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Contents
Introduction 1
Management Recommendations 3
Channel 3
Flood Plain 5
Watershed 5
Summary 6
Background Reading 7
Cover photos by Roxanna Johnston
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Primary Contributors to this Status Report were:
Dan Karig, Professor Emeritus, Department of Earth and Atmospheric Sciences, Cornell University
Todd Miller, Hydrologist, U.S. Geological Survey, Ithaca,New York
Kate Hackett, Senior Planner, Tompkins County Planning Department
Roxanna Johnston, Watershed Coordinator, City of Ithaca,New York
Sixmile Creek: A Management Overview
DRAFT—February, 2008
Introduction
According to recent studies, the Sixmile Creek ecosystem is fairly healthy. The watershed remains
largely undeveloped and the water quality is rated the best of the local streams by the Community
Science Institute. The largest problem in Sixmile Creek is a high load of suspended sediment, a result
of erosion along the main channel and tributaries, predominantly from Brooktondale downstream to
the City of Ithaca dams.
Much or most of this erosion is probably a result of removal of sediment stored along the channels
that was deposited when there was more intensive deforestation and agriculture in the watershed from
the mid 19' to early 20th centuries. This activity caused high rates of soil erosion, primarily in the
upland areas, and resulted in deposition of some of this material in the valleys and in channel
aggradation(the build-up of sediment). In the early 20th century,the watershed began reverting to a
more forested condition, leading to leading to less erosion in the uplands but to erosion of the
sediment deposited in the lower reaches of the watershed together with some underlying clay-rich
glacial deposits. This change in channel behavior,together with an increase in channel sinuosity, is a
natural attempt of the stream channel to attain equilibrium in response to the reduced sediment yield
of the reforested watershed.
The water chemistry of the creek, as measured by the Sixmile Volunteer Monitors and others, shows
lower concentrations of nitrate/nitrogen and dissolved phosphorus than do watersheds with more
agricultural activity, such as Salmon Creek. Chloride concentrations are relatively low but increase
downstream. Much of the increase can be attributed to road salt but the sharp increase in chloride
concentrations from Slaterville Springs to Brooktondale probably reflects natural discharge of
brackish water originating from salt layers in the bedrock.Escherichia coli, (E. coli) levels, an
indicator of pathogens, are usually low but at times are quite high.
Channel sinuosity, at least in the reaches between the dams and Brooktondale, appears to have
increased since 1936, when the earliest aerial photographs were taken. This increase in sinuosity
suggests that the channel is moving toward an equilibrium representing a more"natural" character
with the return of a forested watershed.
The riparian corridor along the creek is minimally impacted by transportation and utility structures
and residential and commercial development from Middaugh Road to the City dams, as well as above
Slaterville Springs, where the restoration project known as the Barille Project is located. There is
significant interaction between the riparian corridor and human activity through Slaterville Springs
and Brooktondale and slight to moderate interaction further downstream to Middaugh Rd.
The watershed beyond the riparian corridor has improved greatly because reforestation has stabilized
soils and reduced the amount of sediment supplied from the watershed. At present the area devoted to
agriculture constitutes only a small fraction of the watershed. Housing and industrial development is
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not yet a significant factor, but definitely could threaten these improvements, particularly where clay-
rich sediments underlie the watershed.
Although many behavioral aspects of Sixmile Creek are now fairly well understood, others are not.
Several priorities for future research were identified. These include:
1) Quantifying the amount and source of bedload sediment moving through the watershed;
2) Describing how channel sinuosity and channel cross-sectional shape has evolved during the
20th century;
3) Quantifying the amount and source of the sediment input to Sixmile Creek from its
tributaries,
4) Determining the effects of road drainage ditches on storm-water runoff and channel erosion.
