Rivers
Last updated: June 8, 2009.
Why do rivers enthrall us?
Maybe it's because they're teeming with life—everything from nibbling midges to
flashing kingfishers and trout and salmon wiggling beneath the
ripples. Perhaps it's because water is so central to our existence
and so vital to life. Possibly it's to do with the tranquility of a
river; in countryside or in cities, the sight of water has an
instantly calming effect. All these things come to mind,
instinctively or very consciously, whenever we think of rivers or
spend time near them swimming, fishing, boating, or canoeing. Let's
take a look at what makes rivers so amazing!
Photo: Cheat River by Ryan Hegarty, courtesy of
US Fish and Wildlife Service.
What is a river?
Water is constantly circulating between
Earth's surface (the land
and oceans that make up our planet) and the atmosphere up above in a
never-ending conveyor belt called the water
cycle. Rivers are the
main parts of the conveyor that carry water from the higher parts of
Earth (the mountains and hills that we call uplands) to the lower
parts (lakes and seas). You can think of rivers as drains, if you like:
channels with fairly well defined banks,
carved through millennia,
that can be thin and shallow or very deep and wide. Powered by
gravity, rivers are always flowing downhill (even when they look
absolutely flat, they're really flowing down a gentle incline). Unlike
seas, which are salty, rivers are filled with freshwater to which the lives
of many different creatures (insects, animals, and humans too) have
become perfectly adapted. Far from the oceans, rivers bring the land
to life.
Photo: Aerial view of the Black River Yukon Flats National Wildlife Refuge. Photo by M. LeFever courtesy of US Fish and Wildlife Service.
The path of a river
You might think a river is a dead thing, because it's not a living
organism. But you can think of a river as a living thing in at least
three important ways. First, it's full of living creatures like
otters and fish: it may not be alive, but it's certainly full of life.
Second, it's living in the sense that it's
constantly adapting to Earth's changing climate, geology,
ecology—and even the changes that humans bring. Third, it's living
because it changes its character from the place in the uplands where
it begins its life (known as its source)
to the place where it ends
its life (known as its outlet
or mouth, where it flows into
the sea).
The path that a river takes in its journey over Earth's surface is a
bit like the
life a human leads between birth and death but, where a human's life
is spread out in time, a river's spreads out in geographical space.
Upper river courses (youthful stage)
A river begins life high in the hills or mountains. In a cold
region, a river may be created by melting snow or a
glacier. In warmer places, rivers typically form when water drains from
a whole series of upland slopes known as a basin. Water drains from
each slope to form a small trickle called a rill. Rills from many
slopes combine to form brooks,
which join together to make creeks
(small streams) and larger streams,
before all these things eventually merge into a river.
The brooks, streams, and creeks that form a river are called its
tributaries. Flowing down from
high hills and mountains, the upper
part (or course) of a river is usually narrow, steep, and marked by
sharp valleys and abrupt, zig-zag changes of direction. The steepness
means the water flows quickly, often forming dramatic features such
as white-water rapids and waterfalls (great for canoeists).
Rapid flow means the
water has high energy to cut through rocks, wearing away deposits in
a process called erosion.
Photo: Tower Falls. A waterfall is a typical feature of the upper reaches of a river. Photo by Bruce Halstead courtesy of US Fish and Wildlife Service.
Middle river courses (mature stage)
As rivers leave the hills and mountains where they're created,
they take on the classic pattern of the mature rivers we see in the
landscapes around us. They're wider, slower, less steep, and change
course more gradually. The features they form are bigger and more
substantial: wider lakes,
wandering S-shaped bends called meanders,
and deeper, broader cuts in the landscape called valleys. Sometimes two rivers will join together
at a point called a confluence.
Sediment carved from the upper reaches of the river and carried downstream can build banks called levees that keep the water level higher than the landscape around it. When
flows are high, water spills over the banks carrying mud and sediment
with it and creating marshy floodplains.
As rivers cross floodplains, they snake from side to side eroding the
landscape in some places and building it up other places through a
process called deposition. Rivers are often surrounded by lush
grassland areas called meadows.
Photo: Meanders occur in the middle and lower reaches of a river.
These meanders are at the confluence (joining point) of the rivers Alatna and Koyukuk near Allakaket. Photo by Steve Hillebrand courtesy of US Fish and Wildlife Service.
Lower river courses (old-age stage)
There are no strict boundaries between the upper, middle, and lower
courses of a river, and many middle-course features (like meanders) are also
found in the lowest reaches of a river. But the lower course of a
river is less steep again than the middle course and the water runs
even more slowly. Rivers finally reach the sea at estuaries (wide,
deep, open river mouths) and triangular-shaped deltas (where a river
deposits sediment at its mouth creating many narrower channels called
distributaries instead of a
single, wide mouth).
Photo: Deltas occur at the lowest part of a river's course. Or, to put it another way a delta is the last stage in a river's life before it meets the sea. Aerial view of the Noatak River Delta in Alaska. Photo courtesy of US Fish and Wildlife Service.
How rivers flow
Think of a huge torrent of water flowing under the power of
gravity, for thousands of years. Like the water from a pressure washer, it has enough energy to cut sediment
from the hills of its upper reaches and carry it down to the middle
and lower reaches. Rivers gradually slow down because the water has to
overcome
friction as it flows—friction as the moving water rubs against the
banks and bottom of the
river channel itself and friction as layers of water flow past one
another. The rate at which a river flows is also affected by the
climate and the seasons. Some smaller streams (known as winterbornes)
dry up entirely in summer. Humans also have a huge effect on river
flows through
abstraction (removing water for
many different uses),
from irrigation (watering crops in the countryside) to supplying
factories and homes with the water they need.
Why do rivers matter to people?
Thousands of years ago, human settlements grew up around great
rivers such as the Tigris and Euphrates in the Middle East—and this
was how modern civilization began. Rivers remain, in a sense, the "open
pipes" that supply many towns and cities with the
water people need. Rivers have also been hugely important in helping
people travel across Earth, especially before modern roads were
constructed, both for exploration and trade.
People have been harnessing the energy in rivers since ancient times
and
modern-day environmental problems such as global warming have led to
renewed interest in such things as hydroelectric power (made by
damming rivers and forcing them to flow at high-speed past turbines
to generate electricity).
Photo: Villagers gather on the banks of a river
in Bangladesh waiting for humanitarian aid.
Photo by Anthony J. DeCapite courtesy of US Marine Corps and
US Navy.
Rivers are incredibly important to people,
but we've not always looked after them properly. Removing sand and
gravel from river beds for building materials can damage them, for
example, while pouring untreated sewage and other wastewater into
rivers causes pollution, lowers
water quality, and reduces how much
life they can support. Thankfully, many people recognize just how
important rivers are.
Lots of people use them for recreation, from fishing and canoeing to
walking their dogs along riverbanks. Many people have formed
themselves into community groups to help keep their rivers clean and
protect them from damaging industrial developments. Rivers give us
life and help to look after us—and it's vitally important that we
return the favor!
Further reading
