Photo courtesy of Rukshan Kuru-Utumpala
Today is World Rivers Day
“Rivers are the arteries of our planet: they are lifelines in the truest sense.” Mark Angelo
World Rivers Day is celebrated on last Sunday in the month of September. Rivers provides us many goods and services. Yet the value of rivers has been ignored by both people and policy makers alike that has resulted in degradation of rivers. In recognizing the urgent need to address this issue, United Nations has declared the period from 2005 to 2015 as the decade of “Water for Life” to promote nations to use their water resources in a sustainable manner. Along with this the World Rivers Day was declared in 2005 in order to raise awareness among the general public of the values of rivers and to encourage them to take better care of rivers. The aim of this article is to provide an overview about the present status of rivers, the goods and services provided by them and to encourage all to become active partners in caring for our rivers.
Origins of rivers
Water on the planet Earth appeared during the Archean epoch, nearly four billion years ago. Once the water appeared, the water cycle came into operation due to evaporation driven by the solar energy and precipitation driven by gravitational forces of the planet. Precipitation that falls on the land flows on the surface as runoff, some of which is retained by surface water bodies such as lakes and marshes, and some infiltrates into the ground and forms groundwater. The excess runoff gives rise to streams, rivulets and rivers that eventually drain the excess water into the ocean and the cycle continues in perpetuity as long as there is water. Therefore, a river or a stream can be defined as a channelized flow of water on the Earth’s surface.
It can be inferred that primordial rivers may have existed on the planet for nearly 4 billion years. However, the river flows and the river courses may have undergone significant variations during different epochs of Earth’s history due to movement of continents, variation in climate, and astronomical influences on the planet. With the invasion of the land by algae like organisms around 430 million years ago, the rocky surface of the earth was transformed in to one that comprised of soil that enabled further colonization of land by plants, insects and vertebrates, converting Earth from a blue planet into a blue-green planet. This probably had a significant influence on rivers as the vegetation cover in the catchment of a river affects the quality and quantity of water flowing in a river.
Types of rivers
There are a large number of rivers around the world. Yet no two rivers are exactly alike. Amazon is the largest river in the world with a length of 6,516 km and a catchment that encompasses eight nations (Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Suriname and Venezuela) and extends over7,500,000 km2. The longest river in the world is Nile with a length of 6,695 km. Most rivers flow on the surface. However, some rivers tend to flow underground in caves or caverns, or beneath glacial beds and these types of rivers are referred to as subterranean or sub glacial rivers. Most rivers originate in lakes, springs, wetlands, or glaciers located at higher elevations and discharge into oceans. However, some rivers have their source and mouth both inland. For instance, Amu Darya and Syr Darya, two large rivers flowing through central Asia, originate in Tajikistan and Kyrgyzstan respectively and flow into the Aral Sea, which is a large saltwater lake. This type of rivers is called inland rivers or virgin rivers.
Rivers show a great deal of diversity and can be classified based on number of characters such as nature of their substrate, type of water flow, interactions with ground water table, sediment load carried by the river, particle size of the dominant bed sediments and the river morphology. Some rivers flow directly on bed rock while others flow on a bed of sediments. The former is called a bedrock river while the later an alluvial river. Most rivers will have a mix of these two characters, where generally the upper part of the basin comprising bedrock rivers which gradually transform into alluvial rivers as they descend towards the ocean. If the water surface of a river lies above the groundwater table, water will flow from the river to groundwater and is called an effluent river. On the other hand, if the water surface of a river lies below the groundwater table, it will receive water from groundwater and is called an influent river. Most rivers that undergo seasonal fluctuations will behave as an effluent river during the wet season and an influent river during the dry season.
Water flow in a river can vary based on the rainfall received. Some rivers flow all year round and are called perennial rivers while the others flow only for a short time after rainfall and are called ephemeral rivers. Most rivers tend to oscillate between these two conditions, where during the wetter part of the year they flow as a perennial river and during the drier part of the year they flow as an ephemeral stream. These types of rivers are referred to as intermittent rivers.
The rivers carry sediments in two forms, particulate load, and dissolved load. The sediment load can be further divided into bed load and suspended load. The bed load comprises of coarse sand and gravel whereas the suspended load comprises of clay, silt and fine sand. Depending on the type of loads carried by rivers they can be classified as bed-load rivers, suspended-load rivers, or mixed-load rivers. Finally, rivers can also be classified based on their morphology into several types such as straight, braided, meandering and anastomosing rivers. Sometimes different sections of a river may show one or more of these morphologies.
