Wetland Inventory and Assessment for the City of Tigard, Oregon WETLAND INVENTORY AND ASSESSMENT
FOR THE CITY OF TIGARD, OREGON
PREPARED FOR:
City of Tigard
Community Development Department
13125 SW Hall Boulevard
Tigard, Oregon 97223
PREPARED BY:
Randall A. Jones
Steven R. Helm
Leslie J. Anderson
Scientific Resources, Inc.
11830 SW Kerr Parkway, Suite 375
Lake Oswego, Oregon 97035
SRI PROJECT 89034
November 16, 1989
1.0 INTRODUCTION AND APPROACH
Scientific Resources, Inc., (SRI) was asked by the City of Tigard to prepare
an identification and assessment of wetlands within the City of Tigard city limits and
area of interest. The overall purpose of the study was to produce a broad scale
treatment of the extent, location, and habitat value of the wetland resources within
the city and area of interest. The results of the study will be used in city-wide
planning.
SRI began the work in July 1989, beginning with a preliminary mapping
effort to maximize later field data and observation collection. As the area within
the city limits and area of interest (hereafter called the "Study Area") is quite large,
sufficient detail could not be portrayed on the City's standard smaller scale 1"=800
feet maps because of scale and physical size limitations. SRI identified and
"Study detailed Area Unit" maps which are at a scale and size more
appropriate for both reporting the necessary information and for better use in
planning.
The data collection period began in early August and continued into early
November. Data and observations were collected to sufficiently characterize the
size and composition of each wetland area and a Wetland Wildlife Habitat
Assessment (WVVHA) was conducted for each wetland area/system identified. The
maps, data, and WWHA forms are presented in Appendix A.
2.0 DEFINITIONS
Wetlands are defined separately at the federal level for various laws,
regulations, and programs. At the federal level, four agencies are involved with
wetland identification and delineation. The U.S. Army Corps of Engineers (CE)
and the U.S. Environmental Protection Agency (EPA), for administering section
404 of the Clean Water Act, define wetland as:
"Those areas that are inundated or saturated by surface or groundwater at a frequency
or duration sufficient to support, and that under normal circumstances do support, a
prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands
generally include swamps, marshes, bogs, and similar areas."
The U.S.D.A. Soil Conservation Service (SCS) uses a similar definition of
wetland but for identification purposes under the Food Security Act of 1985
"Swampbuster" provision. The application of the definition primarily targets
agricultural lands where farmer eligibility for this program's benefits are concerned:
"Wetlands are defined as areas that have a predominance of hydric soils and that are
inundated or saturated by surface or groundwater at a frequency of duration sufficient to
support, and under normal circumstances do support, a prevalence of hydrophytic
vegetation typically adapted to life in saturated soil conditions, except lands in Alaska
identified as having a high potential for agricultural development and a predominance
of permafrost soils.
The U.S. Fish and Wildlife Service (FWS) conducts inventories of the
nation's wetlands and for that purpose defines wetland as:
"Wetlands are lands transitional between terrestrial and aquatic systems where the
water table is usually at or near the surface or the land is covered by shallow water. For
purpose of this classification [ Classification of Wetlands and Deepwater Habitats of
the United States" (Cowardin, et al. 1979)], wetlands must have one or more of the
following three attributes: (1) at least periodically, the land supports predominantly
hydrophytes, (2) the substrate is predominantly undrained hydric soil, and (3) the
substrate is nonsoil and is saturated with water or covered by shallow water at some
time during the growing season of each year."
Like the CE, EPA, and SCS definitions, the FWS definition encompasses
hydrophytic vegetation, hydric soils, and hydrology, but expands the term wetland to
include shallow aquatic areas, where all three mandatory criteria may not be visually
evident, but are assumed to be present. Specifically, in freshwater systems the lower
boundary of wetlands, from terrestrial to true aquatic habitats, is usually above
water depths of 6.6 feet. In summary, all four agencies recognize that wetlands have
three primary components-- sufficient hydrology during the growing season, a
predominance of hydrophytic vegetation, and hydric soils.
