Comparison of Two Riparian Wetlands in Big Bear Creek over an Urban Gradient
Kristy Brady
Jaime Fleckenstein
Maeve McBride
Jeannette Prazen
Wetland Ecology
December 6, 1999
Introduction
The State of Washington, and especially the greater Seattle area, is experiencing
rapid growth and urban sprawl. Eliot (1993) has estimated that nearly half of the
wetlands in the state have been lost as a direct result of various kinds of development,
since the turn of the century. What’s more, continued anthropogenic disturbances have
further degraded many of the remaining wetlands.
Urbanization can have significant effects on wetland hydrology, soils, plant
communities, and water quality that in turn can dramatically alter how the wetland
system functions. In our project, we have considered the effects of urbanization on
riparian wetlands, those wetlands adjacent to a stream or river that are characterized by
periodic flooding and a high water table, by studying an urban wetland and a rural
wetland along the same stream. We aimed to determine if there is a significant difference
in the water quality, soils, and plant communities between a riparian wetland close to the
headwaters of a stream and one further downstream in a more developed area. We
hypothesized that a wetland system in an urban region would be degraded as compared to
a wetland in a more rural section of the watershed. We believed that the urban wetland
would have less plant diversity and more non-native, invasive plants, and it would also
show signs of an altered hydroperiod. Typically in an urbanized watershed, a stream will
receive more surface runoff during storms, resulting in “flashier” stream flows, that is,
flows that quickly rise and recede. We suspected that the urban site would have more
sediment accumulation, due to an increased sediment delivery as a result of increased
erosion; and lower water quality, characteristic of urban runoff. Our intent was to identify
signs of urban impacts with regards to the flora community, the soil profile, water
quality, and hydrologic patterns.
Background
Riparian systems
Riparian systems are a key link between the upland regions of a watershed and the
stream channel. The components of a riparian ecosystem, hydrology, geomorphology,
vegetation, and biota, are closely linked together by means of complex interactions. In
other words, each component relies heavily on the others to continue functioning
optimally. Riparian zones have the ability to act as buffers, protecting water resources
from anthropogenic influences of nearby altered landscapes. Riparian wetlands form
linearly alongside channels and they tend to process great fluxes of energy and materials
from the upstream watershed (Mitsch and Gosselink 1993).
Puget Sound Urbanization
The Puget Sound area shows a dynamic gradient from dense urban areas to
sparsely populated rural areas. Prior to anthropogenic influences, this region was
blanketed with conifer forests growing atop glacial till and outwash deposits. Streams
received water flow primarily through subsurface drainage. However, as a result of
growing and spreading populations, forests have been replaced with buildings, roads, and
lawns. These new and impervious surfaces, as well as extensive alterations to the soil
profile and composition of the plant community, have significantly changed the water
flow regime of the area, from a dominant underground flow to a dominant surface flow.
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Wetlands and Urbanization
The urbanization of natural landscapes impacts the functioning and value of
wetland systems in many ways. Such activities as dredging, filling, and draining can be
extremely detrimental to a wetland ecosystem. For example, a study of white-cedar
swamps in undeveloped and suburban watersheds of New Jersey revealed that changes in
species composition correlated directly with the number of altered chemical and
hydrologic parameters due to urbanization (Ehrenfeld and Schneider 1993).
Study Area
We chose to study two Big Bear Creek (BBC) riparian wetlands because BBC is a
relatively “healthy” stream in an area rapidly becoming urbanized. Our first site was
BBC 12, as listed in the King County Wetlands Survey (1990), near where the
Woodinville-Duvall Road crosses BBC. This wetland is in the upper portions of the
creek’s watershed, which has not yet been heavily impacted by development. There are
some subdivisions near this site, but much more extensive development is found
downstream. It should be noted that while we used this site as our rural and un-urbanized
site, it has been exposed to many anthropogenic impacts (road crossings, agricultural
history, etc.). BBC 12’s watershed is consequently 29% impervious (Morley, personal
communication).