Anatomy of a river
The beginning or the source of a river is called its headwaters. Headwaters can be springs that originate from groundwater or glaciers. In addition, precipitation and fog intercepted by the trees that are present in the headwaters can also serve as a source. Headwaters feed streams that join with other streams to form stream networks which in turn will join with other stream networks to form tributaries that will eventually join the main river. A river may have one or many tributaries depending on the size of the river. For instance, the largest river in the world, Amazon has more than 1,100 tributaries of which 12 are longer than 1500 km. The course carved by the river across a landscape is called the river channel. The shape of the river channel depends on many factors such as the amount of water carried by the river for what period, soil, rock and vegetation in the landscape. The course of the river channel may shift with time leaving the old river channel as a dry riverbed. The land edge on either side of a river is called riverbanks. If one stands facing the downstream of a river, the bank on the right is called the right bank and bank on the left is called the left bank. The vegetation growing on the riverbanks may show special characteristics and is referred to as the riparian zone or the riverine vegetation. As the river descends, more and more tributaries connect with main river increasing its water volume. The low-lying area along the river gets periodically flooded when the water volume increases, especially during the heavy rains. The area that gets flooded is called the floodplain of the river. During flooding, the sediments carried by the river gets deposited in the floodplain. Therefore, the floodplain soils are highly fertile. Further, the floodplain provides breeding sites for most of the species that inhabits the river. The floodplain also serves as a buffer by storing and absorbing floods and thereby protecting human habitations near the river. This is called the flood retention area of the river. The end of the river is the river mouth or the outfall which may open to the sea directly or through a lagoon. When the river discharges directly to the sea the interphase between the river and the sea is called an estuary. When the river mouth comprises of two or more parts the land in between these parts is called the delta.
Functions of rivers
Rivers are the cradle of human civilization as many of the first civilizations were founded on the banks of rivers such as Euphrates, Tigris and Nile (Fertile crescent), Indus (Indus valley) and Yellow River (North China Plain). This could be attributed to the steady supply of water, presence of a fertile land to grow crops, ability to hunt fish and wildlife that inhabits the river, to use the river for transport and as a guide for people to locate their homes. Most of the modern cities are also located next to major rivers for the very same reasons. In many countries, especially in Asia there is a large population of people whose livelihoods directly depend on rivers. Rivers serve as the main source of drinking water for both urban and rural populations. Many industries depend on rivers to meet their water requirements and rivers also serve as the main source of water for irrigated agriculture. Rivers also provide many rural communities with food in the form of freshwater fish, wild birds, and aquatic plants. Rivers are also used for hydropower generation. In many countries rivers support recreation in the form of boating, fishing, white water rafting, and river-based tourism is becoming a popular segment of the tourism industry. Rivers are used for transport of goods and people and in some countries, they are used as floating markets. Rivers carry weathered products of rocks which contains many minerals that are deposited either on the riverbed or in beaches around the river mouth. River sand is another resource that supports many livelihoods as well as the construction industry. In addition to these tangible benefits, rivers offer many intangible benefits such as flood protection, serving as a center of spiritual and cultural activities of people and a free source of water for their daily needs such as bathing and washing. Rivers are also home for large number of living organisms. Even though rivers account for less than one percent of Earth’s flowing water, it is home for 10 percent of all known species, including onethird of all the vertebrates. Freshwater rivers are also home for around 270 species of turtles, more than 1,300 species of crabs and shrimps, and around 5,700 species of dragonflies. Riparian habitats also have unique attributes and support a large number of terrestrial and water associated species while functioning as critical corridors for the movement of animals and seed dispersal agents.
Drivers of change
Although rivers are incredibly important for the survival of humans as well as a large number of other river dwelling and river associated species, most rivers around the world have been degraded due to a number of anthropogenic drivers. Based on the current estimates nearly 70% of the major rivers are impeded due to dams and other manmade infrastructure. Dams are constructed across rivers for establishing reservoirs for hydropower generation, providing irrigation for agriculture, providing drinking water, prevent saltwater intrusions and/or to avoid floods in downstream human habitations. Dams prevent the free passage of species that inhabit the rivers, the process of sediment transport, or transport of material or people along the river.