On March 20, 1989 a new manual for identifying and delineating wetlands
became the guiding document for the four agencies in determining jurisdictional
wetland (FICWD 1989). The manual gives three sets of mandatory criteria for
identifying wetlands and suggests recommended methods for determining wetland-
upland boundaries. The wetland identification criteria are as follows:
Wetland Vegetation Criteria. Generally, an area has hydrophytic vegetation, and
therefore meets the wetland vegetation criteria, when more than 50 percent of the
dominant species from all strata are classified as wetland species. The FWS, in
cooperation with the CE, EPA, and SCS, has compiled a list of plants (for Region 9
which includes Oregon) that are found in wetlands. Based on the frequency that a
plant is usually found in wetlands, each species was assigned an indicator status of
either facultative upland (FACU) if the plant is occasionally found in wetlands (1-
33% estimated probability), facultative (FAC) if equally likely to occur in uplands
or wetlands (34-66%), facultative wetland (FACW) if the plant usually occurs in
wetlands (67-99%), or obligate wetland (OBL) if almost always occurring in
wetlands (>99%) (Reed 1988).
Hydric Soils Criteria. The National Technical Committee for Hydric Soils (NTCHS)
has established criteria for identifying soils that have developed certain
characteristics in response, over time, to saturated soil conditions sufficient to
support the growth and regeneration of hydrophytic vegetation. Hydric soils are
soils that are saturated, flooded, or ponded long enough during the growing season
to develop anaerobic conditions in the upper part (SCS 1987). All organic soils
(Histosols, except Folists) are hydric, as are mineral soils in Aquic suborders, Aquic
subgroups, Albolls suborder, Salorthids great group, or Pell great groups of
Vertisols that are: a) somewhat poorly drained and have a water table less than 0.5
feet from the surface for a week or more during the growing season; b) poorly
drained with a water table within 1.0 feet of the surface for a week or more during
the growing season; or c) are very poorly drained with a water table less than 1.5
feet of the surface for a week or more during the growing season. Hydric soils
include soils that are ponded (standing water as a result of one event) for a period
from at least 7 days to over one month during the growing season and soils that are
frequently flooded (more than 50 percent chance of flooding under normal or usual
weather conditions) from 7 days to over one month during the growing season.
Wetland Hydrology Criteria. An area has wetland hydrology when, if the soils are
mineral soils, the soils are saturated to the surface during an average rainfall year
for a week or more during the growing season. In somewhat poorly drained soils,
the water table must be above 0.5 feet; in highly permeable, poorly drained or very
poorly drained soils, the water table is 1.0 feet or less from the surface; or in low
permeability, poorly drained or very poorly drained soils, the water table must be
within 1.5 feet of the surface to meet the criteria for wetland hydrology. Poorly
drained or very poorly drained organic soils meet the criteria if the water table is
usually at a depth where saturation occurs to the surface more than rarely. An area
also meets the criteria if it is ponded or frequently flooded with surface water for a
week or more during the growing season.
The three criteria, vegetation, soils, and hydrology, must be met if an area is
to be determined as wetland. A range of wetland indicators for each of the criteria,
collected indirectly from aerial photographs, published maps, and other literature,
or collected directly in the field at a particular site, either satisfy the mandatory
criteria and the area is wetland or fail the criteria and the area is upland.
3.0 METHODS
Wetland Identification and Delineation. The specific methods used to identify and
delineate wetlands in the inventory process for the City of Tigard closely follows the
recommended "Routine Off-site" approach in the Federal Manual for Identifying
and Delineating Jurisdictional Wetlands (FICWD 1989). Comprehensive, in-field,
three-parameter technical boundary determinations were not conducted and were
beyond the scope of this work. Although the routine off-site approach is a
recommended method in the new manual for the identification and delineation of
wetlands, we refer to the wetland areas identified in this study as "Potentially
Regulated Wetland". Therefore, wetland boundaries identified (as shown on the
enclosed maps) are only approximate. They are approximate because the method
itself relies heavily on a compilation of available aerial photography and mapped
data only (e.g., black and white, color and color infrared aerial photography,
U.S.D.A. Soil Conservation Service Soil Survey Maps, a list of Hydric Soils for
Oregon, U.S. Fish and Wildlife Service/National Wetland Inventory maps, large-
scale topographic maps, etc.), thereby indicating the presence of wetlands, but not
their exact boundaries. However, the method, and therefore the quality of the
wetland assessment results, has been improved and strengthened by SRI site visits
which provided "ground truthing" for aerial photographs, verification of wetland
plant communities and species composition, drainage and drainage patterns, and
topography. Further, we have identified some wetlands that are located in areas
mapped as non-hydric soil. Such areas were identified and mapped as wetland on
the basis of our on-site inspections where, in most cases, observations of a
dominance of hydrophytic vegetation and proximity to significant hydrologic
features (and, hence, sufficient water available for soil saturation) were recorded.
