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Lower Gales Creek Enhancement Planning
Geomorphic Assessment • Technical Study
Stream channels function in a physical sense to transport
watershed products, including water, sediment, woody
debris, and nutrients, to the lower end of the catchment.
All of the fundamental characteristics of the channel,
such as planform, capacity, and width-depth ratio, are
reflective of the quantity and characteristics of watershed
products supplied to the channel, and eventually transported
through it. Changes in the quantity or characteristics
of watershed products supplied to the channel are likely
to result in changes in fundamental channel characteristics,
although the link between the watershed and the channel
is complex and specific channel response to watershed
changes may be difficult to predict (Lisle, 1999).
The supply of watershed products to the stream channel
is to a great extent determined by geology and climate.
Often termed independent variables in models of channel
response, geology and climate do not respond to other
factors governing channel behavior, and are not influenced
by human management. The influence of these independent
variables on channel behavior is felt across the entire
watershed. Topography and watershed gradients, which
sensitively control the rate of erosion, are dictated
by tectonic activity and subsequent fluvial erosion.
The quantity and size of bedload and suspended load
sediments available for transport by the channel are
a function of the erodibility of rocks in the watershed
and their mode of transport from hillslope to stream
channel. Climate-driven precipitation determines the
amount and timing of water and sediment supplied to
the channel. Geologic and climatic histories are also
important influences on the delivery of watershed products;
for example, the effects of higher past erosion rates,
driven by a wetter climate, still influence how erosion
occurs today.
The transport of watershed products through the stream
system is also highly influenced by climate and geology.
Large-scale geologic features such as faults, landslides
or bedrock constrictions influence the stream profile
gradient, the continuity of sediment transport down-valley
during floods, and the storage of sediment and wood
on the floodplain (Grant and Swanson, 1995; Benda, 1990;
Miller, 1994). The magnitude, timing and duration of
floods have significant influence over rates of sediment
transport. The study area on Gales Creek that is the
focus of this project is located at a point in the watershed
where the gradient declines slightly and the valley
opens up into a broad alluvial plain that merges into
the Tualatin Valley. Consequently, the upper portion
of the study area was historically dominated by coarse
sediment deposition that created a complex channel and
floodplain valley form.
Channel morphology through the four mile study reach
that is bounded by Prickett Creek on the downstream
end and Iler Creek on the upstream end is much different
today than it was when Europeans arrived in the 19th
Century. Historically, the channel through the study
reach most likely consisted of a mosaic of primary and
secondary channels that avulsed and changed coarse in
response to high flow events that delivered coarse sediment
and debris. Gales Creek was most likely at, or nearly
at-grade with the existing valley floor and had extensive
backwater channels and wetlands that were formed by
preceding flood events. The vegetation on the valley
floor most likely consisted of a mix of hardwood and
coniferous species that formed a dense understory and
canopy. The dense understory was most likely thick with
downed logs that created a rough channel and floodplain
surface that obstructed flow, encouraged formation of
new flow paths, and resulted in deposition of coarse
sediment delivered from large landslides and debris
flows in the upper watershed and adjacent tributaries.
Dunne and Leopold (1978) define the floodplain as
the “flat area adjoining a river channel constructed
by the river in the present climate and overflowed at
times of high discharge”. Again, although this
appears to be a simple definition, on closer examination
the reality is more complex. For example, the flat area
in this definition is a landform constructed primarily
by slow lateral migration and overbank deposition. In
developing a technique for channel classification, Rosgen
(1994), working from the Dunne and Leopold concepts
of bankfull discharge and floodplain formation, notes
that the active floodplain is the area of the valley
flat above bankfull discharge and below a flood prone
stage, twice the maximum bankfull flow depth. He notes
that this may include both active flood plain and low
terrace (a former floodplain abandoned due to climatic
or other changes) (Rosgen, 1994).
During floods, localized erosion and deposition occurs
on the floodplain, resulting in a highly varied microtopography.
