Physiography of Flowing Water - Freshwater Ecology: Concepts and Environmental Applications - page 82

also fragment riparian habitat, leading to distinct changes in plant com-
munities by altering patterns of dispersal and recruitment (Nilsson et al.,
1997; Jansson et al., 2000). Perhaps surprisingly, effects of reservoirs and
other disturbances can extend to organisms upstream (Pringle, 1997). For
example, interruption of salmon runs can lower nutrient input released
from rotting salmon carcasses into small
streams.

Sidebar 5.1.
Human Impacts on Rivers and Streams
from Damming, Channelization, and
Flood Control Measures

MOVEMENT OF MATERIALS BY RIVERS
AND STREAMS

Many rivers have been channelized and ripar-
ian (streamside) vegetation has been removed
to allow rapid boat travel, increased drainage,
and agricultural and urban expansion. This has
dramatic influences on the shoreline habitat.
For example, a 25-km stretch (as the crow flies)
of the Willamette River in western Oregon had
250 km of shoreline in 1854 (Fig. 5.14), but hu-
man activity decreased it to 64 km by 1967
(Sedell and Froggat, 1984). There was a con-
current loss of at least 41% of the riparian wet-
lands during this time period (Bernert et al.,
1999). This removal of virtually all slow-moving
portions and straightening of meanders is
common in rivers in areas where humans live.
Such alteration has been well documented on
the Missouri River (Hesse et al., 1989). In addi-
tion, removal of large instream obstructions,
such as logjams and stumps, is common and
destroys vital habitat for aquatic organisms.
Flood control measures on many large rivers
include levees to contain high discharge. When
floods do occur, the levees constrain the water,
making it move faster and deeper in the main
channel instead of spreading out across the
floodplain and flowing with a lower average
velocity as it would naturally. If the flood does
breach the levy suddenly, it causes consider-
able damage because of the rapid current ve-
locity when the levee breaks. In addition, such
levees constrain flows and act like dams to up-
stream regions that do not have levees. In the
Mississippi basin, mean annual flood damage
has increased by 140% during the past 90 years.
This increase is probably attributable to in-
creases in numbers of levees and removal of ri-
parian wetlands (Hey and Philippi, 1995).

Rivers carry materials dissolved from
land to the sea. In addition, they move larger
particles by erosional processes. The move-
ment of dissolved materials to the sea has
been altered by human activities (Table 5.4).
Dissolved materials in rivers have increased,
particularly the nutrients nitrogen, (Vi-
tousek, 1994), phosphorus, and sulfur. Thus,
the transport of nutrients in rivers may alter
productivity of coastal marine systems
(Downing et al., 1999). Streams and rivers
play a part in the global carbon cycle by
moving dissolved and suspended organic
materials (e.g., woody debris and leaf frag-
ments) from the terrestrial habitats to the
sea (Table 5.5). In this context, tropical rain
forests, where a large proportion of conti-
nental runoff originates, are extremely im-
portant. Desert and semiarid habitats are
not as important because of their low runoff
(Table 5.3). Thus, understanding global cy-
cles of materials and the effects of global
change requires knowledge of how rivers
move materials through the environment.
Dissolved materials move down streams
as a function of discharge and exchange with
the biotic and abiotic components of stream
channels. If a chemical is added in a defined
pulse to a stream, the pulse will become less
coherent as it moves downstream (Fig. 5.15).
Streams have areas with relatively slow ve-
locity and others with high water velocity,
and this causes spreading of the pulse. In
addition, dissolved materials can interact
with the sediments or biota as they flow
downstream. Departures from the ideal
transport of dissolved materials (i.e., a co-