Under these conditions, and on a site specific basis, it is our opinion that the
wetland soils criteria would likely be met. As site specific land development or
wetland conservation plans arise where definitive wetland boundaries are required
for regulatory purposes, additional site data collection may be needed.
Identifying wetlands in the City of Tigard study area was conducted in several
phases. These phases included a review of color aerial photography, intensive study
of detailed topographic sheets, identification and compilation of both hydric soils
only (see SAU descriptions, maps, and summary SAU data sheets) and all soil types
found in the Study Area (a set of summary soils data is provided in Table 1), review
of 100-year floodplain maps, significant natural resources (previous Goal 5 work),
parcel base/tax lots, and topographic quarter-section maps (1 inch = 100 feet), and
National Wetlands Inventory (NWI) maps. All other known mapped data was
acquired.
Study Area Unit (SAU) base maps were developed to provide sufficient
detail for wetland areas identified (see Appendix A). The 6/30/89 Parcel Base for
the City of Tigard, scaled to 1" = 400 feet, was used as a base map for the study
because of the ease of locating streams and wetland areas both on the map and in
the field and also because of the significance of wetland areas to adjacent property
owners. The overall study area was divided into seven SAU's: A, B, C, D, E, F, and
G. An attempt was made to keep neighborhoods intact therefore the maps were
divided at significant roadways and railways where possible, and always along
property boundaries. The following information has been plotted as overlays to the
base maps:
1. 100-year floodplain boundaries.
2. Hydric soils.
3. USFWS NWI wetlands.
4. Potentially Regulated Wetlands.
5. Potentially Regulated Wetlands plotted over a black and white aerial
photograph base.
The City of Tigard provided a color aerial photograph, at approximately 1" =
700', of the entire study area on which clear acetate overlays were used to draft
hydric soils, floodplain, and NWI boundaries. An intensive study of the detailed
topography of each of the SAU's provided clues to potential wetlands. Stream
channels, depressional areas, and ponds (where indicated)were marked for further
field study. Some areas of potential wetland identified on the topographic sheets
either post dated the NWI information or were simply not mapped in that effort.
Based on the degree of overlap between primarily topography and hydric
soils, but also with relation to NWI and floodplain boundaries, an approximate
wetland area was outlined on working field maps. These areas, roughly outlined,
provided the starting point for on-site inspections. SRI began the field work portion
of the study both concurrent with and subsequent to the mapping phase(s). In
addition to collecting data for input to the WWHA analysis, field observations were
made on weather conditions, topography, drainage, vegetation community
composition and species dominance, and human activities. The boundaries of each
potentially regulated wetland were then refined using the site specific observations
made. Potentially regulated wetland boundaries identified in the field were marked
on acetate overlays of 12 April 1989 black and white aerial photographic stereopairs
to clearly indicate the boundaries. These identified wetlands were then transferred
to the study area maps and to the larger parcel base map, scaled at 1" = 800', which
includes the entire study area.
Wetland Wildlife Habitat Assessment. A Wetland Wildlife Habitat Assessment
(WWHA) was conducted. Qualitative descriptions for comparison purposes were
made of each wetland area. The analysis was generally restricted to individual
wetland areas, but where large, continuous wetland systems were encountered, such
as occurs along Summer Creek in the western portion of the City and along Fanno
Creek, these areas were subdivided at points at which significant changes in habitat
were found. These "break points" are, generally, where either the natural vegetation
Map unit m 1 y E _ 5 5 SG -- 7 B _
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v 48
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q q Inclusions 1) 3}C. IJ 3D i
10%0R3/3 1OYR 3/3 IO YR 3/1 IO YR 3/3 IO YR 3/3
loam.. low.-- s;14. loav-.. S;141 clay 10ar►. .