Bedload is intrinsically very difficult to measure but is a very important factor in stream channel
behavior. The"silt dam"traps all bedload from above that point in the channel, as well as some
fraction of the suspended load. Measuring the total amount of sediment removed from the silt dam
and of sediment deposited in the City reservoir would permit the calculation of the total sediment
load that is transported by Sixmile Creek to that point along the stream. An estimation of the ratios of
material having different grain size trapped by the silt dam would be necessary to differentiate
bedload from suspended load in this trap. Unfortunately, none of this information was obtained when
a contractor excavated the sediment from the silt dam.
Quantification of the bedload above the Barille project(Slaterville Springs) is an important objective
because there are questions concerning the role of bedload in the design and function of Natural
Channel Design (NCD) projects such as Barille and Barille II. Several ideas as to how to constrain, if
not quantify this bedload have been discussed, including tracking painted rocks in the bedload and
measuring the movement of bedload "waves".
Measurement of channel sinuosity and width over time and along the stream can provide information
as to how the channel is evolving as the watershed has returned to a more forested condition. These
data could help improve channel design projects. An initial study of sinuosity from the dams to
Brooktondale, using imagery from 1936 and 2003 was made for the Status Report, but this should be
expanded to include air-photo imagery from intervening dates and also to include channel width data.
Analysis of data upstream from Brooktondale would also be useful.
Indirect evidence indicates that most of the suspended sediment load in the Sixmile channel is
derived from tributaries entering below Brooktondale, but it would be valuable to document this with
data collected in the field. This could be done by a synoptic collection of suspended sediment
samples and flow measurements from all these tributaries, but the logistics behind such an effort are
daunting. However, even selective measurements of suspended sediments in selected tributaries
would be valuable for comparison with samples collected at the same time by the Sixmile Volunteer
Monitors.
Dr. Rebecca Schneider, Cornell University Natural Resource Department, has been investigating the
role of road ditches on watershed behavior, with some data collected in the Sixmile drainage. These
data should be acquired and analyzed.
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Another factor with largely unknown effects on the Sixmile system is climate change. This is
predicted to increase annual precipitation in this region, which has already been recognized. There
might also be a change in the intensity of rainfall events. Both effects might change the character of
the stream channel through changes in the size of the mean annual flood.
Although some aspects of the Sixmile system remain relatively unknown or debatable, a number of
watershed/stream channel management recommendations can be made with some confidence based
on the information at hand.
Management Recommendations
Management in the Sixmile system must be integrated among its various hydrologic components:
channel, riparian zone and off-channel watershed.This is a well-recognized paradigm and was
explicitly recommended in Flood Mitigation Needs Assessment for Sixmile Creek(Milone and
MacBroom, Inc.2003).
Channel
The reach between Middaugh Rd. and the silt dam is largely without structures or streamside
development and seems to be returning to an equilibrium state of moderate sinuosity with pools and
riffles. Measurements at German Crossroad indicate that channel degradation has significantly
decreased, although several zones of active incision are still observed. Future degradation will be
inhibited by the massive structures at the pipeline crossings, which act as local base levels. As long
as the channel doesn't threaten homes and critical structures such as pipelines and bridges,
there is no reason to interfere with this natural process and it is obviously the least expensive
approach.
This do-nothing solution will not address the problem of the large suspended load in this section of
the creek. Most of this sediment is derived from the tributaries and would not be addressed by work
done in the main channel. Remedial efforts in these tributaries might be possible but would be a
daunting and very expensive task. Continued incision of and slumping along these channels will
follow the equilibrium trend now occurring in the main channel and will lead naturally to a reduction
in sediment supply, although the time frame is uncertain.
Reaches between and below the dams are highly erosive. This is a case of"clear water erosion",
which occurs when sediment, especially bedload, is removed from the stream. Downstream of that
point, the stream has excess energy and capacity to transport sediment, and thus attempts to erode its
bed and banks. Degradation is impeded by bedrock exposures in much of the streambed along these
reaches of Sixmile Creek and the channel response has been lateral erosion and avulsion (sudden
shifts in channel position). The channel will remain in this condition as long as bedload continues to
be trapped behind the dams. The most reasonable solution to erosional problems in these reaches
is the use of hard engineering structures to control the channel.