Our urban wetland site was BBC 30, located approximately 2 miles downstream
of BBC 12. This wetland has a considerably higher proportion of developed lands in the
surrounding area (residential, commercial, roads, etc.). BBC 30’s watershed is 41%
impervious (Morley, personal communication). From the land cover map it is apparent
that forests dominate the contributing watershed to BBC 12, whereas the contributing
watershed for BBC 30 contains significantly more low- and medium-intensity developed
areas (see land cover map, Figure 1). These wetlands were classified by King County as
primarily palustrine emergent persistent and palustrine scrub-shrub deciduous
respectively (King County Parks 1991).
Methods
Vegetation
The plant community was measured using percent cover of species in one-metersquared plots along twenty-foot transect lines. In total, there were four transect lines with
three meter-squared plots along each line. Each side of the creek had two transect lines
thirty feet apart from each other and perpendicular to the stream. The location for the
first transect was chosen arbitrarily and the remaining three transects established
according to the method described. The zero-foot mark was established at the stream’s
edge and meter-squared plots were centered over the one-foot, ten-foot, and twenty-foot
marks.
After measuring the percent cover as described in the previous paragraph, a walk
through the research area was also completed in order to observe and record any species
present that were not found in any of the plots. The area was surveyed and all species
found were recorded and percent cover for all species in the area was estimated.
The data from the two percent cover analyses were then combined and species
were placed in percent cover classes as follows: 1: 1-2%; 2: 3-10%; 3: 11-25%; 4:2650%; 5: 51-75%; 6: 76-100%. These classes were designed in order to aid in
2
Figure 1
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
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recognizing the diversity of species in the area as well as their relative abundance, and
how these characteristics differed between the two areas.
Hydrology
Water quantity
We measured the stream flow at both of our wetland sites. An average velocity
reading was found using a Columbia Digital Stream Meter. We estimated the crosssectional area of the channel at the location where the velocity measurements were made.
Multiplying the velocity by the channel area produces the stream flow.
Water quality parameters
Temperature and dissolved oxygen (DO) were measured using YSI Model 57
Oxygen meter. DO measurements can be used to indicate the degree of pollution by
organic matter, the destruction of organic substances, or the water’s self-purification. It
is important to determine temperature and DO since they influence almost all of the
chemical and biological processes within the water (Chapman 1992). Conductivity, a
measure of the water’s capacity to conduct an electrical current, was determined with a
Hanna Portable Conductivity meter. Conductivity is related to the concentrations of total
dissolved solids and major ions; it is a rough indicator of mineral content in the water
(Chapman 1992). Turbidity was measured with a 2100P Hach Turbidimeter. Turbidity
is a measure of the transparency of a water sample and it indicates the concentration of
suspended matter in the sample. The water’s pH was determined with a Beckman pH
meter. The pH is an important water quality parameter because it affects biological and
chemical processes (Chapman 1992). All of these measurements were taken in the field
in order to be as accurate and representative of the actual conditions as possible. Water
quality samples were taken in running water, in the middle of the channel.
Since our group lacked the expertise to complete a benthic survey, we instead
obtained the Benthic Index of Biological Integrity (B-IBI) values for the stream segments
adjacent to our study wetlands. B-IBI is an index that combines measurements of
taxonomic diversity, taxa tolerance/intolerance, trophic structure, and population
attributes to assess biological conditions of streams (Booth 1998). Index values range
from 10 to 50, where 10 represents degraded conditions and 50 represents pristine
conditions.
Soils
At both the rural and urban wetland sites, holes were dug at two different
locations. The first location at each wetland was close to the shore of Big Bear Creek, and
the second location was 20 feet inland, in the stream’s flood plain. As each hole was dug,
and the soil profile became apparent, measurements were taken for the depth of each
horizon, and a sample of each horizon was collected and labeled for the purpose of
classification at a later date. Each horizon was then looked at closely and given a proper
soil classification to examine the profile from each location. The texture of each horizon
was also determined using the ribbon method and the use of Table 4.4 in The Nature and
Properties of Soils (Brady and Weil 1996).
