What comes to your mind when you imagine a river? A stream of water flowing through landscapes? While that's partly true, rivers are so much more than just water in motion. They are dynamic ecosystems channelling energy and carrying life, nutrients, rocks - elements that shape not just the river but also the land, lives, and cultures that surround it. Imagine this: the Ganga emerging from the Himalayas, carving its way through the mountains before it begins a slow, laborious meandering through the plains. What shapes these rivers, allowing them to nourish civilisations, and sustain life? Sediment.
Sediment Formation: The unsung architects of rivers
Sediments play a crucial role in the river’s health and ecosystem. Sediment dynamics influence the river’s pace, as well as the shape and course it takes. The process of sediment formation, transport, deposition, and natural erosion not only affects the river’s flow but also moulds islands and sandbanks - with this constant interplay of deposition and erosion shaping the riverine system itself.
Water in rivers moves with an immense energy that allows it to wear down even seemingly hard rocks, given sufficient time. It is important to note the long geological timescales this process of sedimentation takes, especially when considering the ‘replenishment’ and ‘renewability’ of sediment in the system.
The Journey of River Sediments: From Mountains to the Sea
Headwaters: The Birthplace of Sediments
At a river’s origin—often in steep, mountainous regions—water rushes downhill with tremendous energy and a swift pace. As it makes its way along the rocky substrate at immense speeds, the river’s energy erodes rock into small stones and finer sediments. These sediments are then transported downstream, carried by the flowing river.
Plains: Where Sediments Settle
As the river enters the plains, it slows down and moves with less energy. Here, in flatter terrain, the river relieves its load by depositing sediments in floodplains, riverbanks, and islands. Its energy goes to carrying and depositing sediment and to eroding at its banks - resulting in the iconic meandering pattern of rivers like the Ganga, Yamuna, Ghaghra, and others.
At this stage, the work being done by the river (to carry, deposit, and erode) tempers its energy and regulates the intensity of flooding. Natural flooding, often seen as a disaster, plays a crucial role in the plains. Floods spread silt, clay, and sand across the land, replenishing the topsoil and sustaining diverse ecosystems. These deposits make the soil nutrient-rich, supporting agriculture and aquatic life.
Deltas And Estuaries: The River’s Grand Finale
As the river travels its course and eventually meets the sea or ocean, its speed reduces further, energy dissipates, and a lot of sediment deposition takes place at the mouth of the river, often forming a delta or estuary. Sediments are very important for a delta, as they help in the growth of mangroves, mitigate the impact of cyclones, and can reduce saline intrusion.
Anthropogenic changes in sediment flow
Human activities on rivers such as the construction of dams and barrages, sand mining, hampers the flow of sediments, which affects the rivers’ energy and flow, and consequently its floodplain.
Sand Mining and the Removal of Sediment
Sediment creation and deposition are processes that take place on geological timescales, shaped over many millions of years. On the other hand, mining sand for construction and dredging for creating waterways are some anthropogenic activities that remove sediments from the river at a pace with which the river’s natural replenishment system cannot keep up.
As we saw earlier, sediments help regulate the river’s flow, as its energy gets dissipated from carrying its sediment load. When sediment is removed through sand mining or dredging, the river’s flow becomes much faster. These sediment deficient rivers are also known as ‘hungry rivers’ - with untempered energy, eroding at banks and prone to flash floods.
The river’s floodplain acts like a sponge. The adjoining sandbanks allow water to percolate into underground aquifers and smaller water bodies. Excessive and unregulated sand mining removes dangerously large amounts of this sand, and significantly reduces the river's capacity to recharge and store water in the floodplains.
2. Dams And Restricting Sediment Movement
Dams and similar hydropower projects are lauded as symbols of development, and a source of renewable power that controls and taps into the immense energy of rivers as they flow downstream.
However, dams trap a lot more than just water in their reservoirs - they also trap sediment. This deprives areas downstream of sediments and soil nutrients, as the sediment accumulates within the dam reservoir.
The accumulation of sediment in the reservoir, called siltation, also affects the efficiency and functioning of dams by reducing the holding capacity of the reservoir. Desilting large reservoirs takes time, money and energy - currently, a large number of India’s mega-hydel projects are operating at greatly reduced capacities due to unchecked siltation.
3. Concretisation And Restricting Sediment Deposition
Borrowed from the image of the Thames in London, the idea of a river in a ‘developed city’ has frequently determined the treatment of urban river edges by creating riverfronts and restricting rivers into concrete-edged canals. When riverbanks and river beds are concretised, it hinders the river’s natural processes of sediment transportation and deposition. The natural interaction of water, sediments, organic matter, and vegetation on riverbanks is completely absent in such stretches of river.
These concrete edges that ‘channelise’ the river, also harm the river, its riverbanks, floodplains, its sediment interplay, and the ecosystem by eliminating any interaction between them, and erasing habitats for insects, birds, reptiles and aquatic life.
Conclusion
The vital role that sediment plays in the hydrology of rivers highlights a key problem with our current approach to river management and governance in India - neglecting sediment as a key component of the river system, deserving of consideration if we are to holistically protect our rivers, agricultural floodplains, freshwater sources and aquifers. This lack of consideration has led to poorly formulated policies on flood management, hydropower, riverfront development and more, while allowing unchecked sand mining and disastrous water management strategies (like river-linking) to proceed unprotested.
“Rivers are dendritic networks which are interconnected. When you look at things from this source-to-sink perspective, you see that flow and sediment are two sides of the same coin, as processes that interplay with each other.”Shishir Rao, river ecologist (on Sediment Stories 02 by Veditum X India Sand Watch)
It is imperative that we move beyond this closed perception of rivers as a ‘channel of water’ and understand the river as a fluvial system in its entirety, including riparian vegetation, natural flooding and drought cycles, its energy, and the sediment load it carries.
This article is part of the ‘Sediment Stories’ series, highlighting the role of sediments in river ecosystems and communities. Veditum India Foundation and India Sand Watch track sand mining, advocate for sediment-inclusive policies, and share knowledge on this critical resource. Learn more at sandwatch.in