If the City of Ithaca decides to abandon Sixmile Creek as a water supply, dam removal becomes a
management option. This would result in the transport of several hundred thousand cubic yards of
sediment stored behind the dams downstream into the Cayuga Inlet, where efforts are underway to
dredge sediment already accumulated there. If abandoned for water supply and not removed, the
dams must still be maintained, although they could be allowed to fill. . Once filled with sediment, the
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stream will transport sediment from the watershed past the dams and into Cayuga Inlet. If the Inlet
is to remain navigable,a decision will have to be made whether to remove sediment from
behind the dams or from the Inlet.
The reaches above the Barrlle Project at Slaterville Springs are nearly"pristine"and here again the
best management seems to be to allow the stream to continue to equilibrate with watershed
conditions without engineered interference. Only along the headwaters of the east branch(Irish
Settlement Rd) has there been disturbance of and encroachment toward the channel. Management of
this reach should follow the recommendations for riparian buffers described under"Floodplains."
The most intensive interaction between the channel and human activity occurs along the reaches
through Slaterville Springs and Brooktondale. It is in these reaches that pressure for channel
manipulation is and will be the greatest because the processes of channel equilibration now and for
the foreseeable future will cause bank erosion. Channel erosion control projects such as Natural
Channel Design (NCD) projects, in particular those following the Rosgen protocol, have been
implemented (Barrile project), and are being planned for other reaches of Sixmile Creek, but several
issues should be evaluated before embarking on these projects..
First, there are many benefits in living along a stream but it should be recognized that this is a
hazardous environment. The consequences of living by the creek, with both its beneficial and
hazardous aspects, are primarily the responsibility of those living there, rather than that of the public.
Second, it must also be pointed out that the effectiveness and cost benefit of the channel projects,
even the NCD projects, are still debatable. A NCD project may have the geometries of a natural
channel but differ in one very important aspect: it is designed not to migrate or change its geometry,
as do natural channels because of the fixed constraining structures in the former. Both natural
channels and NCD's have geometries that reflect the flow at bankfull conditions because this
condition reflects the maximum work done on the channel over time. Although NDC structures are
constructed so as to remain stable during floods,their track record in larger floods(e.g. FFI of
years) is not great. They often fail under these conditions. This may be due to the fact that an NDC
cannot dynamically change its position,as a natural channel often does.
Third, owners and developers of streamside properties should be aware of the limitations of NCD and
other channel remediation projects. There are many cautionary anecdotes about municipalities
developing land adjacent to a channel and subsequently suffering financial and even human loss
when a large flood event occurs. At the very least, some sort of riparian buffer or"setback"
should be incorporated into stream channel "restoration" projects. Finally, although seldom
done, cost-benefit studies might be considered for these projects. The costs involved in NCD projects
usually run several hundred dollars per foot of channel.
A significant fraction of channel management occurs in the sections adjacent to public bridges.
This work is unavoidable but should maintain the natural channel parameters as much as
possible.
Several past and existing management techniques are counterproductive and are to be avoided
in the Sixmile Creek channel. These include removing gravel from the channel system,
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reducing the channel sinuosity, reducing channel roughness,or other practices that increase
water velocity in the channel.
Removal of bedload (aka"gravel") from the channel and floodplain only increases the incision
(downcutting and bank erosion) downstream of the removal point because Sixmile already has
insufficient bedload for its energy. This energy depends upon several factors, but the most
controllable is the water velocity. The quantity of sediment transported increases exponentially as
velocity increases. Velocity increases as the channel slope increases(sinuosity decreases)and as
channel roughness decreases. The procedures of removing gravel and bulldozing a smooth
straight channel that increases slope and decreases roughness should definitely be abandoned.
