembly of instream biotic community in a river’s downstream reaches is linked to that in the upstream. The biotic community is orderly placed based on their feeding habits. Headwater streams chiefly harbor organisms known as “shredders”, which break coarse organic particulate matters; the mid-rivers contain largely algae and diatom consumers called “scrapers”, while the lower reaches have the dominance of “collector-filterers”, which consume fine organic particulate matters (Figure). Linkages between upstream-downstream, as well as with the lateral landscape, ensure the integrity of a river ecosystem.
River Continuum Concept (Vannote et al. 1980).
Fresh Water and Biodiversity
Freshwater, covering 0.01% of the earth’s surface, is the habitat of about 10% of all recorded species and over 30% of vertebrates. Humans’ use of rivers and other water bodies for meeting their drinking, food, irrigation, and hydro-energy needs has modified the stock and flow of rivers. Activities like dam construction and river diversion for irrigation, hydropower generation, and other uses alter the natural flow regimes of rivers, their morphology, and habitats.
(Copper Mahseer) Neolissocheilus hexagonolopis
Credit: Ramdevi Tachamo Shah
Interventions for flood control such as embankments alter the dynamics between floodplains and rivers. Beyond a threshold, the impacts become irreversible and interfere with local, ecology, freshwater ecosystem, and dependent local community. Changes in the health of natural ecosystems affect the linkages between biodiversity and human lives. Any disturbance to this connection cascades on to human well-being.
Since the beginning of the 20th century, infrastructure has been built in rivers across the world to get irrigation, hydropower, and flood control benefits. In South Asia also, interventions have been made in the Ganga River’s tributaries in Nepal. Sarada Barrage in the Mahakali in 1928, Girijapur Barrage in the Karnali in the late 1970s, Gandak Barrage in 1969, and Kosi Barrage in 1959 to derive irrigation, flood control, and some hydropower benefits. Many other dams have been built in recent years.
In the last two decades, the construction of hydropower dams and other infrastructures has boomed in Nepal. Many hydropower plants have been planned for every river, and the majority of them are under construction. Operation of multiple hydropower and irrigation projects in a single river has resulted in the drying up of stretches of the river with high cumulative environmental and social impacts in the basin. In the already dam-regulated Modi and Trishuli rivers of the Gandaki Basin, the fish population has dramatically declined and many fish species are threatened. Improper fishing practices are other threats to river ecosystems and aquatic lives.
Today, only 37% of the world’s large rivers are free-flowing. A dam disrupts a river’s continuity, halting upstream-downstream migration of freshwater species. In addition, the increasing urbanization and commercial agriculture have led to the discharge of point and non-point pollution sources, lowering the quality of river water. The rising rates of urbanization and infrastructure development have skyrocketed the demand for construction-grade sand and aggregates, leading to haphazard river bed mining. Sand and gravel mining in rivers have broken their functional capacities such as waste assimilation and purification. The Bagmati River in the Kathmandu Valley and others flowing across cities are examples. Increasing flash floods impact freshwater ecosystems and damage aquatic habitats. Prolonged droughts lead to lower surface and interflow, which reduce the supply of quality freshwater to rivers. Collectively these actions destroy aquatic habitats,.
River Fragmentation Jhimruk River Nepal
Credit: Deep Narayan Shah
Not only have human actions fragmented rivers but also destroyed other water bodies like ponds, lakes, and wetlands. The destruction of water bodies has shrunk the habitats of organisms and threatened many biotas. Between 1700 and 2000 CE, over 85% of wetlands have been lost, which is three times higher than the loss of forests. A global study conducted by World Wild Life Fund (WWF) states that, in the last four decades, the population of global freshwater species has declined by 83%. Another study, carried out by the International Union for Conservation of Nature (IUCN), concludes that about 40% of the invertebrates are considered threatened.
In many rivers, iconic species have already become extinct. Species such as the Yangtze River Dolphin, Baiji, and the world’s largest fish, Chinese paddlefish, are almost extinct; so are the Near Threatened Himalayan Epiophlebia laidlawi in India. It is estimated that climate change is likely to shrink their habitat by 83% by 2080. The rate of extinction of freshwater biodiversity globally is higher than that of terrestrial biota. In most South Asian countries, including Nepal, freshwater biodiversity is being lost at a high rate. For example, the Himalayan relict dragonfly, which is common in many headwater reaches of rivers today will likely be found only in the limited headwaters of the rivers of Nepal and Bhutan.
Balancing the use of freshwater to economic needs with social and environmental concerns is key to well-being across the world and also for Nepal. Indeed, irrigation and hydropower will play a role in the development of the country, but it is also important to account for the environmental costs associated with their development. Economic development today should not take place at the cost of the future. Such myopic strategies will jeopardize the future of our children and grandchildren forced to bear the cost of our actions and inaction.
Hydropower plants not only add energy to the national grid but also fragment rivers and decimate, if not eradicate, important fish species and other aquatic life. The development of hydropower should be based on a baseline assessment of a river’s ecosystems, consisting of instream organisms such as macroinvertebrates, fish, amphibians, and mammals. Hydropower development should be guided by the goal of incurring minimal impacts and maintaining healthy river ecosystems. Regular monitoring and compliance with the e-flow policy are the starting points to sustainable hydropower development.
Nepal must include energy development platforms like solar, wind, and other technologies in addition to sustainable hydropower to meet various needs. In recent times, the cost of energy generated by renewable technologies has come down substantially. Nepal’s challenges are widespread adoption of renewable technology, building in-country capacity to implement them, and in the process also creating new jobs. The increasing mix of different renewable energy technologies while providing carbon mitigation benefits can help in beginning the reversal of river degradation, ultimately helping in nature-friendly living. Alternatives exist to energy types but rivers have no alternatives.
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