Assessing Freshwater Ecosystem Resilience in North Carolina
The Nature Conservancy – North Carolina Chapter
Dr. Catherine E. Burns
Background
Ecosystem resilience is the ability of an ecosystem to retain essential processes and structure in the face
of disturbances like climate change. As the pace of environmental change picks up due to climate
change and to land use change associated with a growing human population, identifying areas that are
likely to be highly resilient will be increasingly important for identifying priority areas for conservation
that will be effective in the long term. Although the precise species composition in a given area will
undoubtedly change in response to environmental changes, resilient systems will continue to sustain
high levels of biodiversity and ecosystem function, and it is these areas that hold the most promise for
conservation in the coming decades.
The Nature Conservancy’s North Carolina Chapter (TNC – NC) plans to carry out a terrestrial resilience
analysis in collaboration with other states in the Southeast, guided by TNC scientists who pioneered this
work in the Northeast (FY12 & FY13, pending funding from Doris Duke). This analysis will inform our
decisions regarding where we invest our resources in terrestrial protection and management, and where
we encourage our partners to engage. As in terrestrial ecosystems, freshwater systems will be strongly
shaped by increasing changes in climate, as well as by changing land use in areas surrounding aquatic
systems. The ability to accurately identify those rivers and streams that are likely to have the capacity to
adapt to these changes (i.e. those that are highly resilient) is a critical step towards protecting healthy
freshwater systems.
We propose to conduct an analysis of freshwater ecosystem resilience that will complement our
terrestrial resilience work and that will guide our selection of freshwater priorities for conservation and
restoration. We propose to conduct a freshwater resilience assessment during FY12 and FY13 that
encompasses all of the freshwater systems of North Carolina (NC). Though this will be the first study of
its kind in the Southeast, similar work has been pioneered by TNC in the Northeast and we aim to use
methods that will facilitate comparison of results with the assessment in the Northeast as well as with
other states in the Southeast, many of which are also planning to examine freshwater resilience in the
coming years.
Recent evidence (Rieman & Isaak 2010, Palmer et al. 2009) suggests that the resilience of freshwater
systems can largely be characterized by six elements: linear and lateral connectivity, water quality as
shaped by surrounding land use/cover, instream flow regime, access to groundwater, and the diversity
of geophysical settings in the area. We aim to quantify each of these components for the rivers and
streams in NC to develop a comprehensive assessment of resilience across the state’s freshwater
systems. Specific details are described in the methodological overview below.
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Objectives
We aim to identify the most resilient stream networks in NC that will collectively and individually sustain
native freshwater biodiversity even as the changing climate and land use alters current distribution
patterns. We will achieve this by analyzing at several spatial scales the six primary factors essential to
maintaining resilience, and then creating a combined resilience score for each system in NC. This work
will guide TNC’s freshwater efforts in NC, and will be shared broadly with chapters in the surrounding
states and with our many partners who are working towards freshwater conservation.
Scales of Analysis
Each of our analyses will be conducted at multiple spatial scales. We will do this to facilitate decisionmaking about where to invest resources in freshwater conservation across NC – decisions which require
information both about broad regional patterns as well as site-specific information. Specifically, we will
work to identify whole basins (HUC6) and sub-basins (HUC8) where the capacity to adapt to changing
conditions is greatest, as well as to identify those smaller regions within basins (HUC12; connected
stream networks, and catchments) where our data indicate truly outstanding potential for resilience.
Note: a HUC is a Hydrologic Unit Code that identifies drainage basins in a nested arrangement from
largest to smallest – the lower the number, the larger the area included (HUC6 > HUC8 > HUC10 >
HUC12, etc.).
Methodological Overview
For its work in the Northeast, TNC developed methods for measuring each of the six primary factors that
contribute to freshwater resilience, and this proposal expands on that research. We will separately
assess each of these components, and then will use these data to distinguish systems that appear to
have a large capacity to adapt from those that may have a limited capacity to adapt to future change.
The methods used for each of these characteristics, and examples of their use in other areas, are given
below.
(1) Linear connectivity
A connected stream network is the set of
streams that are bounded by fragmenting
features (dams, waterfalls, impassable culverts)
and/or the topmost extent of headwater
streams. Stream networks are shown in
unique colors in Figure 1 with dams indicated
by black dots. Connectivity within a network of
streams is essential to freshwater organisms. It
enables individuals to move throughout the
network to find the best conditions for feeding
and spawning, and in times of stress it enables
them to move to locations where the
conditions are more suitable for survival
Figure 1. Connected stream network generated using TNC’s
Barrier Assessment Tool.
