Geoscience Reference
In-Depth Information
upper reaches ranks as one of the worst natural disasters in US history, destroying or ser-
iously damaging more than 40,000 buildings. Heavy rain caused the river to breach the
levées in more than 1,000 places, and in many locations flooding was prolonged because
levées prevented the return of water to the channel once the peak had passed. It also seems
very likely that efforts to manage the flood hazard on the Mississippi have contributed to
an increased risk of damage from tropical storms on the Gulf of Mexico coast. The levées
built along the river have contributed to the loss of coastal wetlands, starving them of sed-
iment and fresh water, thereby reducing their dampening effect on storm surge levels. This
probably enhanced the damage from Hurricane Katrina which struck the city of New Or-
leans in 2005.
Urban rivers
Cities have had numerous impacts on rivers, starting with the rise of the first urban civil-
izations which emerged on the floodplains of large rivers in several parts of the world a
henjo Daro in the Indus Valley have revealed ceramic pipes designed to supply water and
brick conduits under the streets for drainage that are thought to have been in operation
5,000 years ago. The Romans are also well known for their sophisticated water-supply sys-
tems. Water was brought to Ancient Rome from distant streams and springs via nine major
aqueducts. Some of these were more than 60 kilometres in length and involved tunnelling
through difficult hillsides with vertical shafts dug for inspection and cleaning.
Large amounts of water were involved in these early municipal systems, but they were
ultimately limited in the volume of water managed by the force of gravity. Water could
be transferred from one place to another only as long as the direction was down a slope.
Modern civilization has hugely increased its ability to move water by using energy to pump
water. In the southwestern USA, for example, water from the Colorado River is pumped
nearly 500 kilometres across the Mojave Desert to large cities on the west coast of Califor-
nia, including Los Angeles and San Diego.
The growth and development of urban areas - the process of urbanization - is frequently
associated with such changes to river systems, some deliberate, others inadvertent. Delib-
erate manipulation of rivers can be on a significant scale. In Japan, for instance, the city
of Tokyo began to develop rapidly after the mid-17th century realization of the Tone River
Easterly Diversion Project, a grand scheme that took more than 50 years to complete and
involved diverting the Tone River more than 100 kilometres to the east to prevent flooding