River Ecology and Restoration - River Dynamics and Integrated River Management - page 655

levels are often reduced in urban streams (Simmons and Reynolds, 1982). Another modification unique
to urban streams is the installation of sanitary sewers underneath or parallel to the stream channel.

Fig. 10.32 Effects of different amounts of impervious cover on the water balance for a watershed (after FISRWG, 1997)

The water quality of urban streams during storm events is consistently poor. Urban storm water runoff
contains moderate to high concentrations of sediment, carbon, nutrients, trace metals, hydrocarbons,
chlorides, and bacteria (Schueler, 1987). Large woody debris is an important structural component of
many small rivers, creating complex habitat structure and generally making the stream more retentive. In
urban streams, the quantity of large woody debris found in stream channels is reduced due to the loss of
riparian forest cover, storm washout, and channel maintenance practices (May et al., 1997). Many river
crossings can become partial or total barriers to upstream fish migration, particularly if the streambed
erodes below the fixed elevation of a culvert or a pipeline. The important role that riparian forests play in
stream ecology often is diminished in urban watersheds since tree cover is often partially or totally
removed along the stream as a consequence of development (May et al., 1997). Figure 10.33 shows a
tributary of the Jialing River (a tributary of the Yangtze River) flowing through the Guangyuan that has
been greatly affected by urbanization. Riparian forest has been replaced by residential buildings and the
banks are hardened with concrete. The flow discharge in low the flow season has been reduced due to
water diversion.
Agriculture and land-use change —Land-use change is the most common human-induced stress on
the ecosystem. Agricultural activities have generally resulted in encroachment on stream corridors.
Producers often crop as much productive land as possible to enhance economic returns; therefore, native
vegetation is sacrificed to increase arable areas. As the composition and distribution of vegetation are
altered, the interactions between ecosystem structure and function become fragmented. Vegetation removal
from stream banks, floodplains, and uplands often conflicts with the hydrologic and geomorphic functions
of stream corridors. These disturbances can result in sheet erosion, rill erosion, and gully erosion, reduced
infiltration, increased upland surface runoff and transport of contaminants, increased bank erosion,
unstable stream channels, and impaired habitat.