Geoscience Reference
In-Depth Information
3. A storm hydrograph showing river discharge changing in response to a rainfall
event
Measuring the flow of rivers and analysing the records is important for evaluating water
resources and the assessment of flood and drought hazards. No river has a longer hydrolo-
gical record than the Nile, where a water-level gauging structure was built on Roda Island
at Cairo in AD 641. The official in charge of the Roda 'nilometer', the Sheikh el Mikyas,
had the duty of observing the water level and during times of flood announcing the daily
rise via public criers. This was always a tense time of year in Egypt. If the river did not
reach a certain level, much cropland would go without water and famine could be expec-
ted, but at a certain higher level irrigation was assured, and with it taxes to the government.
The position of Sheikh el Mikyas continued for more than 1,000 years. The last holder of
the post died in 1947, and in the 1950s the Egyptian government decided to construct a
major dam on the Nile at Aswan, thus significantly changing the country's intimate rela-
tionship with its river. The record from the Roda nilometer was invaluable in calculating
the required storage capacity of the Aswan High Dam which was finally completed in 1970
River flow is dependent upon many different factors, including the area and shape of the
drainage basin. If all else is equal, larger basins experience larger flows. A river draining a
circular basin tends to have a peak in flow because water from all its tributaries arrives at
more or less the same time as compared to a river draining a long, narrow basin in which
water arrives from tributaries in a more staggered manner. The surface conditions in a basin
are also important. Vegetation, for example, intercepts rainfall and hence slows down its
movement into rivers.
Climate is a particularly significant determinant of river flow. It is the major factor con-
trolling the different types of flow identified above: perennial, intermittent, ephemeral, and
interrupted. All the rivers with the greatest flows are almost entirely located in the humid
tropics, where rainfall is abundant throughout the year. These are the Amazon, the Congo,
and the Orinoco, each of which discharges more than 1,000 cubic kilometres of water into
the oceans in an average year.
Rivers in the humid tropics experience relatively constant flows throughout the year, but
perennial rivers in more seasonal climates exhibit marked seasonality in flow. The Indus
River receives most of its water from the Himalayan mountains and the maximum summer
discharge is over 100 times the winter minimum due to the effect of snowmelt. Minimum
discharge is often zero in rivers flowing largely in areas of high latitude and high altitude,
