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2007
HYDROLOGIC CONNECTIVITY AND THE CONTRIBUTION OF HYDROLOGIC CONNECTIVITY AND THE CONTRIBUTION OF
STREAM HEADWATERS TO ECOLOGICAL INTEGRITY AT STREAM HEADWATERS TO ECOLOGICAL INTEGRITY AT
REGIONAL SCALES REGIONAL SCALES
Mary C. Freeman
Catherine M. Pringle
C. Rhett Jackson
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HYDROLOGIC CONNECTIVITY AND THE CONTRIBUTION OF STREAM
HEADWATERS TO ECOLOGICAL INTEGRITY AT REGIONAL SCALES
1
Mary C. Freeman, Catherine M. Pringle, and C. Rhett Jackson
2
ABSTRACT: Cumulatively, headwater streams contribute to maintaining hydrologic conn ectivity and ecosystem
integrity at regional scales. Hydrologic connectivity is the water-mediated transport of matter, energy and
organisms within or between elements of the hydrologic cycle. Headwater streams compose over two-thirds of
total stream length in a typical river drainag e and directly connect the upland and riparian landscape to the
rest of the stream ecosystem. Altering headwater streams, e.g., by channelization, diversion through pipes,
impoundment and burial, modifies fluxes between uplands and downstream river segments and eliminates dis-
tinctive habitats. The large-scale ecological effects of altering headwaters are amplified by land uses that alter
runoff and nutrient loa ds to streams, and by widespread dam construction on larger rivers (which frequently
leaves free-flowing upstream portions of river systems essential to sustaining aquatic biodiversity). We discuss
three examples of large-scale consequences of cumulative headwater alteration. Downstream eutrophication and
coastal hypoxia result, in part, from agricultural practices that alter headwaters and wetlands while increasing
nutrient runoff. Extensive headwater alteration is also expected to lower secondary productivity of river systems
by reducing stream-system length and trophic subsidies to downstream river segments, affecting aquatic com-
munities and terrestrial wildlife that utilize aquatic resources. Reduced viability of freshwater biota may occur
with cumulative headwater alteration, including for species that occupy a range of stream sizes but for which
headwater streams diversify the network of interconnected populations or enhance survival for particular life
stages. Developing a more predictive understanding of ecological patterns that may emerge on regional scales as
a result of headwater alterations will require studies focused on components and pathways that connect head-
waters to river, coa stal and terrestrial ecosystems. Linkages between headwaters and downstream ecosystems
cannot be discounted when addressing large-scale issues such as hypoxia in the Gulf of Mexico and global losses
of biodiversity.
(KEY TERMS: aquatic ecology; rivers ⁄ streams; environmental impacts; hydrologic connectivity; biodiversity; eco-
system function.)
Freeman, Mary C., Catherine M. Pr ingle, and C. Rhett Jackson, 2007. Hydrologic Connectivity and the Contri-
bution of Stream Headwaters to Ecological Integrity at Regional Scales. Journal of the American Water
Resources Association (JAWRA) 43(1): 5-14. DOI: 10.1111/j.1752-1688.2007.00002.x
1
Paper No. J06011 of the Journal of the American Water Resources Association (JAWRA). Received February 1, 2006; accepted October 4,
2006. ª 2007 American Water Resources Association. No claim to original U.S. government works.
2
Respectively, Ecologist, U. S. Geological Survey, Patuxent Wildlife Research Center, Athens, Georgia 30602 (address for correspondence:
Institute of Ecology, University of Georgia, Athens, Georgia 30602-2202); Professor, Institute of Ecology, University of Georgia, Athens, Georgia
30602; and Associate Professor, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, Georgia 30602
(E-Mail ⁄ Freeman: mary_freeman@usgs.gov).
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 5 JAWRA
JOURNAL OF THE AMERICAN WA TER RESOURCES ASSOCIATION
Vol. 43, No. 1 AMERICAN WATER RESOURCES ASSOCIATION February 2007
U.S. government works are not subject to copyright.
INTRODUCTION
The hydrologic connectivity of small headwater
streams to navigable waters is clear and unambigu-
ous to ecologists. Every important aspect of the
river ecosystem, the river geomorphic system, and
the river chemical system begins in headwater
streams. Other papers in this issue focus on the
contribution of headw ater streams to stream flow
(Winter, this issue), nutrient cycling and watershed-
scale water quality (Alexander et al., this issue;
Triska et al., this issue), regional biodiversity
(Meyer et al., this issue), and as provide rs of
organic matter subsidies to downstream reaches
(Wipfli et al., this issue). Our purpose is to consider
and provide examples of large-scale ecological effects
of headwater alteration.
Headwater streams clearly dominate surface water
drainage networks. Definitions of headwater streams
vary, but if we define headwaters as all first- and sec-
ond-order streams, then, in aggregate, these streams
compose over two-thirds of the total stream length in
a river network (Leopold et al., 1964). A first-order
stream is an intermittent or perennial stream with
no temporary or perennial tributaries, while a sec-
ond-order stream is created by the confluence of two
first-order streams. Every large river is fed by liter-
ally hundreds of thousands of small headwater
streams (Leopold et al., 1964). First- and second-order
streams may be too small for boating and fishing, but
they connect upland and ripari an systems with river
systems.