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6 loYR3/3 _ LO`(R3/3 w►otflts u — — -
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w/611.04- w(skok IoYR 672. s:lfy tIol loa...+ Io a 3/1
s;I4- 100.4.•1. Lot ►-,°ales
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w/ dark yeltowtS4- loYR3/y
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Hydric/Nonhydric Nen, 14y d,ri`C Na,...1,y ofriC No+.IyGIric. Mrs.. otric
Ai 4
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q q Inclusions I�
to YR 3/3 10`(R 3/3 1OYX3/3 s; 14 I.a,-
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— _
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u '6 13; 1Y; 157 `i3; 3- 9; lob 13; 1Y;.z9; 30 2.; 15; '1513 I ; 2.; is; t915; ' HA 1 ; 2; IS) NB; "3
q q Inclusions
10YIe34 IoYK "1/7, Io YR /Z Io YR 3/2 10YR14
si11-Y cLQi (caw Si ff Ioa sill- (c0-0-• 5'i 1f /oa,
Si1it' (JAI too.,
6T — — 10 YR 314 — lo YR —
Iv YR 272. IoYR }/x 5'.1U Ioaw, silf 'ca.,.
Iu YR -
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ts1 > IDY12 3/1 black S-6;vts Si 1f /oaµ. Si If (0a'^
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0
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c.:
— — 10 YR 3/3
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si117c ID`(►Z �/3 — LDYR `t/3
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Map Unit ID 1/5C YS D 1-16 F 'II O --
Soil Series do pa1.irvt 1,30oot •LLnrh
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Hydric/Nonhydric ;00,,, I1 y of✓i c Wisni 1'y of r i c
(4-/2./0) (/z -7o%3 ..54-f--4.?
Landscape PositionTerraces Tr✓�a ct5 Escarp 1,-,-e-A--4s
J;Z; IS; l4c • S'�/G 1' z •/s'/9D' qy -21 L,D) 3.3-c,D; 44c2 -s3B,C)'DJ EJ F
q q Inclusions i D; yscif
I0 Y2 34. Io YR
si 14 loa.�.. 5;1+ low
6— loYR 3/L - 10 YR Ya -_ — — —
s;I+ Io0.•-, s1 14 106.-
12 Io YRy3 — 1oYR�3 — — — —
-
a T Si 14 Ioaw, 5ili 104.
nn
E i 18 lot Ip `1/3Io YR `U3 — _ — —
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o
O
o 24 — — — —
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5i Hy CIAy /041 IA"- Si/4 c Lo y Iba.w.
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loyrz '16 "IoYR W3
Si Hy (LI lca w. silly C.lay loco-.
36 w/u w±"les w/ 0,-H-1.es — — — -
41to Y/? `/z --- to YIP y/z — — —
u Si I4 1oa L.. 5114 lou.,.,.
0
,D w1/...i p t les to/ w.0-t-r l es .
48_ — — — —
community changes significantly (e.g., from a forested wetland to an agricultural
field) or where a major roadway or other human development segments the
wetland.
There are two parts to the WWHA methodology: 1) a narrative description
of the site; and 2) a numerical rating of various wildlife habitat parameters. The
guiding basis of the method is to identify the potential a given site has for wildlife.
The system focuses on the fact that wildlife has three basic requirements for
survival-- water, food, and cover. A sketch map of each WWHA area was drawn in
the field and a host of habitat and wildlife observations were made on standard
WWHA forms (developed by SRI with the assistance of Mike Houck, Portland
Audubon Society; Ralph Rogers, U.S. EPA; Dennis Peters and Diana Hwan&, U.S.
Fish and Wildlife Service; Gene Herb, Oregon Department of Fish and Wildlife;
and Esther Lev, Consulting Wildlife Biologist). These observations include:
1. A description of the location of WWHA unit.
2. An approximation of the size of the WWHA unit.
3. Comments regarding the reasoning behind specific numeric ratings or
for potential of the site for rehabilitation.
4. Seasonality of water features. A permanent water source may provide
habitat for a certain type of wildlife on a year-round basis, while a
more seasonal source may coincide with shifts in wildlife usage,
community structure, etc.
5. Visual observation of water quality. Very slow moving water or
stagnant water generally is not considered as having as high a value as
water that is continually flushed through a system. From a wildlife
habitat standpoint, moving water is usually not deficient of dissolved
oxygen-- a condition which can severely limit species diversity.
6. Proximity of water to cover. Distances from a water source to wildlife
cover has predation implications for certain species as escape routes
may become limited, and cover adjacent to water is often climatically
moderated by the presence of a water body.
7. Water type diversity. Some species prefer differing types of aquatic
habitats, be they ponds, streams, or forested or emergent wetlands.
The more diversity in water types a site has, the more species diversity
can be expected.
8. Wildlife food variety. The greater the variety of food, the greater the
potential for meeting thee requirements of more wildlife species.
9. Wildlife food quantity. Although the volume of food available may
not necessarily mean a greater diversity of wildlife at a particular site,
more food does generally mean that more individuals within a given
species, or group of species, may be supported.
