The different sections of this article are
written by three members of the Cobb Country, GA, Stream Monitoring Program.
Three previous articles on the program have appeared in past issues of Stormwater:
“Municipal In-Stream Monitoring” in the September 2008 issue, “Municipal
In-Stream Chemical Monitoring” in the October 2008 issue, and “Municipal
In-Stream Macroinvertebrate Sampling” in the November/December 2008 issue.
Introduction
Although I have summarized and then detailed the
essential components of Cobb County’s Stream Monitoring Program in the last
three issues of Stormwater,
I want to conclude by presenting three important disciplines supporting the
comprehensive nature of stream monitoring: geomorphology, riparian botany, and
geographic information systems (GIS) applications for stream sampling. Robert
Bourne, the program’s long-term biologist from 1994 to 1999, addresses fluvial
geomorphology and riparian botany. Adam Sukenick and Erin Feichtner have
administered the Stream Monitoring Program since 1999, and in this article Adam
addresses GIS applications for in-stream monitoring. Whether considering
geomorphological impacts of flow-sponsored sediment redeposition, riparian
“buffering” of high-sheet-flow and absorption of overabundant plant nutrients,
or GIS management of multilayered water-quality program assets and relevant land
use features, it is not difficult to understand how geomorphology, riparian
botany, and GIS applications directly
support and aid in interpreting in-stream
chemical and biological assessments.
Cobb County’s Stream Monitoring Program is mandated,
firstly, by National Pollutant Discharge Elimination System (NPDES) legislation
requiring a “monitoring program for representative data collection for the term
of the permit that describes the location of outfalls or field screening points
to be sampled (or the location of in-stream stations), why the location is
representative, the frequency of sampling, parameters to be sampled, and a
description of sampling equipment” (CFR 40 122.26). The program also fulfills
biological sampling requirements for the NPDES permits of Cobb County’s Noonday
Creek and other expanding wastewater reclamation facilities. Such sampling
requirements were also recently added to sampling regimens emphasizing chemical
sampling in Phase I communities such as Cobb in the Metropolitan North Georgia
Water Planning District.
 |
| Anthropogenic-influenced, high-flow scoured bank with threatened tree, which, ironically, is needed for stabilization |
The California Stormwater Quality Association (CASQA)
also addresses the call for enhanced water-quality assessment in NPDES programs,
first defining
general
program assessment as providing managers “the feedback necessary to determine
whether their programs are achieving intended outcomes (complying with permit
requirements, increasing public awareness, changing behaviors, etc.), and
ultimately whether continued implementation will result in water quality and/or
habitat improvement” (CASQA 2008). Further distinguishing between the
implementation outcomes of programs like street sweeping, erosion control, and
education and the
desired
result of improved water quality, the document goes on to exhort more
limited programs to “evaluate program implementation and water quality, and seek
to find the relationship between the two.” The CASQA document concludes with
some “Effectiveness Needs and Future Directions,” arguing for “development of
cost-effective water quality assessment tools” and lamenting the “lack of
readily available and understandable methods”; both deficiencies, it says,
contribute to the preponderance of implementation assessment per se over
water-quality (improvement) sampling. From California to Georgia,
direct,
comprehensive water-quality assessment is incumbent upon water resource managers
and planners, as NPDES program implementation
water-quality
impact has been an issue for some time with entities ranging from
nongovernmental organizations to Congress and the Office of Management and
Budget.
Communities with opportunities to enhance their
programs by initiating comprehensive water-quality assessments must become
undaunted opportunists in preserving their most critical resource.
Alternatively, communities already established in water-quality sampling must
not ease into inertia and the easy chair of regulatory adequacy, as there are
kinetic, complex, physical, and political pressures characterizing communal life
journeys that are far from over. Indeed, environmental professionals and the
communities they serve must consider what must be done in the tough terms of
what had to be done.
—Lanse
Norris
Establish GIS for the
Future
Although water-quality analysis has advanced with
technology, water-quality technicians have been using the same collection
methods for some time. Although we have a better understanding of complex
relationships between sampled water quality and biological populations, what has
really changed is our capacity to store, analyze, and interpret the data.