Sediment deposition on the floodplain is a key element
in establishing new riparian vegetation, as is localized
erosion, which provides growing areas in proximity to
the water table. Also, log jams and woody debris act
as hydraulic controls in the channel, and influence
groundwater elevation throughout the floodplain, increasing
the amount of time that soil moisture is available during
the growing season, and increasing the overall density
of vegetation. Woody debris also plays a key role in
stabilizing the floodplain by providing resistance to
erosion in flood channels, storing and sorting sediment
in localized areas, and preventing widespread erosion
by resisting the tendency of flood flows to concentrate.
Individual trees or downed logs break up floodplain
flow paths.
The heterogeneous nature of the floodplain due to
these processes contributes to the future recruitment
of large trees and woody debris. Recent deposits of
flood sediment deliver nutrient rich deposits of fine
sediment onto the floodplain and thus provide suitable
establishment areas for riparian vegetation. Areas of
nutrient rich soil in areas of high roughness become
favorable for the regeneration of large trees, providing
for the next generation of large woody debris. This,
then, perpetuates the long-term supply of woody debris,
and provides for a steady state with respect to the
level of resilience within the system.
Remnants of historic primary and secondary channels
can be seen on the modern valley floor where the primary
land use is agriculture. Today, several of these channel
remain as small backwater channels dense with riparian
vegetation that are cut off from their connection to
the main channel by farm access roads. These channels,
though not functioning biologically or morphologically
as they did in the past, still provide some function
by collecting and filtering farm runoff, thereby reducing
pollutant loads to Gales Creek. These backwater channels
also appear to have maintained a groundwater connection
to the river and therefore may provide some wildlife
benefit. Old primary channels can also be observed on
the valley floor. They primarily persist today as dips
in the valley floor and function primarily as drainage
swales for farm runoff. Over time, these channels have
been smoothed through filling and other modifications.
Their form and relative depth provides indicators that
they once were a primary flow paths for Gales Creek.
To generate a comparison of the spatial extent of
the historic floodplain through the study area as compared
to the present extent we used a combination of recent
and historic aerial photos and USGS 1:24,000 topographic
maps. Mapping historic floodplain extent was based on
observed indicators in the field, assumptions about
the relationship between the primary channel elevation
and the elevation of the valley floor, and the presence
of coarse scale morphological features of the valley
floor such as its relative flatness, a marked break
in slope between the adjacent hillsides and the valley
floor, the presence of alluvial fan morphology at the
mouths of tributary valleys, and its location relative
to the larger Tualatin Valley landform. Based on these
assumptions, the entire valley floor was mapped as floodplain
producing the region presented in Figure 4. Between
Iler and Pickett Creeks it was estimated that a total
of 1,205 acres of floodplain used to exist on the valley
floor.
The mid-19th century to the early 20th century was
most likely a period of rapid change in land use and
stream morphology on the lower Gales Creek valley floor.
The large conifers that were present on the valley floor
were often the first to be removed by an early wave
of settlers to the area. The trees in the valley floor
were large and grew quickly due to the presence of deep,
fertile soils and year-round access to moisture in the
valley bottom. Following removal of much of the marketable
timber, agriculture took hold on the fertile soil. Development
of agriculture requires clearing land, building levees,
and controlling local and tributary drainage. Over time,
this process affected most of the valley floor, confined
Gales Creek to the edges of the valley to maximize usable
farmland, and exacerbated future channel incision along
much of lower Gales Creek. A morphology consisting of
multiple channels, full of large woody material and
high quality spawning and rearing areas for salmon,
was confined to a relatively narrow corridor with a
morphology that is efficient at moving both water and
the watershed products that were delivered to it (e.g. – sediment,
large woody material, etc). Consequently, the watershed
products that historically created habitat complexity
are now transported through the system, or have greatly
reduced residence times, which results in simplified
morphology and habitat.
To evaluate the degree to which floodplain extent
has been reduced over the last several decades, a series
of aerial photos were obtained. Photos were obtained
for 1974, 1989, and 2001 and the extent of riparian
vegetation was mapped using each photo set. The extent
of riparian vegetation was used as a proxy for floodplain
extent since it was the most visible representation
of where floodplain areas may be present and functioning.