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(Pringle 2001). There has been considerable impact on the connectivity of river systems in the
Southeast due to dams and impassible culverts. This has led to a substantial decrease in the length of
connected stream networks throughout the region. These changes will have lasting impacts on the
ability of these systems to respond to climate change and other stressors in the years to come. We
hypothesize that areas with greater linear connectivity will be more resilient to environmental change.
We will compare linear connectivity among NC’s stream networks by calculating barrier density and
Figure 2. The length of
connected stream
networks between
barriers. Low
connectedness is shown in
red, orange and yellow,
and higher connectedness
in green, blue and purple.
functional network length between barriers at several scales. To do this, we will use TNC’s Barrier
Assessment Tool (BAT) with data on natural and anthropogenic barriers from the National Hydrography
Dataset Plus (NHDPlus), the National Inventory of Dams (NID), the NC Dam Safety dataset, and the
Aquatic Obstruction Inventory data from the Wildlife Resources Commission. This analysis will give us a
quantitative assessment and visual representation of the areas of NC that are the most (and least)
connected. Figure 2 shows similar data for the Northeast, with areas in red and yellow indicating those
that are least connected and those in blue and purple those that are most connected. Our analyses will
also indicate which barriers, if removed, would result in the largest gain in functional network length.
Based on recent collaborations with Duke University and American Rivers, we now have a dataset
defining each of the connected stream networks in NC. We propose to build on this dataset to evaluate
network length and barrier density.
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(2) Lateral connectivity
In natural freshwater systems, the floodplain is periodically innundated with water, resulting in the
exchange of nutrients, sediments, and organisms that are necessary for the long-term health of these
systems. These periodic floods also create additional habitat for aquatic organisms that use it for
feeding and/or spawning, and serve to maintain the
stream channel physical habitat and nearby
terrestrial systems. These processes are all
necessary to support a fully functional freshwater
ecosystem and require good connectivity between
the channel and floodplain, or “lateral connectivity”
(Noe and Hupp, 2005). Due to land use change,
channelization and altered flow regimes from dam
operations, the historical extent of flooding has
been much diminished in many systems in the
Southeast. We hypothesize that areas with higher
lateral connectivity will be more resilient to climate
change and other disturbances.
For each watershed in NC, we will use an approach
developed by TNC to identify the Active River Area
(ARA) – the “areas of dynamic interaction between
the water and the land through which it flows”
(Smith et al. 2008). The ARA includes the river
meanderbelt, floodplain, riparian wetlands, terraces
and material contribution zones (Figure 3, top
panel). Naturally vegetated ARAs, especially in
floodplain areas, can store flood waters and
sediment to reduce flooding and erosion damages.
Figure 3. Top panel: The Active River Area (grey) for one
In addition, maintaining and restoring these areas to
region in Halifax County, VA. Bottom panel: Land cover
a more natural condition can foster infiltration that
within the Active River Area for a portion of the Chemung
River in NY. Dark orange indicates areas with greater than
serves to recharge groundwater aquifers which in
10% impervious surface, light orange indicates less than
turns help to mitigate the impact of low flows
10% impervious surface, and beige areas indicate 3% or less
associated with more frequent drought conditions.
impervious surface.
Using data from the National Land Cover Dataset
(NLCD 2006), we will quantify the composition and configuration of different land cover types found
within the ARA of each connected stream network to identify those systems with a greater proportion of
natural land cover, and hence with greater lateral connectivity and likelihood for resilience (Figure 3,
bottom panel).
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(3) Land cover and impacts on water quality
Water quality, and consequently the biotic
condition in the stream, declines with
increasing watershed imperviousness (CWP,
2003, Cuffney et al. 2010, King & Baker 2010,
Wenger et al. 2008), and is also impacted by
other aspects of land cover in the watershed
such as the prevalence of agriculture (Bolstad &
Swank 1997, Gergel et al. 2002, Mattson &
Angermeier 2007). The ability of freshwater
systems to adapt to disturbance relies on high
water quality which in turn relies on the land
cover and land uses surrounding the river
system. The rate of population growth in NC is
Figure 4. Land cover for each of NC’s watersheds.
high (18% in the past decade) and with it comes
rapid land use and land cover change. This means that in the future many freshwater systems will
experience conditions with more impervious surface and worse water quality than they do today. The
NC Chapter of TNC has recently completed an assessment of the land cover found in each of NC’s
watersheds, conducted at the full-basin (Figure 4) and sub-basin (HUC8) scales, as well as an analysis of
the rates of land cover change in each basin and sub-basin.
