Riparian Landscapes - Mountains and Plains: The Ecology of Wyoming Landscapes - page 57

the Snake River south of Jackson, blue spruce and red
osier dogwood have formed a relatively stable commu-
nity. in eastern Wyoming, ash and boxelder form simi-
lar woodlands.

Many people living along creeks and small rivers ben-
efit from soil laid down by generations of beaver work-
ing for thousands of years. Some have suggested that the
beaver is a keystone species, affecting ecosystem structure
and function far beyond what might be expected, con-
sidering animal size and population numbers. others
have referred to beaver as bioengineers because of the
structures and services they provide. to illustrate, sedi-
mentation behind their dams provides clearer water
downstream, and greatly enhanced bank storage leads
to sustained streamflow throughout the year (figs. 4.12
and 4.13). A study in colorado found that a valley with
an active beaver colony had eighteen times more water
storage in the spring and higher streamflow in late sum-
mer than a comparable drainage where the beavers had
been removed. 19 Higher late-summer streamflow bene-
fits fish and wildlife as well as landowners. Beavers also
thin woodlands by cutting trees, thereby stimulating
the formation of new sprouts and the growth of under-
story plants. When the beavers are removed, the habitat
can change quite rapidly. 20
Somewhat similar to beaver dams are the debris jams
that commonly occur along wooded creeks (fig. 4.14). 21
Debris jams are initiated most often by a tree that falls
across a stream, or by floating wood and other materi-
als that become lodged in the channel, creating a dam.
Both kinds of dams improve water quality and benefit
some wildlife. 22 However, a common reaction has been
to remove obstructions to streamflow. the dams some-
times appear messy, and they can inundate land that
owners prefer not be flooded. indeed, beavers become
a nuisance when they build dams at road culverts or
when they interfere with irrigation projects.
Beavers and their dams are much less common now
than in the early 1800s. Alluvial sediments originally
deposited behind beaver dams have washed down-
stream in many areas. With fewer dams to dissipate the
energy of spring floodwaters, new gullies are created in
a year or two, sometimes leaving fence posts hanging
from barbed wire in midair. As the channel deepens, the
water table is lowered, and the soils on either side of the
gully become dry earlier in the summer. the riparian
vegetation changes from willows and moist meadows,
which are relatively rare in the landscape and valuable
for livestock and wildlife, to the drier, already wide-
spread vegetation of the adjacent upland. the riparian

Bank Storage, Stream Hydrology, and Beaver

the plant growth on floodplains slows the movement of
floodwater and thereby increases the potential for sedi-
ment deposition when floods do occur. the accumu-
lated sediments increase the amount of water stored in
the bank and floodplain, which, along with other fac-
tors, increases the probability of streamflow through-
out the year. However, maintaining bank storage has
its costs in the form of water consumption by phreato-
phytes—trees and shrubs that live with their roots in
groundwater much of the year. every pound of plant
material produced may require as much as 400 pounds
of water, because up to 99 percent of the water entering
a plant through the root system is lost to the atmosphere
by transpiration. this water is required to keep stomata
open for the uptake of carbon dioxide and to maintain
turgid leaves for better light interception. But water
transpired by plants is not available for downstream
uses. Such losses, along with seepage to groundwater
aquifers and evaporation directly from water surfaces,
are known as conveyance losses. Water consumed by
riparian vegetation is sometimes viewed as unfortunate
in semi-arid environments, but it can also be viewed as
a small price to pay for the benefits of increased forage
and shade for livestock, erosion control, sedimentation
during floods, biological diversity, better wildlife habi-
tat, and sustained late-summer flows—all examples of
ecosystem services.
Few data are available to estimate quantitatively the
amount of water used by riparian vegetation, but two
generalities seem probable: first, streamflow depletion
by phreatophyte transpiration (that is, the proportion
of streamflow used by plants) is higher in small streams
than in rivers; and second, depletion is higher during
late summer, when streamflow is low and transpiration
is more rapid. Preliminary estimates suggest that reduc-
tions in streamflow by phreatophyte transpiration are
less than 2 percent in June when streamflows are high,
and up to about 50 percent in late August when the vol-
ume of low is low.