Geographic information systems have the capacity to
revolutionize what we do with whole, historical data sets. The most commonly
recognized component of GIS is the final product, a graphical representation of
the data—a map. However, to think of GIS as only a map would be tantamount to
thinking of a water-quality study as only a final overall score of “good” or
“poor” for a stream.
If you were tasked with the responsibility to conduct
a study to determine baseline water-quality conditions, you would likely begin
by selecting sampling sites—sites that represent average conditions within the
study area. Considerations in choosing sites might include land use, industrial
discharges, drainage area, topography, stream accessibility, hydrography,
riparian composition, and soil type; the list is really limited only by the time
and resources you have to invest in the project. To begin selection, you
could
find a land use map, determine major land use classes within the study area, and
place a sampling site in each class. Next, you could look at industrial
discharges to make sure sampling sites are not in a direct waste stream.
Certainly, you would also evaluate drainage areas so that various sized
watersheds were being sampled, as well as first-, second-, and third-order
streams. After preliminary site selection, you would also pull out topographical
maps and look at road coverage to see whether the sites selected are
accessible.
Instead of using a drawer full of maps and countless
hours of field reconnaissance, you could employ GIS. GIS can be useful in site
selection by first defining the study area, then adding as many site-influencing
“layers” as you have available. Think of each map in the above scenario as a
layer on a GIS. Layers can be turned on and off to look at multiple images (or
combination of images) at once. By using the hydrography, riparian, and soil
layers simultaneously, you could, for example, determine that sites are likely
situated in wetlands without having to go to the field and spar with aggressive
wetland insects. To optimize time and efficiency, criteria for site selection
are predetermined, and GIS is used to choose potential sites. Later, after sites
are field-verified, you can create a whole new layer to geographically display
your sites.
With site selection finalized, you must now consider
which chemical parameters, for example, to analyze and how to manage data.
Consider that every visual map is really only a graphical representation of some
data set. Management of data is the second, less appreciated, component of a
GIS. A road map is merely a list of road names geographically referenced and
displayed. Water-quality data, like road names, can also be managed and later
displayed using GIS. A database of results from water-quality analysis can be
stored and used to “map” results of a water-quality study. Each site sampled for
a study will be geographically referenced with a name, location, and dataset.
Later, GIS can be used to visually display information contained within that
dataset. For example, fecal coliform data stored within GIS can be queried and
used to create a display that graphically displays each site that fails to meet
the state standard. Beyond that, with proper planning, chemical and land use
data can be cross referenced and used to display geographic relationships, such
as displaying all sampled sites located downstream of agricultural land use
areas that fail to meet state water-quality standards for fecal coliform. Even
further, by analyzing several years of data, you may be able to determine trends
in the data, such as when fecal coliform thresholds were exceeded and the rate
at which they’re increasing.
With enough data entered into the system, the data
relationships you can infer are quite comprehensive. If a water-quality study
includes data from outside sources (such as industrial monitoring, sanitary, and
storm sewer discharge data), as well as collected water chemistry, GIS may be
useful in detecting sources of “hits,” or unusual results, found during routine
sampling. Essentially, field investigation or reported problem inquiries often
begin at your computer. For example, if lab analysis produces a metals result
well above typical baseline levels, you can use GIS to first display topography
and delineate the area draining to the point at which the sample was collected.
Then, using land use coverage, the next step could involve highlighting all
industrial land use (the area with the highest potential to produce metals
contamination). Next, locate known industrial discharges within this selected
area and query industrial monitoring data to determine which industry is likely
to produce waste with metal identification and concentration close to the sample
collected downstream. If this doesn’t immediately narrow your search or identify
the source, it may mean that there is a new industry or previously unmonitored
industry you are not aware is discharging.