10. Wildlife food seasonality is a measure of food on a year-round basis.
Habitats that can support wildlife throughout the year are often more
valuable than habitats only used on a seasonal basis.
11. Structural diversity of cover. Vertical stratification of vegetation (e.g.,
multi-layered systems with a ground layer of herbaceous cover,
intermediate strata of shrubs, and an overlying canopy of saplings
and/or trees) facilitates a stronger basis for support for a greater
variety of species than a less structurally diverse system.
12. The variety of cover types is important to wildlife from an
escapement, foraging, and reproduction standpoint. An area having a
wide variety of species important as wildlife cover (e.g., ash overstory,
alder, willow, and spirea intermediate canopy, and a soft rush/slough
sedge ground cover) will be more valuable than an area having a
single cover type (e.g., a monoculture of reed canary-grass).
13. Seasonality of cover types. As with water and food seasonality, a
habitat cover type will have more importance to wildlife if that cover
is present year-round. Seasonality of cover types is determined
primarily on the basis of the percentage of evergreen species versus
coniferous species.
Notes were also taken on human and other physical disturbances which
included such factors as relative seclusion from or proximity to housing, traffic,
and/or commercial or industrial activities. Removal of the physical components of
habitat (water, food, cover) were also included.
The individual scores among the various habitat components, were summed
to arrive at a final score for a given site. Depending on the final site score, a class
was assigned to the site with classes representing a predetermined range of habitat
quality. The classes, from I through IV, in decreasing order of habitat quality (class
I is highest, class IV is lowest) are defined as:
Class I 76-96 High value)
Class II 59-75 Mod. high value)
Class IR 34-58 Mod. low value)
Class IV 0-33 Low value)
The WWHA rating system is intended as an assessment of the relative value of
wetland for wildlife habitat. It is not intended to provide a comprehensive
environmental or functional analysis of each site.
4.0 STUDY AREA UNIT DESCRIPTIONS
Study Area Unit A. This SAU is bounded by Scholls Ferry Road on the north
beginning at its junction with Old Scholls Ferry Road on the west extending
northeastward to 121st Avenue. From this junction at 121st Avenue, the eastern
boundary runs south to North Dakota Street and jogs east to a point in line with
115th Street and then due south to Fonner Street, following Fonner to 121st again.
On 121st, at approximately the junction with Howard Drive, the southern boundary
begins and runs due west along property lines to the limit of the study area.
The topographic gradients of SAU A are most severe on the south near the
base of the slopes forming Bull Mountain. This area is relatively steep and
dissected, with five primary stream valleys contributing runoff to the northern
portion of the SAU ending at their confluence with Summer Creek in the north.
The extreme western portion of the area is low-relief, rolling hills dominated by
agricultural land. Residential areas are dominant in basically the eastern one-half
of the area, with several such developments underway.
The lowest hydrologic level in the SAU is Summer Creek. Summer Creek is,
generally, a very low gradient stream and is a tributary to Fanno Creek to the east.
Although large portions of the creek margins have been altered due to residential
development and the placement of sewer lines, a large near-natural riparian and
wetland corridor remain in the lower reaches. Summer Lake, a large pond modified
from a pre-existing series of ponds and wetlands by development activities, is a
dominant hydrologic feature of the unit. This is a shallow body of water surrounded
by residential development on the north and open grassy park areas (and additional
development in-progress) on the south. A low concrete dam impounds streamflow
to form the lake. The broad floodplain extends eastward from the lake to 121st
Avenue. Just west of 121st, another set of two smaller ponds exist.
Four hydric soil series have been identified in the SAU (SCS 1987) (see
summary data sheets in Appendix A). In decreasing order of dominance (areal
coverage), these are: Wapato silty clay loam (mapping unit 43), Delena silt loam
(16C), Cove silty clay loam (13), and Cove clay (14). The Summer Creek drainage
is dominated in its upper (southern) tributaries by Delena silt loam. These soils are
poorly drained, wet, organic Inceptisols limited to stream courses. However, a large
area of the extreme western segment of the SAU is dominated also by Delena soils--
an area largely used for agricultural purposes. The middle sections of the tributaries
to Summer Creek, as well as the upper reaches of the creek are dominantly Wapato
silty clay loam. These are poorly drained, very wet, dark colored Mollisols that have
developed in flood plains throughout the county. Scattered areas of poorly drained
Cove clay soils are found also in the middle tributary reaches. These soils have a
very high content of "shrink-swell" clays-- expansive when wet and developing large
surface cracks to a significant depth when dry. Most common in the lower segments
of Summer Creek is Cove silty clay, a poorly drained, wet, finely textured Mollisol
with a high content of shrink-swell clays, but not as high as the Cove clay. In
summary, much of the Summer Creek drainage has hydric soils, but these are
generally limited to floodplains.
