Geoscience Reference
In-Depth Information
Chapter 1
Nature's driver
Water is the driver of Nature.
Leonardo da Vinci (1452-1519)
(Italian painter, architect, and engineer)
We live on a wet planet. Water is the most abundant substance on Earth and covers two-
thirds of its surface. It is also found in smaller quantities in the air we breathe, the plants and
animals we see, and the ground on which we tread. This water is continuously on the move,
being recycled between the land, oceans, and atmosphere: an eternal succession known as
the hydrological cycle. Rivers play a key role in the hydrological cycle, draining water from
the land and moving it ultimately to the sea.
Any rain or melted snow that doesn't evaporate or seep into the earth flows downhill over
the land surface under the influence of gravity. This flow is channelled by small irregular-
ities in the topography into rivulets that merge to become gullies that feed into larger chan-
nels. The flow of rivers is augmented with water flowing through the soil and from under-
ground stores, but a river is more than simply water flowing to the sea. A river also carries
rocks and other sediments, dissolved minerals, plants, and animals, both dead and alive. In
doing so, rivers transport large amounts of material and provide habitats for a great variety
of wildlife. They carve valleys and deposit plains, being largely responsible for shaping the
Earth's continental landscapes.
Rivers change progressively over their course from headwaters to mouth, from steep streams
that are narrow and turbulent to wider, deeper, often meandering channels. From upstream to
downstream, a continuum of change occurs: the volume of water flowing usually increases
and coarse sediments grade into finer material. In its upper reaches, a river erodes its bed and