Using GIS to expose complex relationships illustrates
the third and final component of a GIS—its ability to analyze data and model or
create unrealized or derived datasets. GIS has the ability to manage countless
datasets, and its usefulness as an analytical tool is limited only by the stored
data. The example applications above illustrate how chemical, land use, and
industrial monitoring data can be used to detect trends and locate sources of
pollution, but consider GIS coverage of nonpoint-source information. Biological
and habitat data, for instance, are key elements of any water-quality program
and are very susceptible to change and influence from external inputs. Storage
of biological datasets gives a user the ability to graphically present sites
with the best biological diversity or illustrate locations where rare or
endangered fish were collected. However, by cross-referencing databases, we may
discover relationships previously unknown. More obvious relationships between
biological and chemical results have been explored, but using GIS might quickly
identify subtle thresholds or parameters that limit diversity. For instance, by
geographically mapping all sites where sensitive species, such as stoneflies,
exist and analyzing concomitant water chemistry results versus water chemistry
results where stoneflies are not found might demonstrate the possibility of a
biological limiting factor like pH or low dissolved oxygen (DO). With enough
chemical and biological sampling information, different existing datasets may be
cross referenced, and they may already
hold enough information to conclude that below a certain pH or DO level stonefly
populations will vanish. With thresholds for stoneflies established, one can now
filter the entire dataset to look for downward trends in pH or DO. If results
are found to be approaching stonefly tolerance levels, the identified sites can
be prioritized for additional study or protection.
 |
| Good natural buffer with integrated, stabilizing vegetation |
GIS has broader applications that cover projects from
implementation to assessment. From the initial question to the final answer, GIS
streamlines each step. New components of compliance permits, for example, can be
organized, managed, and evaluated using GIS. After identifying a problem—perhaps
spill containment time—GIS can be used for initial analysis of the problem.
In this case, one would identify all streams and
likely points of entry, such as storm drains or industrial discharges, then
collect input flow data through field work or from local monitoring stations
(such as USGS gages) to determine baseline flow rates at several points within
each watershed. With these data, one can employ GIS to test spill scenarios and
response times. With flow-rate data, and using spatial analysis to derive spill
travel distance, one could determine the dispersion time of a spill from its
point of entry to locations downstream, and, by adding traffic datasets for the
same area, one could also determine response times to locations throughout the
region. With the problem and response times now identified, one could focus on
slow response times and implement strategies to expedite containment. Using GIS,
one could create datasets to identify areas where current response plans are not
adequate. The user could also map areas where the fastest response time is most
critical.
With continued data input, GIS is better able to model
real-world incidents and allow one to continually assess the effectiveness of a
plan. After an occurrence of a spill, one can evaluate the implementation
strategy and determine its effectiveness.
In short, GIS is the only tool that can illustrate the
problem, analyze the data, model the impacts, and evaluate the results. The
answers provided are limited only by the questions one can ask given the
strength of the dataset, and Cobb County stream monitoring personnel often find
themselves framing questions after data distribution answers have already
emerged amid the comprehensive GIS data.
A geographic information system is a finely tuned,
uniquely individualized, and highly adaptable analytical tool with the ability
to store, retrieve, and graphically display any dataset. Information within the
system is readily accessible and easily manipulated for tailor-made results.
Investing time and effort in the beginning to establish a comprehensive GIS
allows one to efficiently produce results for current permit monitoring
requirements and also allows room for expansion as new regulations become
effective. While sample analysis and collection protocols show little growth,
the demands placed on those that impact, regulate, and evaluate water quality
will continue to change with environmental demand-driven permit requirements.
Incorporating GIS now establishes an efficient way to meet current reporting
demands and provides the capacity to expand with future challenges.