Study Area Unit B. The northern boundary of this unit is composed of Scholls Ferry
Road, Hall Boulevard, and Oleson Road. The eastern boundary begins on the
north at the junction of Washington Drive and Taylors Ferry Road, south on
Washington to Hall Blvd. to Hwy. 217. The southern boundary follows Hwy. 217 to
the grade of the old railroad near its crossing of 95th Avenue, continuing southwest
along the railroad grade to Tiedeman Road. The boundary follows Tiedeman to the
property line north of Fowler Junior High School, then west to the limit of SAU A.
The topography of this unit can be described as low gradient, rolling hill and
swales, with the dominant topographic feature being the relatively broad floodplain
and somewhat incised channel of Fanno Creek. Fanno Creek and Ash Creek are
the two major tributary streams crossing the unit.
Cove silty clay loam, Wapato silty clay loam, Huberly silt loam, and Verboort
silty clay loam, are the hydric soil series in the unit. The characteristics of the Cove
and Wapato series are discussed above. Huberly silt loam are poorly drained Typic
Fragiaquepts that have developed on stream terraces. This soil is not on the
National list of hydric soils, nor is it on the State list of hydric soils. But the soil is
listed as "hydric" on the more specific Washington County list of hydric soils and,
therefore, for the purposes of this study, is considered as such. These are Inceptisols
with an aquic moisture regime, having also a slightly cemented hardpan between 25-
38 inches depth. The hardpan layer is about 4 inches thick and is slightly
impermeable to water.
Verboort soils are poorly drained also, but are Typic Argialbolls that have
formed in the lowest "bottomlands". These are Mollisols that have a high clay
content in the surface horizons (argillic layer), and that are extremely dark colored
(almost black), with a low chroma (dark colored) albic horizon immediately over
the clay layer. The implications of the argillic layer to wetland determinations is
that the clays tend to perch runoff above them and also will retain runoff (and
therefore create conditions for anaerobiosis) well into the growing season.
The distribution of the hydric soils in SAU B is primarily restricted to
drainageways and floodplains. Much of the area of the hydric soils in the unit has
been filled by residential and commercial developments. The largest single tract of
undeveloped hydric soil in a floodplain that remains is located adjacent to Hwy 217
just east of Greenburg Road and south of Oak Street. During the summer of 1989,
however, most of this area has been covered by fill material in anticipation of a
development project and two small mitigation ponds were created (see SAU B data
in appendix).
Study Area Unit C. Occupying the northeastern corner of the Study Area, SAU C is
bounded by Taylors Ferry Road on the north, on the east by the Willamette
Meridian (Multnomah and Washington County line), Haines Road and Hwy. 99 on
the south. The boundary continues southwest along Hwy. 99 to the Southern Pacific
Railroad grade to Katherine Street (which parallels the property lines on an
extension east of the southern limit to SAU B).
The topographic character of the unit is best described as rolling, low hills
with low narrow swales and stream courses between them. Ash Creek dominates
the hydrologic features of the unit, with only two other significant drainages
apparent on the large-scale topographic sheets. The trend in slope (and drainage) is
generally from northeast to southwest.
Cove silty clay loam, Huberly silt loam, Verboort silty clay loam, and Wapato
silty clay loam are the hydric soils found in SAU C. The Verboort series dominate
the drainage of Ash Creek, while the Wapato series underlies the tributary that
merges Ash Creek from the east near Metzger Park. Cove silty clay loam is
dominant in a broad band under a small tributary to Ash Creek which roughly
parallels Hwy. 217 on the southern margin of the unit.
Study Area Unit D. SAU D abuts SAU A to the north and has as its eastern
boundary the same north-south extension in line with 115th Street. The southern
and western boundaries are defined by the City limit and area of interest
boundaries.
Bull Mountain and its associated foothills stand out as the most significant
topographic feature of this Study Unit. Indeed, the mountain dominates the
topographic features of the entire Study Area. Rising to an elevation of slightly over
700 feet (msl), steep slopes radiate outward from the crest in all directions. Only
very small and narrow stream courses have developed as a result of the steep slopes,
and no hydric soils occur in SAU D.
