River Ecology and Restoration - River Dynamics and Integrated River Management - page 639

north is made possible by rivers together with other, useful habitats. After all, birds can only fly a certain
distance before they need to eat and rest. For rivers to function effectively as conduits for these birds,
they must be sufficiently connected and be broad enough to provide the habitat required for migratory
birds.
The migration of salmon upstream for spawning has been extensively investigated and is a well known
example of the movement of aquatic organisms and interactions with the habitat. A conduit to their
upstream spawning grounds is very important to the salmon which mature in a saltwater environment. In
the case of the Pacific salmon species the stream corridor depends on the nutrient input and biomass of
dying fish and plentiful spawning in the upstream reaches. So, not only are conduits important for the
movement of aquatic biota but also for the transport of nutrients from ocean waters upstream.
Stream corridors are also conduits for the movement of energy, which occurs in many forms. Heat is
transported with flowing water along a stream, as shown in Fig. 10.13. The potential energy of the stream
is provided by gravity, which alters and carves the landscape. The corridor modifies heat and energy
from sunlight as it remains cooler in spring and summer and warmer in the fall. Stream valleys move
cool air from high to low altitudes in the evening, and, therefore, are effective air-sheds. The energy built
up by the productivity of plants in a corridor is stored as living plant material and it moves into other
systems by leaf fall and detritus.
Seeds may be carried for long distances by flowing water and then deposited. Whole plants may be
uprooted, transported, and then deposited, still living, in a new area by strong floods. Plants are also
transported when animals eat and transport their seeds throughout different parts of the river. Some
riparian habitats depend on a continuous supply and transport of sediment, although many fish and
invertebrates can be harmed by excess fine sediment.
Fig. 10.13 A stream is a flow pathway for heat, water, and other materials, and organisms as shown for a small tributary
of the Songhua River in northeastern China (See color figure at the end of this topic)
10.1.4.3 Filter and Barrier Functions
Stream corridors may act as filters, allowing selective penetration of energy, materials, and organisms,
they may also act as a barrier to movement. In many ways, the entire stream corridor serves beneficially as
a filter or barrier that reduces water pollution, minimizes sediment transport, and often provides a natural
boundary to land uses, plant communities, and some less mobile wildlife species as shown in Fig. 10.14.