—Adam
Sukenick
Introducing Fluvial
Geomorphology
Not long ago, fluvial geomorphology was an obscure
discipline relegated mainly to collegiate research. Though great strides have
been made by numerous researchers in our understanding of fluvial geomorphology,
it is only recently that concepts of geomorphology have been applied. One of the
major proponents of applied geomorphology is David Rosgen. His book, Applied
River Morphology, is the bible of the discipline. In this book,
Rosgen brings together and synthesizes vast amounts of research into a cohesive
and comprehensive whole. He also standardizes and presents terms and
nomenclature for expressing concepts that relate to fluvial geomorphology. This
effort is not unlike what Eugene Odom did for the field of ecology, and the
ramifications may be nearly as important. Needless to say, one cannot discuss
the discipline of modern fluvial geomorphology with out evoking the name of Dave
Rosgen. It is often said that all philosophy is a footnote to Plato; so too
could it be said that all applied river geomorphology is a footnote to
Rosgen.
The study of fluvial geomorphology unites the
disciplines of geology, ecology, and hydrology. The principles of geomorphology
were developed through persistent and meticulous study of the interactions
between flowing waters and the land over which they flow. Applied geomorphology
utilizes these principles developed by researchers to quantitatively evaluate
how physical attributes of a stream impact the stream ecology and how the stream
interacts with the physical environment. In Applied
River Morphology, Rosgen takes much of the historical information on
stream geomorphology along with his own research and unites them into a
well-organized, systematic methodology for stream channel evaluation. Channels
are initially classified and delineated by their most salient physical features.
These features include bankfull, valley slope, sinuosity, belt width, slope,
width-depth ratio, and entrenchment. Rosgen uses these morphological
characteristics to establish a channel typing system that is very useful in
understanding the major forces sculpting a given channel. The typing system uses
four levels of resolution, with more specificity and parameters as one goes from
one level to the next. For details on the Rosgen method of evaluating stream
geomorphology (including definitions of terms), see Applied
River Morphology.
 |
| Bob Bourne setting a transect with Dave Breaden, senior stormwater management engineer, seated |
The fundamental unit of most stream studies is the
study reach. The study reach is a known length of stream segment where most of
the measurements and observations will be made. Study reaches can be chosen for
many reasons including establishing reference reaches, studying land use change,
or as part of a watershed assessment. It is important to map the watershed
upstream of the study reach and to gather information on land use and activities
in the watershed. After establishing the study reach, one is ready to begin
taking measurements. The first feature one should determine is bankfull.
Bankfull is the most important feature in assessing channel morphology. The
bankfull feature is created by a hydraulic event and is described as “the
incipient elevation on the bank where flooding begins” (Rosgen 1996).
The bankfull event is now associated with the concept
of competence, or the ability of the stream to transport sediment. This is an
extremely important concept when assessing the impact of urbanization on stream
channel morphology. In an undisturbed watershed, the largest amount of total
sediment transport is accomplished by bankfull hydrologic events that occur
roughly once every 1.5 years. These events are responsible for sculpting the
channel. In an undisturbed state, most natural channels maintain equilibrium by
aggrading and degrading at constant rates so that the forces of sedimentation
and erosion balance each other. There is clear physical evidence in most
undisturbed channels as to where these flows are transforming the channel.
Typical features include flood planes, the tops of point bars, areas of wrested
vegetation, and sedimentation. These features are used to determine bankfull. In
an urbanizing watershed, however, bankfull can be a very problematic indicator.
This is because the physical signs of bankfull may lose their consistency as the
hydraulic regime of a stream begins to change in response to changes in land
use. The well-documented changes in peak flows caused by changes in land use
result in changes in point bars and wrested vegetation as well as erosion and
deposition. During this state of flux, it may be all but impossible to determine
bankfull. Field personnel must draw on their experience, and it is best to have
two or more participate in the evaluation so they can discuss in depth the
physical observations at the site.
Cross-sectional area is one of the fundamental
dimensional measurements of geomorphology. A cross-sectional area is a slice of
stream channel taken across the width of the channel. These measurements should
be taken at a transect (a line drawn perpendicular to the flow of the stream
from one bank to the opposite bank) located along the study reach. (The number
and location of transects depends on the scope and purpose of the study.) To
measure cross-sectional area, one needs a consistent, level line across the
transect to be used as a reference against which changes in elevation can be
noted (usually with a measuring rod) as one proceeds across the transect
starting on one bank and ending on the opposite bank. Features such as bankfull,
edge of water, and depth of water should be noted at each transection of the
stream. Changes in bankfull lines and fresh scouring can be measured by stable
physical markers like bank pins and scour chains driven into areas of the
channel outside the cross section. Following the completion of the cross-section
measurements, substrate studies are normally conducted at the transect using a
pebble-count method. There are several competing methodologies, but all attempt
to quantify the particle size of streambed material.
