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Content may be subject to copyright.
FEATURED COLLECTION INTRODUCTION: CONNECTIVITY OF STREAMS AND
WETLANDS TO DOWNSTREAM WATERS
1
Laurie C. Alexander, Ken M. Fritz, Kate A. Schofield, Bradley C. Autrey, Julie E. DeMeester, Heather E.
Golden , David C. Goodrich, William G. Kepner, Hadas R. Kiperwas, Charles R. Lane, Stephen D. LeDuc,
Scott G. Leibowitz, Michael G. McManus , Amina I. Pollard, Caroline E. Ridley, Melanie K. Vanderhoof, and
Parker J. Wigington, Jr.
2
ABSTRACT: Connectivity is a fundamental but highly dynamic property of watersheds. Variability in the types
and degrees of aquatic ecosystem connectivity presents challenges for researchers and managers seeking to
accurately quantify its effects on critical hydrologic, biogeochemical, and biological processes. However, protect-
ing natural gradients of connectivity is key to protecting the range of ecosystem services that aquatic ecosystems
provide. In this featured collection, we review the available evidence on connections and functions by which
streams and wetlands affect the integrity of downstream waters such as large rivers, lakes, reservoirs, and estu-
aries. The reviews in this collection focus on the types of waters whose protections under the U.S. Clean Water
Act have been called into question by U.S. Supreme Court cases. We synthesize 40+years of research on longi-
tudinal, lateral, and vertical fluxes of energy, material, and biota between aquatic ecosystems included within
the Act’s frame of reference. Many questions about the roles of streams and wetlands in sustaining downstream
water integrity can be answered from currently available literature, and emerging research is rapidly closing
data gaps with exciting new insights into aquatic connectivity and function at local, watershed, and regional
scales. Synthesis of foundational and emerging research is needed to support science-based efforts to provide
safe, reliable sources of fresh water for present and future generations.
(KEY TERMS: ecological integrity; river networks; streams; wetlands; floodplains; riparian areas; watersheds;
U.S. Clean Water Act.)
Alexander, Laurie C., Ken M. Fritz, Kate A. Schofield, Bradley C. Autrey, Julie E. DeMeester, Heather E.
Golden, David C. Goodrich, William G. Kepner, Hadas R. Kiperwas, Charles R. Lane, Stephen D. LeDuc, Scott
G. Leibowitz, Michael G. McManus, Amina I. Pollard, Caroline E. Ridley, Melanie K. Vanderhoof, and Parker J.
Wigington, Jr., 2018. Featured Collection Introduction: Connectivity of Streams and Wetlands to Downstream
Waters. Journal of the American Water Resources Association (JAWRA) 54(2): 287–297. https://doi.org/10.1111/
1752-1688.12630
1
Paper No. JAWRA-17-0107-P of the Journal of the American Water Resources Association (JAWRA). Received July 24, 2017; accepted
January 22, 2018. ©2018 American Water Resources Association. This article is a U.S. Government work and is in the public domain in the
USA. Discussions are open until six months from issue publication.
2
National Center for Environmental Assessment (NCEA) (Alexander, Schofield, DeMeester, LeDuc, Ridley), and Office of Water (Kiperwas
[formerly], Pollard), U.S. Environmental Protection Agency (USEPA), Washington, D.C., USA; National Exposure Research Laboratory
(NERL) (Fritz, Autrey, Golden, Lane), and NCEA (McManus), USEPA, Cincinnati, Ohio, USA; Southwest Watershed Research Center (Good-
rich), USDA-Agricultural Research Service, Tucson, Arizona, USA; NERL (Kepner), USEPA, Las Vegas, Nevada, USA; National Health and
Environmental Effects Research Laboratory (Leibowitz, Wigington [retired]), USEPA, Corvallis, Oregon, USA; and Geosciences and Environ-
mental Change Science Center (Vanderhoof), U.S. Geological Survey, Denver, Colorado, USA (Correspondence to Alexander:
alexander.laurie@epa.gov).
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA287
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Vol. 54, No. 2 AMERICAN WATER RESOURCES ASSOCIATION April 2018
INTRODUCTION
The principal objective of the U.S. Clean Water
Act (CWA) is to “restore and maintain the chemical,
physical, and biological integrity of the Nation’s
waters” (33 U.S.C. §1362(7)). This objective explic-
itly establishes the central role of ecological (i.e.,
chemical, physical, and biological) integrity to the
attainment of clean water. Connectivity is a key
component of ecological integrity, and has long been
a central tenet in aquatic ecosystem research. Con-
cepts and definitions of connectivity have evolved
over the past several decades (Taylor et al. 2006) as
have metrics for quantifying it (Leibowitz et al.