We propose here to extend this analysis of land cover to assess likely future patterns of land cover in
each watershed using development projections currently underway at NC State University. These
analyses together will allow us to identify freshwater systems that are likely to be highly stressed due to
the surrounding land use/cover, as well as those that are located in areas with comparatively natural
land cover. We will augment this approach with data from NC’s Division of Water Quality that lists each
of the state’s waterways that are “impaired” due to low water quality (DWQ’s 303(d) designation).
New techniques have been developed (Baker et al. 2006) to assess the potential for natural land cover
to buffer the transport of nutrients across the landscape and into the stream system. This type of
sophisticated analysis provides a way to evaluate how the configuration of cover types in the landscape
surrounding a given stream reach can buffer nutrient flow, and consequently to indicate which streams
are best buffered.
We hypothesize that better buffered systems (which should on average have higher water quality) will
be more resilient to climate change and other disturbances, and propose to conduct this type of analysis
for each of NC’s rivers and streams. Such an analysis has recently been conducted for the Northeastern
portion of NC and Southeastern VA, and clearly shows substantial differences in buffering due to
variation in land cover in the surrounding landscape (Figure 5: green shading indicates areas with
substantial buffering, red and orange shading indicates those with minimal buffering).
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Figure 5. Buffered Flowpath
Length for Northeast NC and
Southeast VA summarized at
the NHDPlus catchment scale.
The transport distances from
"source" pixels (e.g., row crop
agriculture) through downslope,
potential nutrient "sink" pixels
(e.g., forest and/or wetlands)
along flow pathways to streams
is computed. The output
represents distance through
buffer cells to the stream along
a flow pathway. The streams in
the green catchments are better
buffered from sources
(agriculture) than streams in the
other catchments.
(4) Instream flow regime
The instream flow regime, including the amount, frequency, duration and seasonality of flow though a
stream, plays a critical role in shaping the communities that live in freshwater systems (IFC 2004, Poff et
al. 1997, Poff et al. 2010, Postel & Richter 2003). Alterations in flow regime due to changes in patterns
of precipitation (e.g. increasing drought frequency), water withdrawals, land use and associated runoff,
and dam operations are common throughout the Southeast. These alterations have had, and will in the
future have, significant negative impacts on the species and communities that live in the region’s
waters. The specific responses of instream biota to altered flow regimes are not well understood,
though a growing body of literature has begun to address this (Carlisle et al. 2010, Fitzhugh and Vogel
2010). We hypothesize that streams with more natural flows (i.e. those with flows that are less altered)
will be more resilient to environmental changes, and to climate change in particular. Therefore, we
propose here to assess the degree of flow alteration in NC and its likely ecological consequences in three
ways:
1) Conduct a literature review to synthesize existing information on ecological responses to flow
alteration in the Southeast, and based on this review, propose hypotheses for a subset of freshwater
species/guilds that describe their response to altered flows (flow-ecology hypotheses).
2) Use a modeling approach to identify areas where the observed stream flow has been highly altered
relative to historic conditions, and to identify areas where stream flow is closer to natural, unaltered
conditions.
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3) Use a modeling approach to forecast how future climate and land use changes are likely to impact
flows.
Combining information on the flow requirements of instream biota (based on the literature review) with
data on the historic, current and future flows within each watershed will enable us to identify areas
where flow conditions are most likely to support resilient freshwater communities now and in the
future. The literature review and formation of flow-ecology hypotheses will be conducted by a postdoctoral fellow who will work under the direction of Cat Burns, and in collaboration with Dr. Martin
Doyle at Duke University who specializes in environmental flow research. The modeling research will be
conducted by the post-doctoral fellow and by Cat Burns, using the WaterFALL™ (Watershed Flow and
ALLocation) Model recently developed by the Research Triangle Institute (RTI; Figure 6).
Modeling approach: The WaterFALL™ model is a user-friendly, web-based platform to quantify flows
using the NHDPlus platform. The model allows the user to quantify different metrics describing the
flow regime at scales ranging from small catchments up to entire basins. It also enables the user to
manipulate land cover in a given area by increasing or decreasing the percent cover of different land
cover types or by importing a new data layer that includes predicted land cover types in the future.
Similarly, WaterFALL™ allows the user to manipulate future climate, including precipitation and
temperature, by increasing or decreasing values by a fixed percentage or increment, or by importing
data from other sources that project future climate scenarios for NC.