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Results
Vegetation
Our survey of the two sites revealed that the rural site, with fourteen species sited,
has twice the species diversity of the urban site, where only seven species were sited. As
well, the urban site is dominated by reed canary grass (Phalaris arundinacea), an
introduced and very invasive species. While reed canary grass is also present at the rural
site, it is not the most abundant; rather Scirpus microcarpus, a bullrush, has the highest
percent cover. Other native species, such as Juncus effusus, Galium, Stellaria, and
Athyrium filix-femina occurred in infrequent patches. (see tables 1 and 2 below)
Table 1: Rural Site Vegetation
percent cover classes: 1: 1-2%; 2: 3-10%; 3: 11-25%; 4: 26-50%; 5: 51-75%; 6: 76-100%
Species
Class
Scirpus microcarpus / small-flowered bullrush
5
Phalaris arundinacea / Reed canary grass
4
Spiraea douglasii / steeplebush or hardhack
3
Rubus discolor / Himalayan blackberry
2
Ranunculus repens / creeping buttercup
2
Rubus laciniatus / evergreen blackberry
2
Juncus effusus / common rush
2
Athyrium filix-femina / lady fern
2
Galium sp.
1
.
Stellaria sp. / starwort
1
Equisetum sp. / horsetail
1
Lemna sp. / duckweed
1
Alnus rubra / red alder (4 trees)
litter: 4
Salix sp. / willow
litter: 2
lichens/bryophytes (on trees and coarse woody debris) 1
*no significant canopy cover since leaves already dropped; however, significant litter cover.
Table 2: Urban Site Vegetation
percent cover classes: 1: 1-2%; 2: 3-10%; 3: 11-25%; 4: 26-50%; 5: 51-75%; 6: 76-100%
Species
Class
Phalaris arundinacea / Reed canary grass
6
Rubus discolor / Himalayan blackberry
3
Rubus laciniatus / evergreen blackberry
2
Equisetum sp. / horsetail
2
Scirpus microcarpus / small-flowered bullrush
2
Solanum dulcamara / bittersweet nightshade
2
Alnus rubra / red alder (19 trees)
litter: 5
lichens/bryophytes (on trees and coarse woody debris) 1
*no significant canopy cover since leaves already dropped; however, significant litter cover.
Hydrology
Water quantity
As would be expected, the wetland downstream, our urban site, had considerably
more flow (2.17 m3/s) than the site upstream (1.55 m3/s). When further downstream,
more watershed area is contributing surface and subsurface water flow to the channel,
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and therefore the resulting stream flow is substantially higher (see table 3). We suspect
that the increased stream flow may be intensified by the fact that the urban wetland’s
watershed is approximately 10% more impervious. Those additional impervious surfaces
have the potential to contribute more surface flow to the channel. To fully understand all
of the occurring processes, we would have to conduct an intensive hydrologic study of
watersheds and their flow regimes over a long time period.
Water quality
Temperature and DO values were nearly the same at both wetland sites. DO was
nearly saturated at both sites, which we believe is due to the turbulent, fast moving
waters, in which we made our measurements. Both conductivity and turbidity recordings
were higher at the urban wetland site. This indicates that this site receives water with a
higher concentration of particulate matter. The pH for both sites was approximately 6.
The B-IBI scores for the rural and urban wetland sites are 32 and 28, respectively.
The rural site had nearly double the percentage of tolerant species, which is a measure of
the relative abundance of organisms classified as tolerant to general disturbance (personal
communication, Morley).
Table 3
Parameters
Air temp (oC)
Water temp (oC)
DO (mg/L)
pH
Conductivity
(S/cm)
Turbidity (NTU)
Velocity (m/s)
Average Channel
Depth (m)
Channel Width (m)
Flow (m3/s)
Wetland Sites
Rural
Urban
(BBC12) (BBC30)
9.5
8
7
8
10.4
11
5.76
6.38
64.3
1.86
0.73
72.5
3.62
0.64
0.71
3
1.55
0.75
4.5
2.17
Soils
Rural Wetland
At the shore location, the profile (from top down) has 1” of slightly decomposed
organic matter (Oi horizon), 1.5” of moderately decomposed organic matter (Oa horizon),
6” of an A horizon made of clay loam, 6” of a Bo Horizon, which has a slight
accumulation of iron oxides, also made of clay loam, and 4” of a Bt horizon, which is an
accumulation of clay, made of clay.
At the floodplain location, the profile has 1” of slightly decomposed organic
matter (Oi horizon), 2.5” of moderately decomposed organic matter (Oa horizon), 9” of an
A horizon made of silt loam, and a Bt horizon of undetermined depth, which is an
accumulation of clay, made of clay (see figure 2).
