River Ecology and Restoration - River Dynamics and Integrated River Management - page 645

Fig. 10.19 Wenjiagou Landslide in Mianzhu City buried the stream and vegetation

vegetation on slopes were buried underneath the 180 m thick landslide. Both faunal and floral communities
have been totally destroyed and the restoration needs a long period of time.
Erosion and sedimentation often are the direct cause of ecology impairment. Figure 10.20(a) shows the
high sediment concentration in a stream in Taiwan, southeastern China, which causes a strong stress on the
aquatic biocommunity. The sediment results from intensive soil erosion caused by a rainstorm. The high
concentration results in low transparency, low dissolved oxygen, and sediment coating the substrate.
Benthic animals and fish may be killed during the high concentration event. Figure 10.20(b) shows the
turbid seawater with a high concentration of sediment on the east coast of Taiwan. The sediment is
transported into the ocean by debris flows and hyperconcentrated flows. Tidal current and waves bring
the sediment onto the shore and bays, which impacts on fish and invertebrate communities.
Stream ecology is influenced by certain animal activities. For example, beavers build dams that cause
ponds to form within a stream channel or in the floodplain. Figure 10.21(a) shows that a couple of beavers
skillfully use natures building materials and construct a wood dam with tree branches on the Spring Pond
in Pennsylvania; and Fig. 10.21(b) shows the 3 m high beaver dam forms a pond, which provides a good
habitat for fish and birds. Without any machines the beavers transported so much building materials and
built the dam within several months. The landlord of the Spring Pond, Mr. R. Devries pronounced that
there is no way for humans “could ever match their dam skills, their dam resourcefulness, their dam
ingenuity, their dam persistence, their dam determination and their dam work ethic”.
Of course the dam construction by beavers disturbs the stream ecology. The pond kills much of the
existing vegetation. Moreover, if appropriate woody plants in the floodplain are scarce, beavers extend
their cutting activities into the uplands and can significantly alter the riparian and stream corridors. The
sequence of beaver dams along a stream corridor may have major effects on hydrology, sedimentation,
and mineral nutrients (Forman 1995). Silts and other fine sediments accumulate in the pond rather than
being washed downstream. On the other hand the aquatic ecological conditions are improved by the
beaver dams. Water from storm flow is held back, thereby affording some measure of flood control.
Wetland areas usually form, and the water table rises upstream of the dam. The ponds combine slow flow,
near-constant water levels, and low turbidity that support fish and other aquatic organisms. Birds may
use beaver ponds extensively.