Mapping can also
provide important information at a study site. The first measurement taken
during mapping is channel length, as measured by the linear length of the
thalweg, or deepest line of the channel. A tape is run the length of the
thalweg, and lateral measurements are taken at specific intervals at right
angles to the tape toward the banks. Features such as bankfull and edge of bank
are noted. Other features such as point bars, riffles, and pools are also
measured, providing detailed information on important physical features of the
reach subject to change over time. Other important features, such as belt width
and sinuosity, can also be established. Both measurements track the deviation of
the stream channel from a straight line between two points on a channel.
Sinuosity is the net difference in length between the meandering stream channel
and a straight line between two points at each end of—usually—the study reach.
Belt width is determined by the amplitude of the meanders. This information,
combined with the transects, can then be compiled and used to plot a plan view
map showing meanders, stream features, bed materials, and variations in the
dimensions of the channel.
Geomorphology
studies have a variety of uses, ranging from quick evaluations of stream
condition to detailed long-term studies designed to trace changes over time.
Geomorphology is very intertwined with the habitat component of biological
assessment. The habitat score for the metrics used in biological assessment is
in large part determined by geomorphological observations. Examples include
particle size embeddedness, undercut banks, and bank stability. Bank stability
is an indication of the capacity of streambanks to resist erosive forces.
(Rosgen’s notes list bank height, bank angle, density of roots, soil
stratification, and particle size as erodibility factors.) Many habitat
assessment protocols address erodibility by addressing bank stability. Bank
stability is defined as a measure of the potential for detachment of soil from
the upper and lower streambanks. Habitat assessment protocols evaluate the
bank’s erosive potential by examining salient signs of erosion along with bank
slope. Steep slopes are considered more unstable than gradual slopes. Banks are
rated inversely to percent slope and also to the amount of observable erosion.
Other factors include soil stratification, particle size, and vegetative cover.
Stratification increases erodibility; this is especially true when fine-grained
material rests upon coarse material. Erosive potential also increases initially
with particle size; for example, the stronger cohesive forces of consolidated
clay particles resist erosion more than sand. This does not hold true as size
increases further, such as when sand particles are compared to cobble and
boulders.
Geomorphology
studies are also important in tracking changes over time within a given
watershed. This analysis can provide important information on the effectiveness
of best management practices and land use ordinances designed to protect stream
channels from the potentially negative hydrologic impacts of land use
change, including flooding and erosion. Factors observed in geomorphology studies such as sediment transport, bank erosion,
undercutting and deposition, width depth ratio, and sinuosity can all be very
important indicators for hydraulic impact to a stream. In instances where it is
desirable to restore or mitigate damage to eroded stream channels and damaged
habitat, geomorphology techniques are essential. Without understanding the
nature of the forces that caused the problem in the first place, success in
mitigating these problems would be unlikely.
Traditional
monitoring efforts have centered on the chemical—and, later, the
biological—components of our flowing waters to assess water quality and stream
health. The addition of stream geomorphology has greatly broadened our
perspective. Geomorphology has played a central role in uniting all the
disciplines involved in studying streams and rivers into a comprehensive whole.
Geomorphology can help planners assess all aspects of flowing waters, including
stream ecological health, erosion, and flooding. With these powerful tools
available, it cannot be argued that we do not possess the means of both
evaluating and mitigating the impact of our land use decisions on our streams
and rivers, and that their quality is the direct result of our
choices.
