Significant Findings
This report summarizes the current understanding
of floodplain processes and landforms
for the Willamette River and its major tributaries.
The area of focus encompasses the main
stem Willamette River above Newberg and the
portions of the Coast Fork Willamette, Middle
Fork Willamette, McKenzie, and North, South
and main stem Santiam Rivers downstream of
U.S. Army Corps of Engineers dams. These
reaches constitute a large portion of the alluvial,
salmon-bearing rivers in the Willamette Basin.
The geomorphic, or historical, floodplain of
these rivers has two zones - the active channel
where coarse sediment is mobilized and transported
during annual flooding and overbank areas
where fine sediment is deposited during higher
magnitude floods. Historically, characteristics
of the rivers and geomorphic floodplain (including
longitudinal patterns in channel complexity
and the abundance of side channels, islands and
gravel bars) were controlled by the interactions
between floods and the transport of coarse sediment
and large wood. Local channel responses to
these interactions were then shaped by geologic
features like bedrock outcrops and variations in
channel slope.
Over the last 150 years, floods and the
transport of coarse sediment and large wood
have been substantially reduced in the basin.
With dam regulation, nearly all peak flows are
now confined to the main channels. Large floods
(greater than 10-year recurrence interval prior to
basinwide flow regulation) have been largely
eliminated. Also, the magnitude and frequency
of small floods (events that formerly recurred
every 2–10 years) have decreased substantially.
The large dams trap an estimated 50–60 percent
of bed-material sediment—the building block of
active channel habitats—that historically entered
the Willamette River. They also trap more than
80 percent of the estimated bed material in the
lower South Santiam River and Middle and
Coast Forks of the Willamette River. Downstream,
revetments further decrease bed-material
supply by an unknown amount because they limit
bank erosion and entrainment of stored sediment.
The rivers, geomorphic floodplain, and vegetation
within the study area have changed noticeably
in response to the alterations in floods
and coarse sediment and wood transport. Widespread
decreases have occurred in the rates of
meander migration and avulsions and the number
and diversity of landforms such as gravel bars,
islands, and side channels. Dynamic and, in
some cases, multi-thread river segments have become
stable, single-thread channels. Preliminary
observations suggest that forest area has increased
within the active channel, further reducing
the area of unvegetated gravel bars.
Alterations to floods and sediment transport
and ongoing channel, floodplain, and vegetation
responses result in a modern Willamette River
Basin. Here, the floodplain influenced by the
modern flow and sediment regimes, or the functional
floodplain, is narrower and inset with the
broader and older geomorphic floodplain. The
functional floodplain is flanked by higher elevation
relict floodplain features that are no longer
inundated by modern floods. The corridor of present-
day active channel surfaces is narrower, enabling
riparian vegetation to establish on formerly
active gravel bar surfaces.
The modern Willamette River Basin with its
fundamental changes in the flood, sediment
transport, and large wood regimes has implications
for future habitat conditions. System-wide
future trends probably include narrower floodplains
and a lower diversity of landforms and
habitats along the Willamette River and its major
tributaries compared to historical patterns and
today.
Furthermore, specific conditions and future
trends will probably vary between geologically
stable, anthropogenically stable, and dynamic
reaches. The middle and lower segments of the
Willamette River are geologically stable, whereas
the South Santiam and Middle Fork
Willamette Rivers were historically dynamic, but
are now largely stable in response to flow regulation
and revetment construction. The upper
Willamette and North Santiam Rivers retain
some dynamic characteristics, and provide the
greatest diversity of aquatic and riparian habitats
under the current flow and sediment regime. The
McKenzie River has some areas that are more
dynamic, whereas other sections are stable due to
geology or revetments.
Historical reductions in channel dynamism
also have implications for ongoing and future recruitment
and succession of floodplain forests.
For instance, the succession of native plants like
black cottonwood is currently limited by (1)
fewer low-elevation gravel bars for stand initiation;
(2) altered streamflow during seed release,
germination, and stand initiation; (3) competition
from introduced plant species; and (4) frequent
erosion of young vegetation in some locations
because scouring flows are concentrated within a
narrow channel corridor.
Despite past alterations, the Willamette River
Basin has many of the physical and ecological
building blocks necessary for highly functioning
rivers. Management strategies, including environmental
flow programs, river and floodplain
restoration, revetment modifications, and reclamation
of gravel mines, are underway to mitigate
some historical changes. However, there are
some substantial gaps in the scientific understanding
of the modern Willamette basin that is
needed to efficiently integrate these blocks and
to establish realistic objectives for future conditions.
Unanswered questions include:
- What is the distribution and diversity of
landforms and habitats along the
Willamette River and its tributaries?
- What is the extent of today’s functional
floodplain—the part of the river corridor
actively formed and modified by fluvial
processes?
- How are landforms and habitats in the
Willamette River Basin created and sustained
by present-day flow and sediment
conditions?
- How is the succession of native floodplain
vegetation shaped by present-day
flow and sediment conditions?
Answering these questions will produce
baseline data on the current distributions of landforms
and habitats (question 1), the extent of the
functional floodplain (question 2), and the effects
of modern flow and sediment regimes on future
floodplain landforms, habitats, and vegetation
succession (questions 3 and 4). Addressing questions
1 and 2 is a logical next step because they
underlie questions 3 and 4. Addressing these four
questions would better characterize the modern
Willamette Basin and help in implementing and
setting realistic targets for ongoing management
strategies, demonstrating their effectiveness at
the site and basin scales, and anticipating future
trends and conditions.