The impact of dams is most pronounced on river dwelling species as the section of the river downstream of the dam will now have low flows or very dry conditions, which will reduce the carrying capacity of the river. Further, reduction in water volume results in number of other negative impacts such as increased predation risk, high temperature fluctuations, reduction in water quality, loss of aquatic vegetation, loss of microhabitats and transformation of the riverbed due to lack of channel maintenance flows. The dam will serve as an obstacle for free movement of river dwelling species, which will disrupt the life cycles of species that require to migrate between freshwater and sea water. Reduction in downstream floods will remove the triggering action required by some species to initiate breeding or transition between life cycle stages. These drivers have resulted in the extinction of at least 80 species of freshwater fish while the actual number is likely to be much higher. Loss of species will lead to loss of ecosystem services provided by these rivers which will directly affect human well-being and livelihoods.
River pollution is another major driver of change. Pollution can result due to point sources such as industrial discharges or non-point sources such as household wastewater discharge, storm water runoff carrying agrochemicals, farm waste, solid waste such as non-degradable plastics and sediments. These pollutants can be toxic to river inhabitants as well as river users. Nutrient loading due to farm waste and agrichemicals in storm water runoff tends to promote algal blooms that reduces oxygen levels in the water creating dead zones as well as harmful effects mediated by toxins produced by these algae.
Overexploitation of rivers such as overfishing or using harmful fishing techniques such as illegal fishing gear, dynamite fishing or use of natural or synthetic poisons that may kill much more than target organisms, unsustainable water abstraction without providing sufficient environment flows to meet the ecological and social demands of the rivers and removal of river sand beyond the rate of replenishment will have detrimental effects on rivers and its inhabitants. Introduction of invasive species into rivers is another major impact on river flow as well as riverine species. Some of these invasive fish can outcompete native species or feed on native species causing reduction in diversity of native species.
Deforestation in watersheds of rivers and removal of vegetation in the riparian zones can be detrimental to rivers. The natural forest cover in the watershed intercepts the rain and fog reducing the speed at which water droplets falling on the ground minimizing the erosion and creates a sponge effect by slowing down the speed of runoff, which allows the absorption of rain water into the soils and groundwater. This will reduce the amount of water that flows into rivers during heavy rains thereby preventing downstream flooding. At the same time slow release of absorbed water by the forest flow and groundwater will ensure that there is perennial flow in the rivers. Thus, the natural forest cover in the watershed areas helps to prevent floods as well as droughts. This becomes especially important in the context of climate change driven changes in weather patterns that result in short, intense rains followed by prolonged periods of droughts. The former results flash floods and the later results in droughts that undermines many of the ecosystem services provided by the rivers. Deforestation also results in increased flow of sediments to rivers that can change the bed structure and the carrying capacity of the river causing changes in the micro habitats of the river as well as the ability of it to serve as a buffer against flooding. Activities such as gem mining in rivers, sand mining for construction and flushing of large dams can also contribute to increased sediment flow into rivers.
Way forward
Rivers are essential not only to humans but also to a large number of living organisms that use them as permanent or transient habitats. Therefore, taking urgent measures to address the drivers of change is vital to ensure the maintenance of healthy rivers which in turn will ensure human well-being as well as a continued flow of goods and services provided by this very valuable resource. Sri Lanka, even though a small island, has an extensive river network that comprises of 103 river basins with a collective length of around 4,500 km. Almost all the rivers originate from the Central Massif. Most of the major rivers in Sri Lanka are dammed for hydropower generation or to support irrigated agriculture. Also, many of our rivers are polluted due to industrial and urban wastewater and heavy use of agrochemicals in our croplands. Unplanned disposal of solid waste near rivers have also contribute to pollution of rivers with waste as well as leachate arising from such waste disposal sites. There are a number of invasive alien plants and animals that are affecting most of our rivers. Deforestation of most of our critical watersheds have affected the flow regimes of our rivers which are likely to become exacerbated due to climate change driven changes. Thus, as we celebrate our rivers on this day, we must also make a silent pledge to ourselves to reduce our impact on rivers as much as possible as they are the lifeline of terrestrial life on this fragile planet.