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Figure 2
Shore
Location
surface
Flood Plain
Location
texture
surface
texture
1"
Oi
1"
Oi
1.5"
Oa
2.5"
Oa
6"
A
Clay Loam
9"
6"
4"
Bo
Bt
A
Silt Loam
Bt
Clay
Clay Loam
Clay
Urban Wetland
At the shore location, the profile has 1” of slightly decomposed organic matter (Oi
horizon), 2.5” of highly decomposed organic matter (Oe horizon), 8” of an A horizon
made of sandy loam, 4” of a Bt horizon, which is an accumulation of clay, made of clay
loam, and a Bg horizon of undetermined depth, which shows mottling, and is made of
clay loam.
At the floodplain, the profile has 1.5” of slightly decomposed organic matter (Oi
horizon), 1” of highly decomposed organic matter (Oe horizon), 4” of an A horizon made
of sandy loam, 4” of a Bc horizon which has concretions and is made of loam, 2” of a Bd
layer which has dense, unconsolidated material and was mostly gravel, and a Bg horizon
of undetermined depth, which shows mottling and consists of clay loam (see figure 3)
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Figure 3
Shore
Location
surface
texture
0"
Oi
1"
Oe
3.5"
11.5"
15.5"
Flood Plain
Location
A
Bt
Bg
surface
texture
0"
Oi
1.5"
Oe
2.5"
A
Sandy Loam
6.5"
Bc
Loam
10.5"
Bd
Gravel
12.5"
Bg
Clay Loam
Sandy Loam
Clay Loam
Clay Loam
Discussion
Vegetation
The rural site has much higher species diversity than the urban site, which is
dominated by the invasive and exotic reed canary grass. The reed canary grass at the
rural site was dwarfed compared to the height it was at the urban site, where it stood at
least seven feet tall. While plants are not particularly biased as to where water comes
from, they do respond to many environmental factors including moisture gradients; thus,
it is reasonable to propose the more diverse the moisture gradient, the larger variety of
species will be present. This seems to be the case in the two BBC sites we observed.
The rural site has a wide floodplain that is either inundated or at least heavily saturated
for long periods of time. The urban channel, on the other hand, is very constricted by
surrounding pavement and mowed lawns. Consequently, floodplains seem to only
become saturated after heavy rainfall, which cause peak flows that override the
streambanks. The moisture regime of the urban site, manipulated by the area’s
development, could be the reason reed canary grass was able to rapidly overrun the area
and eliminate any competitors, which might have inhibited it from achieving such
dominance.
Spiraea douglasii, another invasive species, which thrives in disturbed sites, is
also present at the rural wetland. Should heavy development continue to alter and disturb
the historic nature of the rural wetland, it can be reasonably predicted that Spiraea will
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flourish and begin to out-compete other species for resources. It will then become a
dominant species along with reed canary grass.
The decreased diversity and increase in introduced, invasive species at the urban
site is most probably a direct result of urbanization factors such as the input of roads, and
lawn and home construction altering those gradients, such as moisture, to which plants
respond. This is consistent with our hypothesis for urbanization effects on plant
communities.
Hydrology
The stream flow at the urban site was considerably greater than the stream flow at
the rural site. This may be due to the location of these sites within the BBC basin, or in
part due to the additional developed areas in the lower basin creating greater surface
flow. Our measurements were taken directly after a significant storm event, so the stream
flows were not at their base level. Most of the water quality parameters were nearly
equivalent for the two sites. However, turbidity and conductivity were both higher for
the urban site. This, too, may be a result of the situation of the sites in the basin. Since,
the urban site is further downstream it will naturally have the potential to carry more
sediment, transported either from the channel or from the uplands. However, the
impervious areas may have changed the water flow regime from subsurface flow to
surface flow. This rerouting of the water may result in accelerated erosion upland.
Increased storm flows in the channel may also contribute more sediment downstream if
the channel experiences increased sediment loading due to streambank erosion or
streambed migration. The B-IBI data show a distinct difference for the two sites. The
change in species composition illustrates that the biota are responding to some aquatic
degradation.
