Geoscience Reference
In-Depth Information
induced by huge volumes of water. Nurek Dam on the Vakhish River in central Tajikistan
is one of the best-documented examples of a large dam causing seismic activity. This part
of Central Asia is tectonically active anyway, but initial filling of the reservoir and each
period of substantial increase in water level was mirrored by significant increases in earth-
quake frequency during the first decade of the dam's lifetime.
The building of a new dam means that any previous inhabitants of the area designated for
the reservoir must be moved. The numbers of people involved can be very large and some
of the biggest schemes in this respect have been in China. The Sanmen Gorge Project on
the Yellow River involved resettling 300,000 people, and the Three Gorges Dam on the
Yangtze River has displaced about 1.2 million people from 13 cities, 140 towns, and more
than 1,000 villages. Governments usually offer compensation to people who are displaced
by a new reservoir, but in many remote areas inhabitants do not possess formal ownership
documents for the land they live on, a problem that can slow or actually prevent legal com-
pensation.
Downstream of a reservoir, the hydrological regime of a river is modified. Discharge, ve-
locity, water quality, and thermal characteristics are all affected, leading to changes in the
channel and its landscape, plants, and animals, both on the river itself and in deltas, estuar-
ies, and offshore. By slowing the flow of river water, a dam acts as a trap for sediment and
hence reduces loads in the river downstream. As a result, the flow downstream of the dam
is highly erosive. A relative lack of silt arriving at a river's delta can result in more coastal
erosion and the intrusion of seawater that brings salt into delta ecosystems. Downstream
changes in salinity due to construction of the Cahora Bassa Dam in Mozambique threaten
mangrove forests at the mouth of the River Zambezi. One knock-on effect of this is a de-
cline in prawns and shrimps, both of which breed in mangroves.
The most dramatic downstream effects have occurred on rivers dammed in several places.
Construction of a series of dams in the 20th century on the Colorado River, one of the most
intensively used waterways in the USA, has severely cut the river's naturally heavy sedi-
ment load, which had led the Spanish explorer Francisco Garces to name it (Rio Colorado
is Spanish for 'red-coloured river'). Before 1930, the river carried more than 100 million
tonnes of sediment suspended in its water each year to the delta of the Gulf of California,
but it delivered neither sediment nor water to the sea from 1964, when Glen Canyon Dam
was completed, to 1981, when Lake Powell behind the dam was filled to capacity for the
first time. Since then, river water has reached the Gulf of California only irregularly, when
discharges from dams allow. On average, the river now delivers an annual sediment load to
the Gulf of California that is three orders of magnitude smaller than the pre-1930 average.
The decline in fresh water and nutrients brought by the river to its estuary and the Gulf