2018). For the purposes of this featured collection,
we define connectivity as follows: Connectivity is the
degree to which components of a watershed are
joined and interact by transport mechanisms that
function across multiple spatial and temporal scales,
and is determined by the characteristics of both the
physical landscape and the biota of the specific sys-
tem. Here, “physical landscape” refers to abiotic fea-
tures of the landscape, i.e., the scalable habitat
template established by climate, geology, landform,
and human activities (Southwood 1988). Within the
abiotic template, connections established by
exchanges of matter, energy, and living organisms
are influenced by the spatial structure and arrange-
ment of habitats and land uses (Turner and Gardner
2015; Leibowitz et al. 2018). This definition of con-
nectivity reflects a systems perspective of watersheds
as heterogeneous mosaics of interacting ecosystems
in which variations in the duration, magnitude, fre-
quency, timing, and stability of flows form dynamic,
spatiotemporal continua of connectivity (Figure 1).
These gradients of connectivity, which vary in
degree from highly isolated to highly connected, sup-
port the wide range of ecosystem functions needed
to maintain the chemical, physical, and biological
integrity of the nation’s waters, and are the focus of
this featured collection.
The papers in this featured collection review the
results of 40+years of scientific research on the
chemical, physical, and biological connectivity of
streams and wetlands to larger, downstream waters,
such as rivers, lakes, and coastal waters (hereafter
collectively referred to as downstream waters). They
synthesize the substantial body of scientific evidence
on the ecological effects of source waters —and in
particular, small or temporary streams and nontidal
wetlands —on the ecological integrity of downstream
waters and, as a collection, summarize the state-of-
the-science on the interrelatedness of these diverse
aquatic ecosystems.
BACKGROUND ON THE FEATURED
COLLECTION
The reviews in this collection were originally devel-
oped as a report (USEPA 2013, 2015) that provided
the scientific basis for a 2015 rulemaking by the U.S.
Environmental Protection Agency (EPA) and the U.S.
Army Corps of Engineers (Corps) to clarify the defini-
tion of “waters of the United States,” which determi-
nes the scope of CWA jurisdiction. The resulting
regulation, titled the Clean Water Rule (FR 80 FR
37054; https://www.federalregister.gov/d/2015-13435/
page-37054) went into effect in most U.S. states in
August 2015, but was stayed nationwide by the U.S.
Court of Appeals for the Sixth Circuit in October
2015 pending the outcome of litigation. At the time of
this writing, the 2015 regulation is in review by the
issuing agencies under direction of an Executive
Order, issued in February 2017, to rescind or revise
it (FR 82-42, 12532; https://www.whitehouse.gov/presi
dential-actions/presidential-executive-order-restoring-
rule-law-federalism-economic-growth-reviewing-waters-
united-states-rule/).
The need for rulemaking to clarify CWA scope
arose from three Supreme Court cases in 1985, 2001,
and 2006 that created uncertainty about the regula-
tory status of some types of waters. In 1985, the
Court’s unanimous decision in United States v. River-
side Bayview Homes (474 U.S. 12) upheld the agen-
cies’ long-standing practice of regulating “adjacent”
wetlands, defined as wetlands that are “bordering,
contiguous, or neighboring” to a “navigable water.”
(Note: Regulatory and legal terms are shown in quo-
tation marks to avoid confusion with other usages.)
In a 2001 case, Solid Waste Agency of Northern Cook
County (SWANCC) v. U.S. Army Corps of Engineers
(531 U.S. 159), the Court decided 5-4 that use by
migratory birds was not, by itself, sufficient for CWA
jurisdiction over intrastate, nonnavigable, isolated
waters, including the many ponds and wetlands along
common flyways and overwintering areas. Most
recently, in 2006, the Supreme Court heard Rapanos
v. United States, 547 U.S. 715, a case challenging the
inclusion of nonnavigable tributaries and their “adja-
cent” wetlands in CWA jurisdiction. The Court’s split
(4-1-4) ruling in Rapanos yielded multiple, conflicting
opinions with no majority decision. Justice Scalia and
three other Justices argued that the scope of the
CWA includes only “relatively permanent, standing or
flowing bodies of water” and wetlands with a “contin-
uous surface connection” to such waters. Justice Ken-
nedy concurred with Scalia et al. that Rapanos
should be remanded to the lower courts, but dis-
agreed with their standard of relative permanence for
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jurisdiction. In a separate opinion, Justice Kennedy
wrote that tributaries, wetlands, and open waters
must “possess ‘significant nexus’ to waters that are or
were navigable in-fact or that could reasonably be so
made.” He argued that a water body has “significance
nexus” if, “either alone or in combination with simi-
larly situated lands in the region, [it] significantly
affect[s] the chemical, physical, and biological integ-
rity of other covered waters more readily understood
as ‘navigable.’” In a third, dissenting opinion, Justice
Stevens and three other Justices agreed with the reg-
ulatory agencies that the waters at issue were juris-
dictional under the CWA, and that the Corps’
decision to assert jurisdiction over the particular wet-
lands in question was both “reasonable” and “permis-
sible.” The dissenting Justices also stated that the
multiple standards issued in these Court opinions
“can only muddy the jurisdictional waters.” For a
more complete history of the CWA and legal chal-
lenges to its jurisdiction, see Downing et al. (2003),
Downing et al. (2007), Nadeau and Rains (2007), and
Adler (2013, 2015).