Figure 6. Research Triangle Institute’s WaterFALL™ model, seen here as a screen shot from their web-interface.
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We will first model the historical, unaltered flows for each watershed in NC. We will then work with RTI
staff to model current patterns of flow by incorporating information on water withdrawals/returns and
dam operations where available or by relying on streamflow monitoring gage trends in the absence of
reliable data on specific human alterations. Next, we will quantify future flows for a range of scenarios
representing low, intermediate and high rates of development, and representing a variety of likely
climate change scenarios (data will be obtained from NC State University and the SE Climate Science
Center). We will compare the unaltered flows with the current (altered) flows and with flows under
future land cover and climate scenarios to identify areas experiencing the greatest hydrologic alteration.
To accomplish these comparisons, we will use TNC’s Indicators of Hydrologic Alteration (IHA) methods
(Richter et al. 1996, Mathews & Richter 2007), which will allow us to map how altered the flows in each
stream/network are currently, or are likely to be in the future. Finally, we will use the results of the
literature review and the flow-ecology relationships along with the data from WaterFALL™ to identify
areas where current and future flow regimes are most likely to support resilient freshwater ecosystems.
(5) Access to groundwater
Access to groundwater is important for stream resilience for two primary reasons. It moderates flow
regimes and leads to less flashy streams, reducing the likelihood that flows will fall below critical levels.
In addition, the influx of groundwater into streams helps maintain cooler water temperatures which are
important for some instream communities and which will become more critical as air and water
temperatures rise due to climate change. We hypothesize that freshwater systems with greater access
to groundwater will be more resilient, and propose to compare groundwater access by calculating the
Baseflow Index for NC’s rivers and streams. Baseflow is the component of streamflow that can be
attributed to groundwater discharge into streams, and the Baseflow Index is calculated as the ratio of
baseflow to total flow. The output from the WaterFALL™ model will be set up to generate this
information. WaterFALL™ estimates streamflow contributions as the combination of runoff from the
surface and baseflow from shallow groundwater. Baseflow consists of water that has infiltrated the
subsurface and reached the saturated zone. Flow from the saturated zone into the stream channel is
controlled by the recession coefficient, a calibrated parameter within WaterFALL™. Baseflow estimates,
as with streamflow estimates, are produced on a daily time step for each NHDPlus catchment.
(6) Diversity of geophysical settings
A resilient freshwater system spans the diversity of geophysical settings in its drainage. A highly resilient
system would ideally include variation in elevation, gradient, geology and stream size. These factors
have long been identified as important in shaping freshwater biodiversity (Higgins et al. 2005). To
conserve freshwater biodiversity, we need to protect these combinations of geophysical factors that
over an evolutionary timescale ultimately drive patterns of freshwater biodiversity (Anderson and
Ferree 2010, Palmer et al. 2009, Rieman & Isaak 2010).
We propose to quantify the geophysical diversity at multiple spatial scales, from the catchment up to
the basin scale. This analysis will highlight areas where the geophysical context is more diverse and
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hence more likely to be resilient when facing climate change and other disturbances. For example,
recent analyses in the Northeast provide a simplified classification (92 classes) of streams that takes into
account the stream size, temperature, gradient and geology (Figure 7, left panel). Tallying the number
of stream classes found within each connected stream network in the region provides an indication of
the variability in geophysical diversity across the region (Figure 7, right panel). We will also compare
this diversity to the expected natural geophysical diversity of the system given no anthropogenic
barriers.
Figure 7. Quantifying geophysical diversity in stream networks. Left panel: stream classification based on size,
temperature, gradient and geology. Right panel: the number of different stream classes found within a connected
stream network, an indication of an area’s geophysical diversity. High diversity areas (those with many stream
classes) are shown in green and blue, and low diversity areas in pink, orange and yellow.
Incorporating information on geophysical diversity will allow us to: 1) take into account the ultimate
drivers of freshwater biodiversity, 2) capture the variety of available microclimates and gradients that
species can take advantage of during rearrangement in response to disturbance, and 3) better integrate
genetic and phenotypic diversity in our resilience planning (e.g. conserve genotypic/phenotypic diversity
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by conserving a diverse representation of habitats across river basins with adequate redundancy
[Rieman & Isaak 2010]).
