Physiography of Flowing Water - Freshwater Ecology: Concepts and Environmental Applications - page 83

herent pulse moving through a frictionless, inert channel) can be used to
investigate the retentive properties of the channel and the effect of the hy-
porheic zone (Webster and Ehrman, 1996). The hyporheic zone can play a
significant role in retention of nutrients (Hill and Lymburner, 1998).
In addition to the removal of dissolved materials, more and larger
particles can be moved downstream as dis-
charge increases (Fig. 5.16). Mobile mate-
rial can be divided into two categories: sus-
pended load and bed load. The suspended
load is the fine material that is suspended
in water under normal flows. This sus-
pended load can also be referred to as tur-
bidity or total suspended solids . The bed
load moves along the bottom by sliding,
rolling, and bouncing. It never moves more
than several particle diameters above the
bottom. The relative movement of both
types of particles and the size of the parti-
cles that can remain suspended depend
on the water velocity and turbulence
(Fig. 5.17). Streams become turbid after a
rain. This turbidity is partially attributable
to increased sediment inputs from land, but
it is also related to the higher water veloc-
ity that can transport more materials from
within the channel.
Turbidity is not evenly distributed in
rivers. The total concentration of suspended
materials is higher near the bottom (Fig.
5.18A). The trend of greater concentration
with depth does not hold with the finest par-
ticles (such as clay), which remain in sus-
pension throughout the water column. The
effect becomes more pronounced with larger
particles such as sand (Fig. 5.18B).
Understanding the movement of solid ma-
terials can be very important to understand-
ing the ecology of streams. Floods cause ero-
sion and can alter habitat. Some species of
fish and invertebrates cannot survive when
the amount of silt is too great. In rivers with
contaminated sediments, transport of those
sediments by flooding can lead to pollution
events because pollutants adsorb to the sur-
face of sediment particles. For example, in the
Clark Fork River in Montana, years of min-
ing have led to contaminated sediments in the
basin. Each time it rains hard, local fisherman
hold their breath; massive trout kills will

Landowners often channelize smaller
streams flowing across their property so they
can develop closer to the edge of the stream
and drain their land more quickly. This chan-
nelization causes the water to move down-
stream at higher velocity, increasing erosion.
Channelization also causes the stream to have
stronger flooding impact downstream, leading
more landowners to channelize their stream
banks. Removal of riparian vegetation creates
an even worse situation because natural re-
tention of sediment and slowing of floodwater
does not occur, thus increasing the severity of
floods.
Human alteration of stream and river hy-
drology has effects across all spatial scales.
At the smallest scales (Paragamian 1987), it re-
moves habitat for aquatic organisms. At inter-
mediate scales, alterations of stream hydrol-
ogy are associated with increased erosion and
more severe flooding. At the largest scales,
human activities lead to global changes in the
transport of materials by rivers. The economic
impact of all these human influences is likely
very great. Effective management requires un-
derstanding of dynamics of natural rivers (Poff
et al., 1997). Complete restoration of large rivers
impacted by humans is unlikely, but partial re-
habilitation may be possible (Gore and Shields,
1995). A new trend in dam removal seems to be
developing with the hope that some of the more
harmful and less useful impoundments can be
removed. There have been at least 467 docu-
mented dam removals in the United States
since 1912 for environmental, safety, economic,
or other reasons (American Rivers 1999). These
removals have had some beneficial effects,
but some of the degradation from dams to
aquatic habitats is irreversible (Middleton,
1999).