Though floodplain may occur outside of these areas,
the additional floodplain most likely only floods during
extremely high events and does not contain the elements
necessary to allow for proper functioning of these areas
as riparian or wetland habitat. Conversely, areas mapped
as floodplain because they contained riparian vegetation
may not be part of the functioning floodplain. They
may in fact be fairly recently revegetated areas that
are on terraces and therefore are not regularly flooded.
Despite these limitations, we feel this approach provides
the best means of estimating changes in floodplain extent
over time.
Figure 5 shows floodplain extent for the 1974 and
1989 images. Riparian extent was mapped onto the 2001
aerial photos since those were digitally registered
and therefore provided a way to compare acreages between
photo sets. The 1974 and 1989 images were provided to
us as hard copies by the Oregon Department of Forestry
office in Salem. Figure 4 shows riparian extent overlain
on the historic floodplain areas. The 1,205 acres of
floodplain present in the early 19th century was reduced
to approximately 292 acres by 1974, to 245 acres by
1989, and to 238 acres by 2001. Floodplain extent was
reduced by 18% between 1974 and 2001, or a reduction
of approximately 2 acres per year.
3.2 Implications of Morphologic
Change on Stream Function
The historic Gales Creek channel and floodplain supported
a healthy ecosystem by building and maintaining physical
habitat that supported salmonids and other aquatic organisms.
Physical habitat can be defined as the structure of
the channel such as deep pools, clean riffles dominated
by recently deposited gravel, and undercut banks. These
physical habitat elements support salmonids in all stages
of their life cycle by providing good quality spawning
habitat, refuges from high flow conditions in the winter,
and hiding places for both migrating adults and rearing
juveniles. The key element in generating and maintaining
good physical habitat relates primarily to two things:
the channels morphologic response to discharge, sediment,
and debris (Bellamy et al, 1992; Benda, 1990; Best and
Keller, 1986; Grant and Swanson, 1995; Harris, 1988;
Lanka and Hubert, 1987; Miller, 1994; Pitlick and Van
Steeter, 1998), and the presence of roughness elements
such as large woody material, bedrock outcrops, and
boulders (Keller and MacDonald, 1995; Poff and Allan,
1995; Keller and Swanson, 1979; Keller et al, 1981).
In the Gales Creek study area, both of these key elements
have been modified over time to maximize economic use
of the valley floor. Constricting the channel and reducing
total floodplain area has created a more homogeneous,
less dynamic environment where the range of physical
habitats necessary to support all life stages of salmonids
have been greatly reduced. Flood flows are now focused
into a single primary channel in most places, with the
presence of few physical obstructions. The lack of physical
obstructions has resulted in higher flow velocities
and more energy focused on the primary bed and banks
of the channel. Consequently, the channel has incised,
exposing steep banks that are prone to erosion. Channel
incision and an increase in the energy focused on the
bed and banks (referred to as shear stress) has created
a channel system that is dominated by a muted pool and
riffle sequence where the pools are fairly shallow and
the riffles consist primarily of large gravel and cobble
and are armored, limiting their usefulness as spawning
areas. The high energy environment of incised channels
has resulted in finer gravels being transported through
the system rather than being deposited within these
reaches.
Roughness elements, such as large cedar and Douglas
fir logs are no longer present to the extent they were
historically and do not play as much of a role in creating
physical habitat. Historically, roughness elements,
especially large woody material, were abundant, creating
obstructions, diversity in the velocity field, and cover
habitat for fish. Large woody material played a major
role in creating a dynamic channel and floodplain dominated
by avulsions, point bars, secondary channels, and backwater
channels by creating obstructions to flow. These historic
channel shifts were important in cleaning old spawning
beds and creating new ones, limiting bed armoring, and
scouring out deep pools and undercut banks. Without
such obstructions present in the channel and on the
floodplain, and limited potential for future recruitment
due to the lack of large conifers on the floodplain,
the opportunity to create physical habitat in the future
through natural processes is limited.