Study Area Unit E. This SAU abuts SAU B and C to the north, and SAU A and D
on the west. The eastern boundary is the Willamette Meridian, and the southern
boundary is Bonita Road and Murdock Street.
Topographically, SAU E is the most complex within the Study Area. The
highest elevation is reached at the hill-crest immediately east of the low pass along
Hwy. 99 just north of King City. The slope gradient trends from this maximum
elevation of over 400 feet generally north-northeast and east-northeast to the Fanno
Creek floodplain (at or near 140 to 150 feet) that parallels the Southern Pacific
Railroad line near the downtown sector. The Fanno Creek floodplain is narrower at
the northern margins of SAU E and becomes significantly broader approaching the
southeastern portion of the unit. Within this unit Summer Creek enters Fanno
Creek on the north. Minor streams drain into Fanno Creek from the southwest near
SW Tiedeman and Walnut Streets, and near SW Burnham and Main Streets. Red
Rock Creek is also tributary to Fanno Creek, but the creek stems from the western
slopes of Mt. Sylvania draining westward entering Fanno Creek immediately south
of the railroad grade (approximately 1/4 mile south of Hunziker Road) and east of
SW Hall Blvd.
Four Hydric soil series are found in SAU E. Cove silty clay loam occupies
and dominates the drainages in the northwestern sectors of the unit, while Huberly
silt loam and Verboort silty clay loam dominate the areas of hydric soil in the east
and southeast. In contrast to the distribution of Cove soils which are restricted to
drainageways, the Huberly and Verboort series are relatively widespread and are
not solely limited to existing stream courses, but also are found in broad
depressional areas.
Study Area Unit F. The southern boundary of SAU F is the Tualatin River,
beginning at the point where Hwy. 99 crosses the river. The western boundary is
Hwy. 99, the eastern limit is 85th Avenue, and the northern is Murdock Street.
Slopes within this unit are primarily south-facing, grading toward the
Tualatin River in a slightly concave form. Slopes in the extreme northeastern
corner, however, trend toward the Fanno Creek drainage. The major drainage
network within the unit is tributary to the Tualatin near SW 113th Street. The
southern arm of this dendritic drainage is, in part, a relict flood channel of the
Tualatin River. The depression now functions as an active tributary stream channel.
Three smaller creeks drain into the larger stream just north of the confluence with
the Tualatin.
As a result of saturated conditions over time within this network of surface
water, hydric soils have developed. Verboort silty clay loam dominates within the
drainage network on the south, and is restricted to the area immediately adjacent to
the stream channels. A minor area of Huberly silt loam occupies one arm of this
network. Large areas of hydric soils exist in the eastern an northeastern quadrants
of the unit, composed mainly of Huberly silt loam and with minor areas of Cove silty
clay loam and Dayton silt loam (see Table 1 for major characteristics of the Dayton
series). In the extreme southeast corner of SAU F, Cove clay and Wapato silty clay
loam are the two hydric soil series that are found.
Study Area Unit G. SAU G also has as its eastern boundary the Willamette
Meridian, but the boundary follows Interstate 5 where the Meridian and the freeway
join. The southern boundary is defined by the city limits, and the remaining
periphery of SAU G abuts SAU F and E.
The Fanno Creek floodplain dominates the topographic features of this unit.
All slopes within the unit direct surface and groundwater flow towards the creek.
Within the unit, Fanno Creek flows through an incised channel both as a result of
natural processes and human activities. It is of interest and significance to wetland
determination that very little area of hydric soil occurs in association with the creek.
The only hydric soil that is found adjacent to Fanno Creek here is Wapato silty clay
loam. The area underlain by the soil is located near the extreme northern margin of
the unit. Huberly silt loam occurs along a minor tributary in the northeast and
across a broader area immediately north of the junction of SW Carmen Drive and
72nd Avenue.
5.0 DISCUSSION AND CONCLUSIONS
Although it is apparent that a significant amount of wetland areas have been
lost due to development, the City of Tigard, within the city limits and the its area of
interest, has a considerable wetland resource base remaining. The U.S. Fish and
Wildlife Service under the National Wetland Inventory (NWI) program developed
maps of wetlands in the Study Area based on 1981 color infrared aerial
photography. Generally, riverine (creeks and streams), emergent (swamps,
marshes), and forested wetlands were identified in the NWI for the Study Area.