Flood Plain
Stream systems include more than the channel at normal (low flow) conditions. Another component
is the flood plain, which is that area flooded by an event of some chosen Flood Frequency Interval
(FFI). For example,the 50 year flood plain is that area covered by a flood that statistically occurs
once in 50 years. For the health of the stream and for the safety and welfare of those living along
the stream,it is important to control development along the floodplain. This can be most
logically done by creating a riparian buffer or setback. This is an area adjacent to the channel
within which certain forms of development should not be undertaken. The widths of such zones must
be discussed and legislated, but certainly should include the flood plain of some fairly large flood
event(FFI of 50 to 100 years).
There are many benefits of riparian buffers, not the least of which is financial. Often the long period
between major floods, especially when channel restoration projects have been completed, leads to
complacency and to development too close to the channel. There are abundant data showing the
constantly increasing financial loss resulting from flooding in the US, much of which is due to
unwise construction too close to the channel. From a cost-benefit perspective, it is usually cheaper to
allow a stream channel to have an undeveloped floodplain in which to expand and to move. Other
positive aspects include improved water quality, enhanced ecological conditions, increased flood
amelioration, and a more aesthetically appealing river environment.
Creation of a riparian buffer does not mean that nothing at all can be done in that area. There has
been much discussion concerning the things that should and shouldn't be in riparian buffers but some
of these are commonly agreed upon. Inhabited structures and impermeable surfaces,as well as
potentially polluting activities(e.g.,manure spreading) should definitely be avoided in the flood
plain. Fill material and other obstructions on the flood plain should also be avoided. Vegetation
adjacent to the channel is beneficial because it resists erosion and trees provide shade, which reduces
water temperatures.Natural flora is preferred, but even untilled land and lawns are better than
unvegetated surfaces. Of course, wider is better, but even a narrow strip of bankside trees offers
significant ecological benefits.
Watershed
At present,the Sixmile watershed is in fairly good shape, with a large percentage in woodland or
abandoned agricultural land; future development poses the greatest problem.Nevertheless, some
current practices should be addressed.
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Even though agriculture is now a minor land use, agricultural practices have an effect on soil yield.
Tillage methods are important. Contour plowing reduces runoff and erosion, but this practice appears
to be much less prevalent than in nearby areas such as Cayuga County. If there are cogent reasons to
resist contour plowing, reduced and conservation tillage should be considered. Current and future
agricultural practices should specifically protect riparian and wet areas,soil loss should be
minimized from working fields, and working fields should be buffered by permanent vegetated
areas.
Road ditches and field drainage systems have been cited as responsible for disruption of surface flow
and for reduction in infiltration. Local highway departments should be appraised of the studies
on the effects of ditching and trained in practices that minimize their negative effects.Rapid
hydroseeding of scraped roadside ditches should continue.
Analyses at the City of Ithaca Water Treatment Plant have shown that sodium concentrations have
increased from 2005 to 2007 (Averages; 15 to 19mg/L). Public notification is required for values
>20 mg/L. Sodium concentrations are strongly correlated with chloride concentrations, which have
also been rising in local streams. Increases in these chemicals have been regionally linked to road de-
icers, usually sodium chloride. Highway departments should minimize the negative effects of de-
icing compounds through product choice(favoring chemicals that require lower application
rates),changes in practices(anti-icing instead of de-icing)and public service announcements
reminding drivers to anticipate compromised road surfaces.
Perhaps the most threatening practices are residential and commercial construction, during which the
unvegetated surfaces can temporarily supply large amounts of runoff and soil to the channel system.
To some degree this may be mitigated by the stormwater regulations now being developed in the
town legislatures. Enforcement of these ordinances is key to their effectiveness.