—Bob
Bourne
Riparian Stream
Buffers
Stream corridors are an important part of the
terrestrial environment containing their own unique ecology. Plants and animals have historically migrated along the
corridors because of the availability of water, the ease of movement in flatter
terrain, and the moderating influence of water on air temperature. The interface
between aquatic and terrestrial habitats produces many opportunities for plants
and animals by providing a rich and diverse environment. The terrestrial region
near a stream can have a profound impact on the quality of the stream itself. It
is important that this region remain undisturbed and vegetated with a diversity
of native plants in order to maintain both good water quality and healthy stream
ecology.
Vegetation
performs many essential functions in the riparian zone and is important to the
stream environment even far away from the riparian zone. Woody shrubs and trees,
though a biological component of the environment, have a profound effect on the
physical integrity of the stream channel. For this reason, riparian vegetation
is included in most habitat assessment protocols. The habitat assessment
evaluates plants in terms of both vegetative cover on streambanks and riparian
corridor width in order to evaluate streambank stability as well as potential
for sedimentation, erosion, and water-quality-degrading runoff. Higher scores
are given for woody plants such as trees and shrubs, and lower scores are given
for grasses. This is because woody plants possess more substantial roots that
are more effective at holding and stabilizing soil and provide more habitat and
shading than do grasses. Most habitat assessments give lower scores when there
are breaks in the natural vegetative zone, especially when the zone has been
impacted by human activity.
As discussed,
vegetation is essential in protecting the physical integrity of the streambanks
and preventing erosion in the riparian zone. The root systems of plants play the
most important role, with larger roots providing structural support and bracing
and smaller roots providing a mesh, which holds soils and resists erosion.
Exposed roots and woody debris reduce stream flow velocities by increasing the
roughness of the banks and the streambed. Smaller plants, including grasses,
mosses, and ferns, also help hold and cover soil, especially in lower regions of
the banks. Ground cover, including leaf litter and humus stabilized by root
mats, protects the soil in the upper flood plain and terraces, preventing
sediment runoff into the stream from stormwater and flooding. All of these
elements together—trees, shrubs, smaller plants, and natural ground
cover—support the physical integrity of the streambank.
Vegetation reduces
overland stormwater flows by increasing the absorptive efficiency of the soil.
Leaves, whether on plants or on the ground, interrupt the kinetic force of
raindrops before they can reach the soil. Leaf litter and humus act as a sponge,
absorbing and holding moisture, then slowly releasing it to the soil. Roots
penetrate deep into the soil, aid in mixing of the organic and inorganic
fractions of the soil, and contribute organic matter when they die and rot.
Burrowing forest animals also mix the organic and inorganic components of the
soil. As a result, soils in the forest can more readily absorb moisture,
resulting in a larger portion of the rainwater being stored as groundwater.
Soils that have developed under hardwood forests usually have infiltration rates
exceeding the annual maximum rate of rainfall. Groundwater migrates through the
soil to low-lying areas, where it is discharged into a stream. Water entering a
stream in this manner is rarely turbid, due to the low velocity of the water and
the stability of the soil through which it passes. Because the water takes
longer to reach the stream and is discharged over a long time, the stream is
less impacted by rainfall and associated storm surges, mitigating the impact of
peak flow events and thus reducing erosion.
Vegetative buffers
along streams have been documented in numerous studies to decrease the
concentrations of pollutants in stormwater passing through the buffer. Both the
soil and the roots can purify water by removing contaminants. Soil uptake is
accomplished by binding contaminants to its humic organic fraction and through
cation exchange in the mineral fraction. Living plants and fungi and other biota
in the soil directly uptake, among other things, phosphate and nitrate into the
plant’s nutrient processing system, nutrients that cause unbalancing algal
blooms in streams. By impeding and dispersing flow, buffers also facilitate the
removal of sediment. Removal of sediment from stormwater protects the stream
from sedimentation and any potential pollutants that may be present in the
sediment. Pollutant removal efficiency is a function of buffer width: the wider
the buffer, the more effective the removal of pollutants. It is also important
that flows across the buffer are as evenly distributed as possible as sheet flow
over the surface area. Concentrated, channelized flow can short circuit a buffer
as well as damage it through erosion.