While the uncertainties raised by these court cases
cannot be resolved by science alone (Adler 2015;
Alexander 2015; Hawkins 2015), the available scien-
tific literature provides a large body of evidence on
the connectivity and ecological functions of water bod-
ies in question —primarily, small or temporary
streams and nontidal wetlands that can influence the
chemical, physical, or biological integrity of large riv-
ers, lakes, and coastal waters. The authors of this
introduction and featured collection worked together
on a five-year interdisciplinary review of literature on
watershed connections that have potential wide-
spread relevance to CWA programs. In September
2013, a draft of the EPA report (USEPA 2013) was
released for public comment and independent peer
review. The peer review (2013–2014) was conducted
by a panel of 27 experts nominated by the public and
convened by EPA’s Science Advisory Board (SAB). All
SAB panel meetings are open to the public, and all
SAB documents —including comments, meeting
notes, and draft and final reports —are available on
the SAB’s website. The proceedings of this SAB
Water table level
Groundwater flow through local and larger scale aquifers
Overland and interflow
Streamflow and transport of materials, organisms
Overbank flow and transport of materials, organisms
Bank storage
Hyporheic flow and surface-subsurface exchange of water, materials, organisms
Stream and wetland recharge of
local and regional aquifers
Perennial stream
Ephemeral stream
Intermittent stream
Precipitation
Atmospheric losses
(e.g., evapotranspiration, volatilization, denitrification)
Wetland (dry period)
Open-water (dry period)
Active floodplain: Expansion and overbank flow into floodplain
and overflow of wetlands and open-waters during wetter periods
Key
Unsaturated zone
Saturated zone
Local aquifer and hyporheic zone
Confining layer
m
an
d
w
etl
a
n
d
r
e
c
h
a
r
g
e
o
f
a
n
d
r
eg
i
o
n
a
l
a
q
u
if
er
s
S
t
r
ea
m
l
o
c
a
l
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G
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O
FIGURE 1. Hydrologic flow paths. Arrows are representative of surface water and groundwater flows occurring throughout a watershed.
Subsurface flows are shown within the cross section, and by faded arrows outside the cross section. Source: USEPA (2015).
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA289
FEATURED COLLECTION INTRODUCTION:CONNECTIVITY OF STREAMS AND WETLANDS TO DOWNSTREAM WATERS
review, and the panel’s final report to EPA’s adminis-
trator, can be found at https://yosemite.epa.gov/sab/
sabproduct.nsf/02ad90b136fc21ef85256eba00436459/7
724357376745f48852579e60043e88c!OpenDocument&
TableRow=2.3#2. Comments from this SAB panel’s
report (USEPA 2014) were incorporated into EPA’s
final report (USEPA 2015) and were further consid-
ered while writing the papers in this featured
collection.
Collectively, the papers presented here provide a
synthesis of evidence pertaining to the interrelated-
ness of freshwater ecosystems, emphasizing what is
known about the watershed-scale functions of
streams, wetlands, and open waters whose jurisdic-
tional status under the CWA have been called into
question by the three U.S. Supreme Court cases sum-
marized briefly above. While relevant to regulatory
decisions under the CWA, these papers are technical
reviews of peer-reviewed scientific literature. As such,
they do not consider or establish EPA policy and do
not consider or propose legal standards for CWA
jurisdiction.
Because the original intent of these reviews was to
inform the 2015 regulation clarifying the definition of
“waters of the United States,” the papers in this col-
lection address three charge questions that were
developed in 2010 for that effort (Table 1). These
questions were formulated through iterative dia-
logues between scientists in EPA’s Office of Research
(ORD), attorneys and policy specialists in EPA’s
Office of Water, and EPA’s Office of General Counsel
(OGC) to identify the specific nature of the policy
needs, and to determine, within that scope, which
questions could be informed by a formal review of sci-
entific (ORD) or legal (OGC) knowledge. Policy ques-
tions expressed in terms of CWA regulations (which
include categories such as “adjacent waters,” “tradi-
tional navigable waters,” and “other waters”) were
translated into terms that could be used to search the
literature (Table 1). For example, “adjacent” waters
were translated into “wetlands and open waters in
riparian areas and floodplains.” The regulatory term
“traditional navigable waters” is derived from the
statutory term “navigable waters” (Downing et al.
2003) and serves as a reference for many CWA juris-
dictional determinations. It includes large streams
and rivers, lakes, reservoirs, coastal waters, but is
not limited to water bodies that are “navigable-in-
fact” (Downing et al. 2003; Adler 2015). For the pur-
poses of these reviews, we use the term “downstream
waters” as a flexible surrogate for “traditional naviga-
ble waters” (Table 1).
