Geoscience Reference
In-Depth Information
banks, but this removal of earth, pebbles, and sometimes boulders gives way to the depos-
ition of material in lower reaches. In tune with these variations in the physical characterist-
ics of the river, changes can also be seen in the types of creatures and plants that make the
river their home.
Its narrow, linear form and its flow in just one direction provide an obvious spatial di-
mension to how we should describe and understand the physical, chemical, and biological
properties of a river: horizontally from upstream to downstream. But a river is not just a
channel. It is also an integral part of the countryside through which it flows, so a lateral
dimension, to the surrounding landscape, is also appropriate. The links with the landscape,
or riverscape as some would prefer, are innumerable. They range from the simple fact that
most water in a river arrives in the channel after flowing across the surrounding topography
to the importance of salmon in a river, say, as a seasonal source of food for local bears.
A third dimension is vertical. Rivers interact with the sediments beneath the channel and
with the air above. The water flowing in many rivers comes both directly from the air as
rainfall - or another form of precipitation - and also from groundwater sources held in
rocks and gravels beneath, both being flows of water through the hydrological cycle.
The vital fourth dimension, time, also has an important place in river research. This is be-
cause of profound variations in many factors that affect rivers, not least the amount of wa-
ter flowing in them. This varies on a wide range of timescales, from an intense rainstorm
that lasts less than an hour to the effects of tectonic forces that operate over many millions
of years.
Rivers are found all over the world and have left their mark on virtually every landscape.
Certain areas lack surface drainage, but in some of these regions rivers flow beneath the
land surface. In deserts, many rivers remain dry for most of the year, only channelling water
in response to a sporadic rainstorm. Elsewhere, fossil channels and valleys indicate where
rivers have flowed at some time in the more distant past. Such fossilized features also occur
on other planets: channels and valleys have been identified on Mars and on Titan - the
largest of Saturn's moons - and these networks are remarkably similar to river and stream
features on Earth. On the surface of Mars, these features have been sculpted by flowing
water in times past, but the river channels and drainage networks on Titan are thought to
have been formed by the flow of liquid methane. For most of our planet's land surface, a
flowing river of water is one of the most fundamental elements. Supplied with energy from
sunlight and gravity, it is a feature that moulds valleys and slopes and provides a complex
habitat for living communities.