After completion of construction projects, the increase in impermeable surfaces leads to increased
runoff and higher peak flows. Parking lots are perhaps the single largest type of impermeable surface,
but that effect can be reduced by permeable surfaces and retention basins. Municipalities are
encouraged to encourage reduction of erosion and impervious surfaces in land use planning
and review processes
Summary
The recommendations in this report speak to the importance of coordinated management of Sixmile
Creek that follows the logic set forth in the Milone and MacBroom report,Flood Mitigation Needs
Assessment for Sixmile Creek. Stream health depends on both implementing new practices and
stopping practices that are counterproductive.
Below is a list of the recommendations detailed in the body of this report:
• As long as the channel doesn't threaten homes and critical structures such as pipelines and
bridges, there is no reason to interfere with the natural stream process and it is obviously the
least expensive approach.
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• In the erodible areas near the City dams the most reasonable solution is the use of hard
engineering structures to control the channel.
• In the section above the Barille project, best management is to allow the stream to continue to
equilibrate with watershed conditions without engineered interference.
• Channel management adjacent to public bridges should maintain the natural channel
parameters as much as possible.
• Several past and existing management techniques should be stopped. These include removing
gravel from the channel system, reducing the channel sinuosity, reducing channel roughness,
or other practices that increase water velocity in the channel.
• Inhabited structures and impermeable surfaces, as well as potentially polluting activities(e.g.,
manure spreading) should definitely be avoided in the flood plain. Fill material and other
obstructions on the flood plain should also be avoided.
• A vegetation corridor, known as riparian buffers, should flank the creek.
• Agricultural practices should protect riparian and wet areas, minimize soil loss. Working
fields should be buffered by permanent vegetated areas.
• Local highway departments should adopt practices that minimize the negative effects of
roadside ditching and deicing.
• Municipalities should reduce the negative effects of new construction by enacting and
enforcing legislation, land use planning and review processes that control erosion and reduce
impervious surfaces.
Background Reading
Support for and details concerning material in the preceding text can be found in the following
resources.
Milone and MacBroom, Inc. , 2003, Flood Mitigation Needs Assessment; Six Mile Creek, Tompkins
County,New York.
This a report commissioned by the Tompkins County Planning Department that outlines the
hydrology of Sixmile creek. It was written by professional hydrologist and reflects that perspective.
Copies are available from the Tompkins County Planning Department.
Sixmile Creek: A Status Report. 2007. 35 p.
This report was written by a number of local people with scientific or technical backgrounds in
stream related issues under the auspices of the Sixmile Creek Partners. The report includes extensive
information concerning the geologic infrastructure, sediment transport and aspects of aquatic health
(e.g. biological diversity and water quality). Integrating this report with the MMI report provides a
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useful background to Sixmile Creek as of 2007. It is available online for review at the Tompkins
County Planning Department.
Leopold, Luna, 1994, A View of the River, Harvard University Press
This book is one of many on Stream behavior, but one of the best. It covers most aspects of stream
behavior(fluvial geomorphology) in a complete and moderately technical manner, but is quite
readable by most people.
Keller, E. A, 2001. Environmental Geology, Upper Saddle River,New Jersey, Prentice Hall.
This is one of the better texts on environmental geology that covers the human interactions with
streams as well as presenting stream behavior at an elementary level. This and many other such texts
are available at local libraries
Rosgen, Dave, 1996, Applied River Morphology, Wildland Hydrology, Pagosa Springs, Colorado.
This book covers the Rosgen approach to channel management as well as providing underlying
hydrologic principals to that approach. An extensive list of websites related to the Rosgen
methodology can be found at: http://www.wildlandhydrologv.com/html/references .html
Community Science Institute website;
http://www.communityscience.org/SixMile/SixMileCreek.html
See pages at this site concerning water quality and flow. Profiles of various chemical and physical
parameters along Sixmile Creek are posted here. The page on summary of results gives a brief
analysis of most of these data.
Langen, et. al., 2006. Environmental Impacts of Winter Road Management at the Cascade Lakes and
Chapel Pond. Clarkson Center for the Environment, Report#1.
This is a recommended resource for the effects of road salt on the environment and on methods to
manage the problem.
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