Although extensive "ground truthing" of photography is part of the methodology
used by the USFWS in identifying wetlands under the program, the NWI does not
extensively address, however, the occurrence of site-specific wetland hydrology
(other than streams or ponded areas recognizable from the photos) or hydric soils
in association with identified wetlands. Wetland hydrology and hydric soils are
mandatory criteria for identifying wetlands as per the new Federal Manual for
Identifying and Delineating Jurisdictional Wetlands (1989), and therefore some
wetland areas identified on the NWI maps for the Study Area do not reflect
accurately wetlands in the Study Area that may be subject to Federal or State
jurisdiction.
Using the Routine Off-site method as recommended by the new manual, in
conjunction with site visits, we have identified as wetland only those areas having
hydric soils (as identified by the SCS soil survey), a dominance of hydrophytic
vegetation (based on color and color infrared photography, and on visual
observations and identifications of species), and wetland hydrology (based on
detailed topographic maps, visual observations, and vegetation composition). As a
result, we have identified a total of 117 individual wetlands totaling approximately 324
acres of potentially regulated jurisdictional wetland within the city limits and area of
interest. SAU E has the highest number of individual wetlands at 34, while SAU G
has only 14. These numbers are, in part, a function of the size of the SAU, but the
actual wetland density is area specific.
The distribution of wetlands in the City of Tigard and its area of interest is
most closely associated with the existing surface drainage pattern. Fanno Creek is
the dominant stream system. It follows a north to south flowing course from its
headwaters in the West Hills to its mouth at the Tualatin River and is fed by two
major tributaries (Ash Creek, flowing from the northeast and Summer Creek, which
flows from the west). Wetlands alone Fanno Creek and its major tributaries are
relatively long and narrow and are limited to the floodplain by topographic features
(e.g., terraces and levies and other sharp changes in slopes gradient). Several small
wetlands areas are located along three minor tributaries of Fanno Creek. One small
stream flows south directly into the Tualatin River along the southern border of the
study area. The most extensive potentially regulated wetland is located in a lowlying
area of agricultural land dominated by hydric soils in the northeast portion of the
study area (SAU A) and is not associated with any floodplain. Where Hwy. 217
crosses Ash Creek and Red Rock Creek, several relatively large wetlands have been
extended and enhanced with ponds. The Tualatin River floodplain, at Cook Park,
has contributed to the formation of several wetland areas also.
A total of 72 Wetland Wildlife Habitat Assessment (WWHA) areas were
identified. These units comprise wetlands that are similar in terms of visually
contiguous habitat types. In terms of WWHA scores, patterns of scores and classes
of the various wetlands in the Study Area emerge. The highest mean WWHA
scores are found for those areas in SAU E and G. Likewise, the lowest (highest
value) mean class scores are also found in SAU E and G. Conversely, the lowest
mean WWHA scores are for the wetlands in SAU F and B, which also corresponds
to the lowest mean class scores for those same SAU's. It is useful to contrast these
results with the percentage, in each SAU, of class I and II wetlands per total number
of wetlands identified within each SAU. Of the four classes of wetland habitat
quality (WWHA classes), 5 class I wetlands were found, 29 class II, 25 class III, and
13 class IV wetlands. Although the mean wetland scores in SAU A did not rank as
high as for SAU E or G, 60 percent of the wetlands identified were class I or II.
SAU F wetlands, however, did have among the lowest mean scores and only 9.5
percent of the wetlands here are class I or II.
6.0 REFERENCES
Federal Interagency Committee for Wetland Delineation. 1989. Federal manual
for identifying and delineating jurisdictional wetlands. U.S Army Corps of
Engineers, U.S. Environmental Protection Agency, U.S. Fish and Wildlife
Service, and U.S.D.A. Soil Conservation Service, Washington D.C.,
cooperative technical publication.
U.S. Fish and Wildlife Service. 1981. National Wetland Inventory maps (see
U.S.G.S. quadrangles).
U.S. Geological Survey topographic quadrangle. Photorevised 1984. Beaverton,
Oregon (1:24,000).
U.S. Geological Survey topographic quadrangle. Photorevised 1970 and 1975. Lake
Oswego, Oregon (1:24,000).
U.S. Soil Conservation Service. 1982. Soil survey of Washington County Area,
Oregon. In cooperation with the Oregon Agricultural Experiment Station.