River Formations
- Devayani Khare
- 2 days ago
- 7 min read
Dear Reader,
For months (dare I say, years?) I’ve looked for a design template that flows like a river, where I could insert blocks of text and images to illustrate different upstream, midstream and downstream formations. I considered commissioning a digital artist but it seemed too ludicrous. I also leaned on my more-artistic sister, but to no avail.
This illustrated feature doesn’t quite do that grand idea justice, but it is what I could muster with my skills, a faint memory of old geography textbooks, and lots of help from diagrams online. I couldn’t feature every possible river-shaped formation from source to sea, nor reflect the diversity across geographies and climate types. Yet I hope the illustrations serve to anchor you on the river’s journey from source to sea, to better understand why rivers are among the most decisive agents of landscape evolution.
In geomorphology, rivers are said to cut down the ambition of mountains to bring it to the level of the sea. ~ Dr Leena Deodhar
This quote from my college days is a powerful, poetic description of the role of a river. I’ve looked in vain for the source, but it seems to have been among the many figurative twists used by my favourite professor, Dr Leena Deodhar. She had a way of personifying a river — often speaking of a river in third person, using ‘she’ to denote its actions — that brought our syllabus to life. This essay is a tribute to her.
The Birth of a River
A river is born in cloud-cloaked highlands, where the rain first kisses the earth, or where a spring bursts forth from the ground. Sometimes, a high-altitude lake may spill over or glacial ice melts flow downhill in rivulets. As rivulets coalesce, they form headland streams or headwaters that gather speed along sharp, jagged mountain slopes. These streams begin downcutting mountainsides into gullies and V-shaped valleys, plucking away any loose material along the way. As streams merge, they are further fed by precipitation, groundwater and tributaries to form gushing, roaring rivers. The area of a mountain that drains all its water into a river system — including the river, its tributaries and groundwater sources — is known as a watershed or catchment.
As the river gains momentum, it erodes the audacity of mountains as they reach for the heavens. Little by little, it tears away at the rocks to grind them into sand, and silt, and clay. As the water races down sheer mountainsides, it carries sediment downriver, which in turn makes the water more abrasive and erosive. The channel’s force also determines the sediment size from soft clay, fine-grained sand or silt, rough gravel, cobbles and pebbles, and when the river is in full spate, it is also known to carry boulders.
The sediment also imparts rivers with a particular hue, which may change with the seasons: silt and clay create muddy brown channels, limestone or carbonate-rich material results in whitish, light, or bluish water, iron-rich sediments lend rust-brown hues, and decomposed organic matter may lead to dark, blackish rivers.
Upstream Formations
As a river flows through landscapes, it may reach a section with alternating bands of hard and soft rocks. The hard rock resists, but the soft rock yields to the river’s downcutting force, creating a sudden, steep drop in the channel. The river is forced to plummet off the edge of the hard rock to continue its journey downslope. And a waterfall is born! Occasionally, waterfalls also hint at the rejuvenation of a landscape — when tectonic upliftment or sea level changes cause the base level of a river to drop, catalysing a mature river’s flow.
River terraces, too, whisper of past fluvial processes, and occur where a river cuts down into its floodplain, leaving behind flat or gently sloping areas elevated above the current water level. River canyons are formed when a river erodes and downcuts its way through an uplifted topography, over millions of years, resulting in a deep, narrow, steep-sided valley. Gorges (often used interchangeably with canyons, by non-geologists) are formed when tectonic or gravitational forces create a narrow crevasse in the earth, which may be further widened by natural erosional agents like water and wind.
Midstream Formations
As the river reaches the foothills and enters the plains, it begins its transitional phase — a mature stage characterised by erosion and deposition. The fast, deep and narrow mountain rivers widen into slower-moving, shallower, meandering channels. With the transition from steep slopes to open plains, the river loses much of its force and vitality and needs to lighten its sediment load, which it does as splayed-out alluvial fans or aggraded, patterned floodplains.
Often, these sediment offloads cause the main river to split into small, intertwining waterways that weave their way through ever-shifting islands of gravel, sand, and silt. These formations are known as braided rivers. Sometimes, braiding also occurs in the downstream stage, just before the river meets the sea.
