Geoscience Reference
In-Depth Information
where temperatures fall below freezing point for a portion of the year. In these intermit-
tent rivers, the distinct contrast between minimum flow during the frozen winter and great
floods during the summer melt season is regular and predictable.
By contrast, the flow of ephemeral rivers, typically found in desert areas, is spasmodic and
unpredictable. This is because ephemeral rivers respond to rainfall which is notoriously dif-
ficult to predict in many deserts. One study of a river bed in the northern Negev Desert in
Israel showed that on average the channel contained water for just 2% of the time, or about
seven days a year. Some desert rivers can go for an entire year without any flow.
Year-to-year variations in river flow are also greatest in dry climates, whereas perennial
rivers in the humid tropics have relatively steady flows from one year to the next. Records
of discharge in the middle reaches of the Kuiseb River in the Namib Desert in Namibia
over several decades show that flow has varied from 0 to 102 days per year.
Over longer periods, changes in rainfall and temperature have also resulted in changes in
river flow regimes, although human interference has confused the picture in many cases
where the desert-marginal belt to the south of the Sahara known as the Sahel experien-
ced a marked desiccation of the climate over the last few decades of the 20th century, a
trend that has continued into the 21st century. The flow of the Senegal River measured at
Bakel, near the meeting of the borders between Senegal, Mauritania, and Mali, showed a
marked decline towards the end of the last century. The average annual discharge at Bakel
for the period 1904-92 was 716 cubic metres per second, but that average was just 379 cu-
bic metres per second over the period 1972-92. The flow in 1984, a particularly dry year,
averaged out at 212 cubic metres per second. A similar picture has been seen on the Niger
River.
Some rivers are large enough to flow through more than one climate region. Some desert
rivers, for instance, are perennial because they receive most of their flow from high rainfall
areas outside the desert. These are known as 'exotic' rivers. The Nile is an example, as is
the Murray in Australia. These rivers lose large amounts of water - by evaporation and in-
filtration into soils - while flowing through the desert, but their volumes are such that they
maintain their continuity and reach the sea. By contrast, many exotic desert rivers do not
flow into the sea but deliver their water to interior basins. In southern Africa, water from
the tropical highlands in Angola flows in the Okavango River to the Okavango Delta, the
large wetland area in the Kalahari Desert in northern Botswana. In central Asia, water from
the Parmir Mountains flows into the Aral Sea via two of central Asia's major exotic rivers:
the Syr Darya and Amu Darya.