Soils
The most apparent difference between the urban and rural sites was the presence
of the A-horizons in the urban soil, which are predominantly sandy horizons. Since
these sandy soils are on top of a clay horizon common to both the urban and rural sites,
this leads us to believe that the sandy soil has been deposited after the clay horizons,
which underlie it. The likely cause of this increased sedimentation is the urbanization of
the areas surrounding this wetland. This indicates that more sedimentation has occurred
at the urban site than at the rural site, which is consistent with our hypothesis. One of the
influences of urbanization on wetland ecosystems, according to the King County Final
Report of the Puget Sound Wetlands and Stormwater Management Research Program, is
the deposition of sediments from development activities, such as clearing and grading.
This deposition is evident in the A horizons of our urban wetland profiles, which are
laden with sand.
Conclusion
While it is difficult to see the whole picture accurately in just a single visit, our
results closely matched our expected outcomes of this study. As urbanization increased,
we did see a decrease in the diversity of vegetation, a subtle decrease in the water quality,
and an increase in the sediment accumulation. The decrease in plant diversity we
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observed can be attributed to the disturbance of the historic plant community and the
introduction of weedier invasive plants. The introduced species, such as reed canary
grass, were more adept to exploiting resources, and were thus able to out-compete native
species, which eventually disappear, as areas such as our urban site become more heavily
developed.
The decrease in water quality and increase in sediment accumulation is due to
increased urban runoff and the complete alteration of the hydrology of the area. The
greater number of roads and other impermeable areas near the urban site, reroutes
precipitation completely so that what would have originally seeped into the earth and
become underground flow, now is forced to flow on the surface. Surface flow, with its
increased velocity, is more capable of eroding streambanks and can therefore deliver
more sediment to a stream or wetland downstream. Thus, we expected to see a buildup
of sediment at the urban site. The sandy A-horizons in the urban soil profile are evidence
of this increased deposition. As well, we expected that increases in surface water would
result in increases in suspended solids. The higher conductivity and turbidity levels at the
urban site support this hypothesis. Urban runoff will generally also carry traces of
nitrogen and phosphorus, which come from chemicals used in urbanized areas (fertilizers,
herbicides and pesticides, detergents, etc.). Being downstream from multiple manicured
lawns, it would be expected to see increased amounts of these elements in the water.
However, time and knowledge restraints did not allow us to perform these tests.
To gather accurate and precise results for this study, it would be necessary to
evaluate these two sites multiple times a year, including times when flow levels are
reduced, as they are after long dry periods, and at peak flows after consistent and
substantial rainfall, for several years. However, this abridged evaluation provides
reasonable insight into what would be the expected results of such a long-term study.
That is, it can be confidently stated that as riparian corridors progress along an urban
gradient, plant diversity will decrease, water quality will diminish, and sediment
accumulation will increase.
References
Azous, A.L. and R.R. Horner. 1997. Wetlands and Urbanization: Implications for the
Future, Final Report of the Puget Sound Wetlands and Stormwater Management
Research Program, Washington State Department of Ecology, Olympia, Washington,
255p.
Booth, D.B. 1998. Urban stream rehabilitation – A progress report on the Center’s 3-year
project, The Washington Water Resource, 9(3):7-11.
Brady, N. and R. Weil. 1996. The Nature and Properties of Soils, 11th ed., Prentice Hall.
Chapman, D. 1992. Water Quality Assessments: A guide to the use of biota, sediments,
and water in environmental monitoring, E & FN Spon, London, 626p.
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Ehrenfeld, J.G. and J.P. Schneider. 1993. Responses of forested wetland vegetation to
perturbations of water chemistry and hydrology, Wetlands, 13(2): 122-129.
Eliot, W., D.G. Gordon, and D. Sheldon. 1993. Restoring Wetlands in Washington: A
guidebook for wetland restoration, planning, and implementation, Washington
Department of Ecology, 110p.
King County Parks, Planning and Resources Department. 1991. King County Wetlands
Inventory, King County Environmental Division, Bellevue, Washington.
Mitsch, W. J., and J.G. Gosselink. 1993. Wetlands, 2d ed., John Wiley & Sons, Inc., New
York, 722p.
Morley, S., Personal Communication. November 19, 1999.
Reinhelt, L.E. and R. R. Horner. 1990. Characterization of the Hydrology and Water
Quality of Palustrine Wetlands Affected by Urban Stormwater: A report prepared for
the Puget Sound wetlands and stormwater management research program, King
County Resource Planning, Seattle, Washington, 37p.
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