The quantity and distribution of roughness elements
in a channel also plays a role in dissipating energy.
The amount of energy a given discharge exerts can be
equated to the unit stream power. Stream power is a
function of the discharge and the water surface slope.
Roughness elements, such as large woody debris or bedrock
outcrops, can resist or deflect flow, increasing the
overall flow length and causing the flow to backwater
as local velocities decrease. Both of these factors
can reduce the local slope, thereby reducing local stream
power. By reducing local stream power the stream is
less likely to incise, less likely to erode banks, and
more likely to deposit gravel which is important to
anadromous fish populations.
3.3 Reach Delineation and
Description
The 2003 Lower Gales Creek Habitat Enhancement Plan
delineated a total of ten reaches (GL01 – GL10)
along lower Gales Creek from the confluence of Prickett
Creek upstream to the confluence of Clear Creek. The
reach delineation for the LGCHEP was based on specific
changes in channel type (Rosgen, 1994) and site specific
geomorphic conditions such as bank condition, number
and size of wood pieces, and primary flow characteristics.
Though these delineations worked well for the analysis
being conducted and will continue to be the primary
delineations used by the Tualatin River Watershed Council
to define locations of future enhancement and restoration
projects, this study developed its own reach delineation
that focused on morphologic character and sediment transport
characteristics (Rosgen, 1994; Montgomery and Buffington,
1993). In addition, our study area was expanded to include
the portion of Gales Creek between Clear Creek and Iler
Creek.
For this study, a total of six reaches have been identified.
Figure 6 and Table 1 delineate the reach breaks for
both the current study and the LGCHEP work. The classifications
and reach delineations are meant to represent average
conditions within each reach with the goal of explaining
the overall trend in channel form and function.
Table 1: Reach delineations and descriptions for the
Gales Creek geomorphic assessment. Refer to Figure 6
for specific locations.
Reach
# |
LGCHEP
Reach ID |
Sediment Transport Regime |
Sinuosity |
Bankfull
Width (ft) |
Bankfull
Depth (ft) |
Width
to Depth Ratio |
Entrenchment |
Slope
(%) |
1 |
None |
Transport |
1 |
58 |
2.9 |
20 |
1.9 |
.01 |
2 |
GL01 |
Aggradation |
1.1 |
71 |
3.2 |
22 |
2.4 |
.004 |
3 |
GL02 |
Transitional |
1.5 |
66 |
3.8 |
17 |
2.4 |
.003 |
4 |
GL03-06 |
Transport |
1.2 |
58 |
3.6 |
16 |
2.4 |
.004 |
5 |
GL07-09 |
Aggradation |
1.3 |
53 |
4.2 |
13 |
2.2 |
.001 |
6 |
GL10 |
Transport |
1.1 |
60 |
5 |
12 |
1.3 |
.001 |
Reach 1
Reach 1 consists of a relatively steep, straight channel
dominated by bedrock outcroppings and a cobble bed.
The riparian corridor is intact and fairly wide through
most of this reach, except in the upper portion where
a farm field abuts the channel. Some floodplain exists
in overbank areas to both the right and left of the
channel and there are several abandoned meander bends
on the floodplain that now support off channel wetland
systems. Due to the relatively steep slope of this reach
and bedrock exposures on the bank and bed of the channel
it has been classified as a transport reach. This classification
has been confirmed by the sediment transport calculations
discussed in Section 4.3. Pool and riffle structure
through this reach is fairly regular with very little
in the way of meandering. The reach also has little
to no woody material in the channel which is most likely
the result of high velocities during peak runoff conditions.
Spawning habitat may be limited through this reach given
the lack of deposition and high velocities. This reach
could provide good rearing habitat if large woody material
is added and secured since the canopy cover provides
good shade and there are several pools and undercut
banks.
Reach 2
Reach 2 overlaps directly with GL01 and consists of
a much wider channel and floodplain than in Reach 1.