As the river meanders, it continues eroding its way through the landscape along its convex edge, while creating natural levees or embankments along its concave shores. Natural levees also serve to keep a river on its course by preventing it from spreading too far across its floodplain — an area of periodic flooding, with rich, fertile alluvial soil.
Sediment deposits along the concave bend may cause a river’s meander to be cut off into a crescent-shaped oxbow lake. In rare cases, a cloudburst or unseasonal glacial lake outburst, or flooding upstream, causes the river’s discharge or volume of water to increase. The river suddenly possesses too much energy for its usual, languid meanders and decides to take a straighter route — leaving an isolated oxbow lake.
The river also lightens its sediment load by depositing sandbars mid-channel, which may serve as islands or refuges for biodiversity. Sometimes, obstacles like fallen trees or boulders may disrupt the river’s flow, generate eddies that trap sediment and become the nucleus for sandbars. If enough fine-grained, fertile silt and clay are trapped, some pioneering species may take root, and further stabilize the sediment and cause the sandbar to grow into a vegetated island.
Downstream Formations
Few rivers end at lakes, some into marshes, yet most rivers flow into the sea. In the final leg of a river’s journey, tired and spent, it enters an estuary — a semi-enclosed area where freshwater from the river mixes with seawater to form a brackish channel. These unique brackish environments include diverse wetlands like salt marshes, mangroves, and mudflats (I’d covered these in the previous edition, Landscape Lexicon: Wetlands), which serve as nurseries for many marine species.
As the river meets the sea, it deposits the last of its sediment into triangular, fan-shaped islets to create a delta, named for its resemblance to the Greek letter (∆).
In some cases, due to seasonal fluctuations or anthropogenic activities like damming and canal diversion, rivers may run dry before they reach the coastline. A rare occurrence is an inland delta, where a river spreads out and disappears across dry land — the Okavango delta in Botswana, Africa, is among the most stunning examples. Seasonal flooding between June and August causes the Okavango river to swell and flow from the Angola highlands to a tectonic depression in the Kalahari desert. It transforms the desert into an oasis and becomes the centrestage for one of Africa’s greatest wildlife spectacles. As the rains abate, the river channel breaks up into numerous lagoons and salt marshes. With the dry spells, the wetlands once more turn into desert, and the flora and fauna wait for the next season.
A River of Life?
Rivers are agents of dynamic equilibrium between erosion and deposition, water flow and sediment load, further subject to tectonic upheavals, seasonal irregularities, and anthropogenic factors. As key drivers of landscape evolution, rivers are not just geographical features; they are part of a living, breathing landscape that forms a crucial link in the earth’s geomorphological, biogeochemical, hydrological and ecological cycles.
Through human history, rivers have been the cradles of civilizations; the remains of our ancestors and their tools lie buried in alluvial sediments. Riverbanks witnessed the birth of religions and the fall of empires, while trade flourished along the waterways. By measuring their annual ebbs and flows, we learned to mark the seasons.
In 2017, New Zealand became one of the first countries in the world to grant rights of personhood to a river: the Whanganui River, described as “an indivisible and living whole, comprising the Whanganui River from the mountains to the sea, incorporating all its physical and metaphysical elements”.
In 2023, researchers discovered correlations between river sediments and the diversification of biodiversity over geological time scales. They conducted global simulations with reconstructions of marine biodiversity spanning the past 540 million years and continental plant diversity over the past 400 million years. They found that increased transfer of river sediment to the oceans led to a diversification of marine species, whereas mass ocean extinctions occurred with decreases in sediment flow. On the continents, too, a greater sediment cover correlated with increases in land-plant diversity. [Source: Nature - the original research | The Conversation - a simpler version by the researchers]
Even today, their socio-economic importance in flood regulation, the creation of fertile floodplains and farmlands, and as a source of hydropower cannot be estimated. Yet we foolishly imperil our lives by polluting, mining, damming and altering rivers, and seem surprised when the river reclaims its land and legacy. Perhaps it is time for our reverence for rivers to move from a philosophical, abstract idea to a more practical, tangible one.