This introductory paper describes the featured col-
lection, briefly reviews the concept of connectivity in
freshwater science, and summarizes major conclu-
sions across the papers in the collection. Leibowitz
et al. (2018) provide a framework to understand
hydrological, chemical, and biological connectivity,
focused on the mechanisms by which wetlands and
headwater streams connect to and contribute to riv-
ers. Fritz et al. (2018) review evidence of the physical
and chemical connections by which streams and asso-
ciated riparian and floodplain wetlands influence the
structure and function of downstream waters (i.e.,
the fluvial hydrosystem; Figure 1). Lane et al. (2018)
review the functions and effects of diverse wetland
and open-water systems occurring outside of riparian
areas and floodplains on downstream waters (Fig-
ure 1). Schofield et al. (2018) review the literature on
how movements of aquatic and semiaquatic biota con-
nect freshwater habitats over various temporal and
spatial scales (Figure 2). Finally, Goodrich et al.
(2018) present a regionally focused case study on the
connections and effects of the intermittent and
ephemeral streams and rivers that dominate the arid
southwestern U.S. Each review also considers biotic
and abiotic factors that influence connectivity, and
synthesizes information on ecosystem functions
TABLE 1. Translating connectivity-related questions between regulatory policy and science. This table crosswalks regulatory and scientific
questions that determined the scope of the reviews in this featured collection. Regulatory and legal terms are shown in quotation marks to
avoid confusion with other usages. Modified from USEPA (2015).
Regulatory question Review question Featured papers
What tributaries have a “significant
1
nexus” to
“traditional navigable waters”?
What are the connections to and effects of
ephemeral, intermittent, and perennial streams
on downstream waters?
Leibowitz et al. (2018)
Fritz et al. (2018)
Schofield et al. (2018)
Goodrich et al. (2018)
What “adjacent” waters have a “significant
1
nexus” to
“traditional navigable waters”?
What are the connections to and effects of riparian
or floodplain wetlands and open waters on
downstream waters?
Leibowitz et al. (2018)
Fritz et al. (2018)
Schofield et al. (2018)
What categories of “other waters” have a “significant
1
nexus” to “traditional navigable waters”?
What are the connections to and effects of
wetlands and open waters in nonfloodplain
settings on downstream waters?
Leibowitz et al. (2018)
Lane et al. (2018)
Schofield et al. (2018)
1
“Significant,” as used here, is a policy determination informed by science; it does not refer to statistical significance.
JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION290
ALEXANDER,FRITZ,SCHOFIELD,AUTREY,DEMEESTER,GOLDEN,GOODRICH,KEPNER,KIPERWAS,LANE,LEDUC,LEIBOWITZ,MCMANUS,POLLARD,RIDLEY,VANDERHOOF,AND WIGINGTON
associated with different types and degrees of connec-
tivity (Figure 3).
The primary pathways of aquatic connectivity con-
sidered here are surface water flows, shallow ground-
water flows, and movements of aquatic and
semiaquatic organisms, all of which connect water-
sheds in four dimensions (Table 2).
Key considerations in our synthesis of aquatic con-
nectivity are ecosystem functions within watershed
components (e.g., streams, wetlands) that alter fluxes
of materials or movements of organisms between
them (Leibowitz et al. 2018). These functions are
broadly categorized into source, sink, refuge, lag, and
transformation (Leibowitz et al. 2018).
In this collection, the term “connectivity” —
which has many contexts and definitions in the lit-
erature —is shorthand for the complex interactions
of landscape, climate, and biota that support ecolog-
ical integrity (Figure 3). Our reviews focus on con-
nections of potential relevance to CWA programs,
but draw from the rich history of research (next
section) that built the foundation for current under-
standing of the role of connectivity in maintaining
the integrity and resilience of downstream waters
(Figure 3).
THE CONCEPT OF CONNECTIVITY IN
FRESHWATER SCIENCE
Connectivity has long been central to the study of
freshwater ecology, hydrology, biogeochemistry, and
geomorphology. The River Continuum Concept (RCC)
(Vannote et al. 1980) portrayed the stream network,
from the headwaters to the river mouth, as a complex
hydrologic gradient with predictable longitudinal pat-
terns of ecological structure and function. The key to
the RCC pattern is that downstream communities are
organized, in large part, by upstream communities
and their processes (Vannote et al. 1980; Battin et al.
2009). The Serial Discontinuity Concept (Ward and
Stanford 1983) built on the RCC to further under-
stand how dams and impoundments disrupt the lon-
gitudinal patterns of ecological function in flowing
waters with predictable downstream effects. The Spi-
raling Concept (Webster and Patten 1979; Newbold
et al. 1981; Elwood et al. 1983) described how river
network connectivity can be conceptualized and mea-
sured as materials cycle from dissolved forms to tran-
siently stored forms in living organisms, then back to
dissolved forms, as they are transported downstream.
Water table level
Aquatic transport or movement
Overland transport or movement (aerial or terrestrial)
Surface-subsurface exchange of water, materials, organisms
Biochemical transformation and transport (e.g., nutrient spiraling)
Perennial stream
Ephemeral stream
Intermittent stream
Wetland (dry period)
Open-water (dry period)
Active floodplain: Expansion and overbank flow into floodplain
and overflow of wetlands and open-waters during wetter periods
Key
Unsaturated zone
Saturated zone
Local aquifer and hyporheic zone
Confining layer
B
FIGURE 2. Biological pathways. Arrows are representative of biological pathways occurring throughout a watershed.