Summary
We aim to conduct a state-wide analysis of resilience for NC’s freshwater systems, and will assess the
capacity of each system to cope with environmental change in each of six ways. Individually, these
elements are each important for informing freshwater conservation in the state, and none of these
analyses have yet been conducted for the entire state of NC. Together, these analyses will highlight
those areas with the greatest potential for resilience, and will provide us a strong platform for making
decisions about where to prioritize freshwater conservation efforts in ways that will continue to be
effective in the future. Aspects of these six components have been investigated in the Northeast by TNC
and are now being used to inform TNC’s actions in that region. We envision that this analysis will
likewise shape our work and that of our partners.
Products
Maps and datasets:
We will produce a set of data layers (with metadata) and associated maps in GIS that show the range of
values for linear connectivity, lateral connectivity, water quality and buffering, groundwater access and
the diversity of geophysical settings. From our modeling work with WaterFALL™, we will also produce
GIS maps and data tables indicating values characterizing instream flow regimes for unaltered, current
and future scenarios. We will also produce a map that synthesizes all of this information into one
ranking system to identify areas most likely to be highly resilient.
Reports:
We will produce two reports, both of which will also be later reformatted and submitted for publication
in a peer-reviewed conservation journal, such as Conservation Biology. 1) A report summarizing all of
the analyses conducted during this investigation and discussing their implications for conservation of
NC’s freshwater systems. This report will be distributed widely both internally and externally, to help
develop a consensus in North Carolina with partners about freshwater resilience and long-range
planning for freshwater conservation, as well as to facilitate discussion with other TNC chapters. This
report will be prepared by C. Burns. 2) A report detailing the results of the literature review on
ecological responses to flow alteration in the Southeast, and describing flow-ecology hypotheses
generated based on the literature review. This report will be distributed broadly within TNC, to
interested partners, and to members of the Department of Environment and Natural Resources’
Ecological Flows Science Advisory Board (of which C. Burns is a member) to inform ongoing efforts to
incorporate environmental flows into water planning across NC.
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Timeline
The entire project can be completed and its products generated in three years. Since each component
will be addressed on different time lines and involves different TNC staff members and partners, details
are provided in the table below, assuming a start date of August 1, 2011. Synthesis of these
components to provide a comprehensive assessment of resilience across the state will be conducted by
C. Burns and will be completed by May 1, 2014. This report and the most relevant data and maps will be
circulated for internal and external peer-review and subsequently revised by July 31, 2014.
TNC staff member(s)
leading analysis
Partners involved in
research and role
Estimated time of
completion
Linear connectivity
Margaret Fields –NC Chapter
Office, Durham
American Rivers, Duke
University: providing
barrier data and initial
stream network analysis
Full analysis completed by July
2012
Lateral connectivity
Analie Barnett – TNC
Southern Resource Office,
Durham
Land cover and water
quality
Margaret Fields and Analie
Barnett
NC State University:
providing future land cover
data
Future land cover analysis
completed by July 2012;
Buffering analysis completed
by July 2013
Flow regime
Catherine Burns and
Post-doctoral Fellow (TBA) –
NC Chapter Office, Durham
Unaltered flows (July 2012),
Literature review (July 2012),
Flow-ecology relationships (Jan
2013), Current and future flow
scenarios (Jan 2014)
Groundwater access
Catherine Burns
Research Triangle Institute:
providing access to the
WaterFALL model and
technical support
SE Climate Science Center:
providing down-scaled data
on future climate scenarios
Research Triangle Institute:
provide access to the
WaterFALL model and
technical support
Diversity of geophysical
settings
John Prince – TNC Southern
Resource Office, Durham OR
Arlene Olivero – TNC Eastern
Resource Office, Boston
Full analysis completed by Jan
2014
Full analysis completed by Jan
2014
Full analysis completed by Jan
2014
Literature Cited
Anderson, M.G. and C.E. Ferree. 2010. Conserving the Stage: Climate Change and the Geophysical
Underpinnings of Species Diversity. PlosOne. July 2010 Volume 5, Issue 7. E11554 p 1-10
Annear, T.I. et al. 2004. Instream flows for riverine resource stewardship, revised edition. Instream Flow
Council. Cheyenne, WY.
Baker, M.E., D.E. Weller, T.E. Jordan. 2006. Improved methods for quantifying potential nutrient
interception by riparian buffers. Landscape Ecology 21: 1327-45.
Bolstad, P. V., and W. T. Swank. 1997. Cumulative impacts of landuse on water quality in a southern
Appalachian watershed. Journal of the American Water Resources Association 33:519-533
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Carlisle, D.M., D.M. Wolock, M.R. Meador. 2010. Alteration of streamflow magnitudes and potential
ecological consequences: a multiregional assessment. Frontiers in Ecology and the
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