Geography Reference
In-Depth Information
greater extent. River floodplain and riparian corridors
at the time of settlement were heavily forested by Ore-
gon ash, black cottonwood, bigleaf maple, white alder,
and several willow species in the lowlands. Montane
rivers in the basin contained coniferous riparian zones
mixed with red alder and willow. More than 80% of
the riparian areas of the Willamette River were covered
by mature forests, which extended 2-10 km wide along
the mainstem Willamette River (Gregory et al., 1998,
Gregory et al., 2002b). In 1850, prairies and oak wood-
lands that were major features of the Willamette Valley
floor were minor components of the floodplain vegeta-
tion. Of the 445 km of mainstem river channels (both
mainstem and side channels), forests and woodlands that
contained hardwoods occupied 68% of the length, mixed
forests 14% and conifer forests 7% (Gregory et al., 2002f).
Prairie and wetland habitats have been reduced exten-
sively throughout the Willamette Valley, but these open
habitats historically accounted for only 6% of the ripar-
ian margin. By 1990, the riparian length of hardwood
forests was 17%, mixed forests 18%, and conifer forest
2%, reducing wooded riparian forests from 89% of the
riparian length to 37% of the riparian length.
Longitudinal patterns of the floodplain forest extent
are illustrated in 1-km bands or slices of the floodplain
from the upper mainstem river to its mouth (Figure 12.2).
By 1990, the extent of floodplain forest was reduced to
less than 10% of the historical floodplain area. Floodplain
bands of 1-km in length generally contained more than
200 ha of forest in the more geomorphically complex
middle and upper river. By 1990, not a single reach of
the Willamette River floodplain contained 200 ha/km
and
4th order) were 25% conifer, 25% hardwood, and 5%
mixed forest (Gregory et al., 2002b). By 1990, these forest
types were reduced to less than 10% for conifer and
hardwood forests and increased to 13% for mixed stands,
representing a loss of more than 40% of the riparian areas
along small streams. Wetlands decreased from 14% to 1%
of the riparian area.
Land conversion and fire suppression around homes,
farms, and towns have altered terrestrial land cover, elim-
inating most of the prairie and oak savanna habitat, both
of which are fire-dependent. Upland forests are in public
and private ownership and are still dominated by conif-
erous forests. However, timber harvest has shifted the age
of the upland forests, reducing the extent of conifers older
than 80 yr by approximately 66%. Consumption of sur-
face water for irrigation, municipal and industrial water
uses removes more than 1,000 m
3
of water/day, dewater-
ing an estimated 130 km of 2nd to 4th order streams in
moderately dry summers. None of these streams would
be expected to become dry in the absence of human
withdrawals (Niemi et al., 2002).
Land conversion between 1850 and 1990 caused major
changes in terrestrial and aquatic habitats in the WRB. All
models of natural resources (fish, aquatic invertebrates,
wildlife, riparian forests, forest cover, surface water avail-
ability) exhibited major declines from pre-EuroAmerican
settlement to the present (Figure 12.4; Baker et al., 2004).
Most of the natural resources we used as ecological indi-
cators were 15 to 90% more abundant historically than
today, depending on the specific resource.
most
had
been
reduced
to
less
than
50 ha/km
12.4 Plan trend 2050 scenario
(Gregory, 2002i).
In 1850, the Willamette River was physically more
complex than in 1990 (Figure 12.3). Channelisation and
bank hardening have straightened the mainstem, reducing
total length of river channels by 22%, area of off-channel
sloughs by 30%, and islands by over 60% (Gregory, 2007).
Prior to settlement and river modification, the floodplains
of the upper Willamette River contained as much as 11 km
of channel length within a 1-km floodplain distance, and
most of the reach included 4-8 km of channel per km
of floodplain (Figure 12.3). By 1995, no reach exhibited
more than 7 km of channel/km of floodplain and most
were less than 2 km of channel/km of floodplain (Gregory
et al., 2002h).
Along smaller streams, riparian areas and wetlands
also changed greatly from 1850 to 1990. In the lowlands,
riparian areas within 120 m of small streams (1st to
In spite of the projected doubling of the human popula-
tion in the WRB from 2 million to roughly 4 million by
2050, land conversion from 1850 to 1990 was far greater
than the additional land conversion projected to occur
by 2050 regardless of the future scenario (Figure 12.4).
Future landscape changes will, to a greater extent, shift
land uses on already converted lands as opposed to con-
verting intact native landforms and streams to additional
urban and agricultural land use. Existing public lands and
unimpacted, natural ecosystems are likely to be conserved.
New urban or rural residential areas will be developed on
lands that previously had been converted to agriculture.
Though future land conversion is not projected to be as
spatially extensive as historical land conversion, projected
outcomes for alternative scenarios differed substantially
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