This figure also includes representative biogeochemical pathways occurring in streams and floodplains. Source: USEPA (2015).
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA291
FEATURED COLLECTION INTRODUCTION:CONNECTIVITY OF STREAMS AND WETLANDS TO DOWNSTREAM WATERS
PHYSICAL & CHEMICAL
INTEGRITY
•Habitat quality
•Water quality
•Toxicity
BIOLOGICAL INTEGRITY
•Community structure
•Indicator species
•Functional groups
•Population attributes
ECOSYSTEM INTEGRITY,
SUSTAINABILITY, RESILIENCY
CLIMATE FACTORS
•Annual water surplus
•Seasonality
•Rainfall intensity
•Temperature
LANDSCAPE FACTORS
•Topography
•Landform
•Soil type
•Aquifer permeability
•Spatial distribution
SPECIES’ TRAITS
•Life cycle
•Dispersal capability
•Dispersal cues
•Dispersal behavior
HUMAN ACTIVITIES
INFLUENCING FACTORS CONNECTIONS PROCESSES EFFECTS
BIOLOGICAL
CONNECTIVITY
HYDROLOGIC
CONNECTIVITY
CHEMICAL
CONNECTIVITY
MATERIAL FLUX
ENERGY FLUX
EVOLUTIONARY
DYNAMICS
COMMUNITY
DYNAMICS
POPULATION
DYNAM ICS
ASSESSMENT
ENDPOINTS & METRICS
Stream and wetland
FUNCTIONS
FIGURE 3. The role of connectivity in maintaining the integrity of water. Climate, landscape, species’ traits, and human activities (Influencing
Factors) interact to form Connections (hydrologic, chemical, and biological) between ecosystems throughout and across watersheds. Fluxes of
materials and energy, and movements of living organisms, are enabled, inhibited, or modified by functions within watershed components (e.g.,
streams, wetlands) that modify the timing of transport and the quantity and quality of resources available to downstream communities (Effects).
Monitoring programs have developed metrics for assessing physical habitat, water quality, and biological assemblages as indicators of the
ecological (i.e., physical, chemical, and biological) integrity of downstream waters (Assessment Endpoints and Metrics). Source: USEPA (2015).
TABLE 2. Dimensions of watershed connectivity. Modified from USEPA (2015).
Dimension Examples and pathways
Longitudinal
(Alexander et al. 2007; Freeman et al. 2007) •Streamflow and downstream transport of materials, organisms (Figure 1)
•Hyporheic flow (Figure 1)
•Groundwater flow through local and large-scale aquifers (Figure 1)
•Aquatic or overland movement of organisms in or along stream channels (Figure 2)
•Biogeochemical transport and transformation (Figure 2)
Lateral
(Ward 1989; Stanford and Ward 1993) •Overbank flow and transport from channels into banks, floodplains, and riparian
areas (Figure 1)
•Spillage and transport from wetlands and open waters into streams (Figure 1)
•Overland flow and interflow (Figure 1)
•Groundwater recharge from streams and wetlands (Figure 1)
•Bank storage (Figure 1)
•Transport or movement of organisms between streams and wetlands or open
waters (Figure 2)
Vertical
(Amoros and Bornette 2002; Banks et al. 2011) •Surface-subsurface exchange of water, materials, organisms (Figures 2 and 3)
•Groundwater recharge from streams and wetlands (2)
•Atmospheric exchanges (Figure 1)
Temporal
(Hewlett and Hibbert 1967; Bohonak and
Jenkins 2003; Zedler 2003)
•Variable source area (Figure 1)
•Seasonal cycles of wetland inundation and outflow to streams (Figure 1)
•Migration by aquatic organisms (Figure 2)
•Dormancy of aquatic organisms (Figure 2)
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These conceptual frameworks focused on the longitu-
dinal connections of river ecosystems, whereas the
subsequent Flood Pulse Concept (Junk et al. 1989)
examined the importance of lateral connectivity
between river channels and floodplains, including
wetlands and open waters, through seasonal expan-
sion and contraction of river networks. Ward (1989)
summarized the significance of connectivity to lotic
ecosystems across four dimensions: longitudinal,
lateral, vertical (surface–subsurface), and temporal
connections; he concluded that running water
ecosystems are open systems because they are highly
interactive with both contiguous habitats and other
ecosystems in the surrounding landscape (Figure 1).
Stanford et al. (2005) related the flows of water and
materials through channels and into floodplains to
dynamic and interconnected habitats (shifting habitat
mosaics) that enable diverse assemblages of aquatic
species to coexist (Figures 1 and 2). As these concep-
tual frameworks illustrate, scientists have long recog-
nized the dynamic hydrologic connectivity that the
physical structure of river networks represents.
