River Ecology and Restoration - River Dynamics and Integrated River Management - page 648

modification of stream hydraulics directly affects the system, causing an increase in the intensity of
disturbances caused by floods. Chemically defined disturbance effects, for example, can be introduced
through many activities including discharging sewage and wastewater (acid mine drainage and heavy
metals) into the stream. Ecological disturbance effects are mainly to the result of the introduction of
exotic species. The introduction of exotic species, whether intentional or not, can cause disruptions such
as predation, hybridization, and the introduction of diseases. For instance, bullfrogs have been introduced
into the western U.S. They reproduce prodigiously and prey on numerous native amphibians, reptiles,
fish, and small mammals and cause biological problems in the ecosystem. Altering the structure of plant
communities can affect the infiltration and movement of water, thereby altering the timing and magnitude
of runoff events.
Dams —Ranging from small temporary structures to huge multipurpose structures, human constructed
barriers can have profound and varying impacts on stream corridors. The effect of disturbances resulting
from barriers used for river impoundment on water quality, sediment transportation, and ecology are
discussed extensively in Chapter 7. Barriers affect resident and migratory organisms in stream channels.
Power plants may kill fish when they swim through the turbines. Figure 10.22 (a) shows that many birds
are searching for dead fish at the outlets of a hydro-power plant in Korea, which were killed when they
swam through the turbine; Figure 10.22 (b) shows that the Baozhusi Dam on the Bailong River in
Sichuan Province has cut off the river flow. The stream ecology of the lower reaches has been greatly
affected. The dam blocks or slows the passage and migration of aquatic organisms, which in turn affects
food chains associated with stream ecological functions.
(a) (b)
Fig. 10.22 (a) Birds are searching for dead fish at the outlets of a hydro-power plant at which fish are killed when
they swim through the turbine; (b) The Baozhusi dam on the Bailong River has cut off the flow and greatly affects the
stream ecology in the lower reaches (See color figure at the end of this topic)
The Colorado River watershed is a 627,000 km 2 mosaic of mountains, deserts, and canyons. The
watershed begins at over 4,000 m in the Rocky Mountains and ends at the Sea of Cortez. Many native
species require very specific environments and ecosystem processes to survive. Under natural conditions,
the basin's rivers and streams were characterized by a large stochastic variability in the annual and
seasonal flow levels. This hydrologic variability was a key factor in the evolution of the basin's
ecosystems. Today over 40 dams and diversion structures control the river system and result in extensive
fragmentation of the watershed and riverine ecosystem.