Clear Creek, a major tributary to Gales Creek, enters
from the right bank at the upstream end of the reach,
contributing both cold summer baseflow and a significant
quantity of bedload in the form of gravel and cobble.
There is a significant break in slope between Reach
1 and Reach 2 creating ideal conditions for bed load
deposition which is the dominant feature within this
reach. The fairly straight and narrow channel present
in Reach 1 changes to a meandering low flow channel
with large bar forms. Active and abandoned off-channel
gravel mines within this reach are a good indicator
of the potential for sediment deposition through this
reach. This reach may have historically been an important
spawning area for salmonids.
Reach 3
Reach 3 overlaps fairly closely with GL02. Though
this reach has the potential to deposit coarse sediment
delivered from upstream, it is slightly more confined
and incised than Reach 2. In addition, much of the sediment
supplied to Reach 2 does not necessarily make it to
Reach 3 to be deposited, which may explain the observed
difference between the reaches. This reach runs primarily
west to east as the main channel crosses the valley
from being confined on one side to the other. Some bedrock
is exposed on the banks and bed of the channel in the
lower portion of this reach though the bedrock consists
primarily of mudstones and is friable and unconsolidated.
A bridge located at the downstream end of the reach
shows evidence of from 1 to 2 feet of channel incision
in the last 30-40 years. This reach may provide some
spawning habitat in certain years where the gravel supply
is adequate. The upper and middle portions of this reach
may provide good rearing habitat if the canopy cover
is improved and large wood is installed and secured.
Reach 4
Reach 4 has similar characteristics to Reach 1 in
that it appears to be a transport reach, is relatively
confined, and has bedrock exposures along the bed and
banks of the channel. In particular, the bedrock exposure
in the vicinity of the Roderick Road Bridge appears
to be a Mafic outcrop consisting of material that is
much more resistance to erosion. Based on this observation,
much of the channel incision and headcut propagation
occurring downstream of the Roderick Road Bridge would
likely be arrested due to this outcrop and therefore
not impact channel or bank conditions upstream. This
is not to say that no incision or bank erosion will
occur, but that this more resistant outcrop creates
a barrier to significant impacts from downstream. Despite
the overall incised nature of Reach 4, there are significant
pockets floodplain and complex in-channel habitat. An
abandoned channel upstream of the Roderick Road Bridge
has potential as a restored secondary channel. The Reach
has a wide and continuous riparian corridor, and some
large wood has generated pool habitat upstream of the
Roderick Road constriction. Though spawning habitat
may be limited through this reach, there is potential
to enhance rearing habitat due to some existing in-stream
complexity and the continuous canopy.
Reach 5
Reach 5 can be classified primarily as a depositional
reach. Reach 5 has experienced significant changes over
the last several decades due to a series of headcuts
that have moved through in succession, apparently due
to channel changes downstream on Gales Creek and on
the Tualatin River. As the headcuts have moved through
the channel has incised. Though the incision appears
to have ended, the system has moved to a new phase of
channel widening, resulting in extensive areas of bank
erosion. The bank erosion has introduced large quantities
of sediment and debris into the channel that has resulted
in the formation of point bars, thereby exacerbating
the intensity of channel widening as new floodplain
is created. Where debris has been deposited in the channel,
pools have formed and gravel beds have been exposed
creating pockets of what appears to be high quality
spawning and rearing habitat. The bank erosion has also
resulted in large gaps in the riparian canopy, thus
limiting future recruitment of large wood. This reach
historically appears to have provided spawning and rearing
habitat for salmonids. It has the potential to provide
both in the future if some of the mentioned issues are
addressed.
Reach 6
Reach 6 is a short reach in the vicinity of the Stringtown
Bridge. The channel through this reach is straight and
incised, lacks channel complexity, and has a very narrow
riparian corridor. Due to these conditions and the fact
that is located downstream of a depositional reach,
Reach 6 was considered a transport reach. It lacked
bar forms and the characteristic pool and riffle sequence
that characterize a meandering low flow channel. This
reach may provide some rearing habitat if the canopy
cover were improved but lacks much spawning habitat
potential.
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