More recently, scientists have incorporated this
network structure into conceptual frameworks
describing ecological patterns in river ecosystems and
the processes linking them to other watershed compo-
nents, including wetlands and open waters (Power
and Dietrich 2002; Benda et al. 2004; Nadeau and
Rains 2007; Rodriguez-Iturbe et al. 2009). The net-
work dynamic hypothesis (Benda et al. 2004) is a
physically-based framework for predicting patterns of
habitat heterogeneity along rivers, based on dynam-
ics that generate potential biological “hotspots” at
tributary confluences. It reflects a more realistic river
network perspective through remixing the earlier,
more linearly driven frameworks with frameworks
describing the patchy and stochastic nature of lotic
ecosystems (e.g., Resh et al. 1988; Townsend 1989;
Rice et al. 2001). Bunn and Arthington (2002) identi-
fied natural flow variability and associated lateral
and longitudinal connectivity of stream channels and
floodplains as two principal mechanisms linking
hydrology to aquatic biodiversity of riverine species
(also Leigh et al. 2010). In their review of integrative
research on river corridors, Harvey and Gooseff
(2015) highlight quantification of connectivity from
river-reach to regional scales in terms of “hydrologic
exchange flows” that bring downstream flows into
contact with hyporheic, riparian, and floodplain envi-
ronments, where many biogeochemical reactions and
contaminant filtration occur.
In addition, application of metapopulation theory
and population genetic theory to natural populations
has greatly improved our understanding of the role of
dispersal and migration in the demographic persis-
tence, community assembly, and evolution of aquatic
species (Figure 2) (Hastings and Harrison 1994; Pan-
nell and Charlesworth 2000; Fagan 2002; Bohonak
and Jenkins 2003; Waples 2010; Bauer and Hoye
2014). Sheaves (2009) emphasized the key ecological
connections ―which include process-based connec-
tions that maintain habitat function (e.g., nutrient
dynamics, trophic function) and movements of indi-
vidual organisms ―throughout a complex of inter-
linked freshwater, tidal wetland, and estuarine
habitats as critical to the persistence of aquatic spe-
cies, populations, and communities over the full
range of time scales.
OVERVIEW OF MAJOR CONCLUSIONS
Based on the review and synthesis of more than
four decades of scientific research into aquatic ecosys-
tem connectivity, the papers in this collection found
strong evidence supporting the critical role of chemi-
cal, physical, and biological connectivity of streams,
wetlands, and open waters in maintaining the struc-
ture, function, and overall ecological (chemical, physi-
cal, and biological) integrity of downstream waters
(e.g., rivers, lakes, estuaries, and oceans) (Figure 3).
Watersheds are integrated at multiple spatial and
temporal scales by flows of surface water and ground-
water, transport and transformation of physical and
chemical materials, and movements of organisms,
which establish varying degrees of connection and
isolation among freshwater habitats in space and
time.
The literature unequivocally demonstrates that
perennial, intermittent, and ephemeral streams, indi-
vidually or cumulatively, exert a strong influence on
the integrity of downstream waters (Fritz et al. 2018;
Goodrich et al. 2018) (Figure 1). The existence of a
continuous bed and bank structure in river networks
is strong geomorphologic evidence for longitudinal
connectivity; lateral connections maintain the struc-
ture of floodplains and riparian areas via recurrent
inundation and deposition of materials during peak
and recession flows (Fritz et al. 2018). Perennial,
intermittent, and ephemeral headwaters make up the
majority of stream channels in most river networks
and cumulatively supply most of the water in rivers
(Fritz et al. 2018). Flows from ephemeral streams are
a major driver of the dynamic hydrology, geomorphol-
ogy, and biogeochemistry of southwestern rivers,
where approximately 80% or more of streams by
southwestern state are ephemeral or intermittent
(Goodrich et al. 2018). Intervening channels connect
headwaters structurally and functionally to large
inland or coastal waters via channels, wetlands, and
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA293
FEATURED COLLECTION INTRODUCTION:CONNECTIVITY OF STREAMS AND WETLANDS TO DOWNSTREAM WATERS
alluvial deposits where water and other materials are
concentrated, mixed, transformed, and stored or
transported (Fritz et al. 2018).
Wetlands located outside of floodplains and ripar-
ian areas (hereafter, nonfloodplain wetlands) are
abundant in some regions and perform many of the
same functions as wetlands in floodplains and ripar-
ian areas. These functions include recharge of
groundwater that sustains river baseflows; retention
and transformation of nutrients, metals, sediment,
and pesticides; export of organisms or propagules to
downstream waters; storage and subsequent release
of floodwaters; and provision of protective habitats
for stream species (Lane et al. 2018). However, the
mechanisms by which nonfloodplain wetlands are
hydrologically connected to downstream waters differ
fundamentally from those for riparian and floodplain
wetlands, and their effects on downstream waters
can be more challenging to detect and quantify (Lane
et al. 2018). Nonfloodplain wetlands can —and typi-
cally do —remain chemically and biologically con-
nected, even in the absence of hydrologic connections,
by overland or aerial movements of aquatic and semi-
aquatic organisms and the materials that they trans-
port (Lane et al. 2018; Schofield et al. 2018)
(Figures 1 and 2).
