Geoscience Reference
In-Depth Information
of California has had a huge impact on ecology. One study suggests that the lack of river-
borne nutrients today may have resulted in a 96% decrease in the population of shellfish in
the Colorado River Delta in Mexico.
The effects of dams on river ecology are numerous. Other important drivers of ecological
impacts include changes in river temperature, the amount of dissolved oxygen carried, and
the barrier effect of dams on the dispersal and migration of plants and animals. The dam-
barrier effect on migratory fish and their access to spawning grounds has been recognized
in Europe since medieval times. A statute introduced in Scotland in 1214 required all dams
to be fitted with an opening and all barrier nets to be lifted every Saturday to allow salmon
to pass. The problem certainly continues, however, sometimes with considerable econom-
ic implications. For example, a dramatic decline in catches of Caspian Sea sturgeon, the
source of caviar, during the late 20th century was attributable primarily to the construction
of several large hydroelectric dams on the River Volga and the consequent loss of spawning
grounds.
Disruption to the movements of fish is one of several reasons for a recent movement in
some countries to decommission dams. The small number of dams removed includes those
that no longer serve a useful purpose, are too expensive to maintain, or have levels of en-
vironmental impact now deemed unacceptable. Most dams that have been removed or con-
sidered for removal are on rivers in the USA, but several European countries have also
been involved in dam decommissioning. For example, two dams were destroyed and the
debris cleared from tributaries of the Loire River in France in 1998 as part of a long-term
government management scheme - the Plan Loire Grandeur Nature - for the river and its
basin. A central aim of the scheme is to ensure the environmental protection of the Loire
and to restore the river's salmon population. Removal of the Maisons-Rouges Dam on the
River Vienne and the Saint Etienne de Vigan Dam on the River Allier was designed to re-
store access to salmon spawning grounds.
Land use
Rivers are intimately connected to the landscapes through which they flow, so it should
come as no surprise to learn that any changes in a landscape inevitably affect its rivers.
The way people use landscapes strongly influences rivers in numerous ways at multiple
scales. Clearing natural forest vegetation to provide land for cultivation, for example, is
well known to cause less interception of rainfall, less infiltration of water into the soil,
less evapotranspiration, and more surface runoff, typically causing enhanced rates of soil