Vegetated buffers
also shade the stream and keep the water cool during hot periods. This, in turn,
maintains higher and more stable dissolved oxygen levels. DO levels are crucial
to stream organisms during the warm summer months. Shade also helps prevent
excessive algal growth during warmer weather. Algal growth, as quantified in the
general literature as chlorophyll a, is greatest in late winter and in the early
spring, pre-leaf-out period, when temperatures have moderated and the leaf
canopy is yet to be established. Although algal growth is documented as a
natural phenomenon, it can be exacerbated in urban areas by application of lawn
nutrients.
Information on
species diversity, density, and age provides extremely important information
about past land use and ecological health of the stream. In a mature, healthy
piedmont forest, one would expect an upper canopy with larger trees of native
lowland varieties and a lower canopy of understory trees, shrubs, and grasses.
Beech, birch, maple, tulip, ash, hackberry, and elm are all desirable streambank
trees. Hornbeams, alder bush, button bush, deciduous holly, swamp dogwood,
elderberry, and native cane are desirable understory plants. These plants not
only hold the soil well, but their leaves also provide a good food source for
macroinvertebrates. Protocols exist to determine tree age, diversity, and
density based on taxonomic keys and quantified field
observations.
Riparian plants
overhanging and entering the stream channel provide important habitat and are
especially important as a source of cover and as a food source for
macroinvertebrates and fish. As a result, plant species in the riparian zone may
have an impact on macroinvertebrate and fish species diversity. Information on
riparian condition and plant species can provide important insight when
evaluating aquatic biodiversity. This is especially true in headwater streams.
(There have been numerous studies published on riparian plant species and
macroinvertebrate diversity, including several studies by the US Army Corps of
Engineers and the University of Georgia.) Plant species composition can also
provide information on the age of the forest. The presence of younger trees and
early successional species, such as loblolly pine, sweet gum, and tulip,
indicate more recent clearing of the banks. These past activities in the
riparian buffer may have long-lasting effects on both fish and macroinvertebrate
communities.
Exotic
Plant Species. Many habitat
assessment protocols give higher scores to native species than to non-native
species. This is because some non-native species are able to outgrow and
out-reproduce native species and thus can dominate the flora of some areas. By
crowding out native plants, these invasives can alter the ecology of an area
negatively impacting the biota dependence on the presence of native species.
Thus, when non-native species dominate a buffer region, it can have a negative
impact on the stream biota. Non-native plants such as kudzu or Chinese privet
can inhibit the growth of native plants that are superior food sources and
streambank stabilizers. Privet is shade tolerant and can survive high soil
moisture as long as there is some drainage. As a result, it thrives on
floodplains even beneath an established canopy and can form dense stands at the
expense of native understory trees and shrubs. Kudzu will smother out all
competing vegetation, and, as its root structure is weak, it offers little
erosion protection during winter months when the plant dies
back.
Large monocultures
of non-native grasses are often planted along and near streams. Although these
plants are usually not hardy enough to significantly naturalize or become
aggressive pests, they may result in the exclusion of native species where they
are aided and maintained by humans. These grasses, such as fescue, were
cultivated in Europe and are adapted to a cool, moist climate with fairly rich
soils. As a result, they have shallow roots and require extensive watering
during dry periods, and they require chemical maintenance, especially when
planted on graded fill dirt consisting mainly of clay with minimal topsoil. The
hard clays beneath these grasses absorb very little water, and the roots are too
shallow and weak to greatly enhance absorption. As a result, runoff containing
lawn chemicals is a strong possibility when ornamental grass lawns descend into
the riparian corridor. The shallow roots also do not secure soils nearly as well
as native trees shrubs and grasses.
Stream buffers not
only help to protect and maintain stream quality but also provide an important
habitat for many aquatic and terrestrial organisms as well as an amenity for
humans. The presence of streamside vegetation and stream buffers is not only
aesthetically pleasing, but also vital to the stream and to all living things
that depend on the stream for their survival.
—Bob
Bourne