There is robust evidence that the movement of
organisms between streams, wetlands, open waters,
and downstream waters is an integral component of
the river food webs that support aquatic life. Streams
and wetlands serve numerous functions for down-
stream waters and their populations, by acting as
sources of colonists, food, and genetic diversity; as
refuges from adverse abiotic and biotic conditions;
and as complementary or obligate habitats for the
maturation and reproduction of fish, amphibians, and
invertebrates (Schofield et al. 2018). Because many
aquatic species are capable of moving overland (e.g.,
by active flight, walking, wind dispersal), biological
connections can be more widespread, complex, and
variable than hydrologic connections, and habitats
that seem to be hydrologically isolated are often
highly connected by movements of aquatic biota (Fig-
ure 2). For practical reasons, research into biological
connectivity is often conducted in single systems, for
example, looking at individual species, assemblages,
or ecosystem types. In reality, biological connectivity
is best assessed as the cumulative effects of multiple
species moving, via multiple pathways and across
multiple habitat types, to make use of the full range
of resources found in spatially and temporally vari-
able wetlands, streams, rivers, lakes, ponds, and
other freshwater habitats found throughout water-
sheds (Schofield et al. 2018).
Natural variation in degrees of connectivity arises
from differences in local and regional climate,
landscape, and biotic factors (i.e., connectivity gradi-
ents), and is needed to support the range of functions
by which streams and wetlands maintain down-
stream water integrity (Figure 3). For example,
whereas large stream channels are highly efficient
transport mechanisms for water and other materials,
smaller tributaries and riparian or floodplain wet-
lands provide optimal environments for functions
associated with water storage and material transfor-
mation (e.g., flood control, denitrification) (Fritz et al.
2018). Therefore, variation in the types and degrees
of connectivity enables different ecosystem functions,
and its quantification is central to our understanding
of how streams and wetlands influence the integrity
of downstream waters. In addition, the incremental
effects of individual streams and wetlands accumu-
late throughout (and across) watersheds and must be
evaluated in context with other streams and wet-
lands, as downstream water quality reflects the com-
bined effects of the functions (e.g., of biogeochemical
cycling) and connections (e.g., transport of nutrients)
of all watershed components (Fritz et al. 2018; Good-
rich et al. 2018; Lane et al. 2018; Leibowitz et al.
2018).
Due to the variable functions and aggregate effects
of tributaries and wetlands, the size, number, and spa-
tial distribution of streams and wetlands in a water-
shed are important factors governing the integrity of
downstream waters (Fritz et al. 2018; Lane et al.
2018; Schofield et al. 2018). For example, while the
probability of a large-magnitude transfer of sediment
or organisms from any given headwater stream in a
given year might be low (i.e., a low-frequency connec-
tion when each stream is considered individually),
headwater streams are the most abundant type of
stream in most watersheds. The actual probability of a
large-magnitude transfer in a given year is therefore
much higher when the cumulative contributions are
considered. Similarly, nonfloodplain wetlands can
store large proportions of snowmelt runoff (30%–60%)
and precipitation (11%–20%) received by a watershed,
and that storage could be increased by wetland
restoration (Lane et al. 2018). Similarly, cumulative
export of invertebrates from headwater to downstream
waters can be substantial, especially in intermittent
and ephemeral streams, as terrestrial invertebrates
accumulate in these channels during dry periods and
are then transported downstream in wet periods (Scho-
field et al. 2018).
Assessments of the functional, watershed-scale
effects of nonfloodplain wetlands on downstream
waters can be especially challenging, as temporal and
spatial gradients of connectivity of this diverse group
of wetlands (e.g., many prairie potholes, vernal pools,
playa lakes) are particularly dynamic (Lane et al.
2018; Leibowitz et al. 2018). Recent research has
JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION294
ALEXANDER,FRITZ,SCHOFIELD,AUTREY,DEMEESTER,GOLDEN,GOODRICH,KEPNER,KIPERWAS,LANE,LEDUC,LEIBOWITZ,MCMANUS,POLLARD,RIDLEY,VANDERHOOF,AND WIGINGTON
quantified the connectivity and cumulative effects of
many individual nonfloodplain wetlands within
watersheds. For example, landscape-scale analysis of
inundation from satellite imagery and watershed
modeling of groundwater flow paths have shown that
apparently “isolated” wetlands can be hydrologically
connected seasonally, cyclically, or continuously over
long (>30 km) distances, via surface water and
groundwater flows that contribute to river baseflow
(Lane et al. 2018; Leibowitz et al. 2018). Data from
these and other studies provide strong evidence that
functions of these wetlands act at multiple spatial
and temporal scales to affect hydrologic and chemical
fluxes or transfers of water and materials to down-
stream waters.
SYNTHESIS AND IMPLICATIONS
The overall objective of the U.S. CWA —to restore
and maintain the chemical, physical, and biological
integrity of the nation’s waters —acknowledges the
critical role of ecological processes in the attainment of
clean water. Interim goals of the CWA related to water
quality, populations of shellfish, fish and wildlife, and
societal needs (e.g., water for human consumption and
recreational uses) recognize the need for advancing the
scientific understanding of aquatic ecosystems with
authorizations to fund “basic research into the struc-
ture and function of fresh water aquatic ecosystems,
and to improve understanding of the ecological charac-
teristics necessary to the maintenance of the chemical,
physical, and biological integrity of fresh-water aquatic
ecosystems” (33 U.S.C. §1254). The ultimate and
interim goals of the CWA are inextricably intercon-
nected. Societal needs for safe drinking water, for
example, are highly dependent on surface water
ecosystem integrity. In an analysis of water use in the
U.S. in 2005, the U.S. Geological Survey found that
77% of the total freshwater used —and 67% of the
public drinking water used —came from surface
waters (U.S. Geological Survey, Surface Water Use in
the United States, 2005. Accessed June 30, 2017,
https://water.usgs.gov/edu/wusw.html). In addition, an
analysis by EPA in 2009 found that intermittent,
ephemeral and perennial-headwater streams are
major sources of water for public water supplies (U.S.
Environmental Protection Agency, Section 404 of the
Clean Water Act. Accessed July 21, 2017, https://
www.epa.gov/cwa-404/geographic-information-systems
-analysis-surface-drinking-water-provided-intermittent).
This study found that a total of 357,404 miles
(575,186 km) of streams in the continental U.S. are
located within “source protection areas” of public
drinking water intakes that have been identified by
state and local governments. Of these stream miles,
the majority (58%) are intermittent, ephemeral, or
perennial headwaters (207,476 miles or 333,900 km).
In the year of that study (2009), approximately one-
third of the U.S. population (~117,000,000 people)
received their drinking water from public sources that
are supplied entirely or in part by freshwater intermit-
tent, ephemeral, or perennial headwater streams (U.S.
Environmental Protection Agency, Section 404 of the
Clean Water Act. Accessed July 21, 2017, https://
www.epa.gov/cwa-404/geographic-information-systems-
analysis-surface-drinking-water-provided-intermittent).
These estimates are conservative, due to the underrep-
resentation of small streams in the nationally avail-
able hydrographic maps (Fritz et al. 2018) used in
these analyses.
As demands on surface water sources continue to
grow, the need for up-to-date scientific information on
natural and anthropogenic controls of freshwater
ecosystem integrity is even greater than it was when
the CWA was enacted in 1972. The literature reviews
in this collection provide a unique synthesis of evi-
dence from the rich history of foundational research
into connectivity, much of which was motivated by
CWA objectives, and from exciting, new studies that
have advanced our scientific understanding of ecologi-
cal connectivity in terms of scale, precision, and com-
plexity. Current research is closing data gaps
identified in this collection and showing promise for
near-term improvements in our ability to assess con-
nectivity and integrity. Examples include network-
based methods for multiscale, multispecies, and mul-
tilevel assessments of ecological networks (Br
asa
et al. 2013; Malvadkar et al. 2015; Engelhard et al.
2016; Rayfield et al. 2016; Pilosof et al. 2017); empiri-
cal observations of spatial and temporal surface wet-
land-stream hydrodynamics at field (Brooks et al.
2018) and landscape scales (Vanderhoof et al. 2016);
models that integrate observational data into mecha-
nistic simulations of hydrologic function (Evenson
et al. 2016; Ameli and Creed 2017; Golden et al.
2017) or integrate those data with network-based geo-
statistical analysis (McGuire et al. 2014); and biologi-
cal connectivity (Bauer and Klaassen 2013). Many
questions about the effects of streams and wetlands
on downstream water integrity can be answered from
the literature reviewed in this featured collection.
Emerging research is rapidly closing existing data
gaps with new insights into aquatic ecosystem con-
nectivity and function at local, watershed, and regio-
nal scales. The syntheses of foundational and
emerging research in this collection provide a
resource for science-based efforts to restore and main-
tain safe, reliable sources of fresh water for present
and future generations.
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA295
FEATURED COLLECTION INTRODUCTION:CONNECTIVITY OF STREAMS AND WETLANDS TO DOWNSTREAM WATERS
ACKNOWLEDGMENTS
We thank Rose Kwok for her excellent comments on all manu-
scripts in this featured collection. We also thank Dan Auerbach and
three anonymous reviewers for comments that improved this intro-
duction to the collection. Lastly, we again thank the 47 peer review-
ers acknowledged in USEPA (2015), whose contributions to that
assessment also improved the papers in this featured collection.
Although this work has been reviewed by the USEPA and approved
for publication, it does not necessarily reflect Agency policy.
DISCLOSURE
Former JAWRA editor Kenneth J. Lanfear served as acting edi-
tor in chief for all articles in this featured collection. Parker J. Wig-
ington, Jr., an author on some of the collection papers and who
was JAWRA editor in chief at the time the collection was submit-
ted, had no role in the review or editorial decisions for any part of
the collection.
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