In this paper, we illustrate mechanisms by which
the cumulative alteration of headwater streams is
likely to affect ecological function at larger scales.
We begin by defining hydrologic connectivity, consid-
ering the need for the legal definition of ‘‘connected’’
to be based on scientific measurements of water,
energy, material, and organism transport between
waterbodies. We then cite an example of unantici-
pated, large-scale environ mental changes resulting
from human impacts on hydrologic connectivity. We
follow by considering large-scale consequences of
headwater stream alteration. Using examples of
coastal eutrophication, diminished riverine produc-
tivity, and lowered viability of river biota, we sug-
gest that headwater alteration has the potential to
reduce ecological in tegrity at large spatial scales,
particularly where river systems are already affec-
ted by landscape changes and downstream modifica-
tions including dams, levees and flow regulation.
We close by discussing specific research needed to
improve our ability to understand and predict the
large-scale consequences of altering headwater
streams.
HYDROLOGIC CONNECTIVITY: DEFINITIONS
AND THE IMPORTANCE OF CONSIDERING
LARGE-SCALE EFFECTS OF ALTERATION
Longitudinal connections within riverine ecosys-
tems have long been recognized by both aquatic and
terrestrial ecologists, as illustrated by the widespread
use of the term river corridor in the literature. The
term connectivity did not emerg e in the freshwater
literature until the early 1990s (but see Amoros and
Roux, 1988). A review of 20 major journals in fresh-
water ecology and management from 1945-2003 indi-
cates that connectivity surpassed the use of corridor
by the late 1990s, with the trend continuin g into the
2000s (Pringle, 2006). In contrast, the term connectiv-
ity was widely used a decade earlier in journals in
the fields of landscape ecology and conservation bio-
logy (e.g., Merriam, 1984). Connectivity is also a
fundamental concept of metapopulation ecology
(Moilanen and Hanski, 2001). A metapopulation is a
group of individual populations that are connected by
migration and dispersal. Metapopulation models were
initially designed and tested on terrestrial biota (typ-
ically insects and small mammals); metapopulation
theory has more recently been applied to riverine
biota, such as fishes and mussels (Stoeckel et al.,
1997; Policansky and Magnuson, 1998; Gotelli and
Taylor, 1999; Fagan, 2002).
Freshwater ecologists frequently use the term con-
nectivity to describe spatial linkages within rivers
(Stanford and Ward, 1992, 1993; Ward, 1997; Amoros
and Bornette, 1999; Wiens, 2002). Ward (1997) defines
riverine connectivity as energy transfer acros s the riv-
erine landscape. Ward and Stanford (1989a) define riv-
ers as having interactive pathways along one temporal
dimension (time scales) and three spatial dimensions
[i.e., longitudinal (upstream-downstream); lateral
(channel-bank ⁄ floodplain); and vertical (atmosphere-
channel-subsurface)]. Consideration of dynamic inter-
actions along these four dimensions has proven to be
an effective conceptual framework to understand
human impacts on river ecosystems (e.g., Ward and
Stanford, 1989b; Boon et al., 1992; Pringle, 1997, 2000;
Tockner and Stanford, 2002).
In contrast to riverine connectivity, hydrologic con-
nectivity (Pringle, 2001, 2003a,b) encompasses
broader hydrologic connections, beyond the water-
shed, on regional and global scales. Hydrologic con-
nectivity refers to the water-mediated transport of
matter, energy, and organisms within or between ele-
ments of the hydrologic cycle (sensu Pringle, 2001), in
essence combining the hydrologic cycle with riverine
connectivity. Aspects of hydrologic connectivity are
essential to maintaining the ecological integrity of
ecosystems, where ecological integrity is defined as
F
REEMAN,PRINGLE, AND JACKSON
JAWRA 6 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
the undiminished ability of an ecosystem to continue
its natural path of evolution, its normal transition
over time, and its successional recovery from pertur-
bations (Westra et al., 2000). Conversely, hydrologic
connectivity also directs and facilitates the flow of
exotic species, human-derived nutrients, and toxic
wastes in the landscape. Hydrologic connectivity at
large scales is a formidable concept because of the
inherent complexity of water movement within and
between the atmosphere, surface-subsurface systems
and the ocean (e.g., Winter et al., 1998); and the
extent and magnitude of human alterations (e.g.,
Pringle and Triska, 2000).
Scientific concepts of connectivity differ from legal
definitions. Hydrologists view connectivity as a con-
tinuum because the entire landscape is hydrologically
connected (Figure 1). Moreover, biological connections
among waterbodies are not restricted to pathways of
water flow; e.g., migratory birds, amphibians, and
winged aquatic insects travel across watershed
boundaries. Legally, however, stream navigability
and the influence of headwaters on the integrity of
interstate waters have played prominent roles in
legal que stions over Federal jurisdiction of small
streams and wetla nds. The interstate commerce
clause of the U.S. Constitution gives the Federal gov-
ernment authority to regulate river-based commerce,
which includes regulating water quality. In the past,
the concept of connectivity extended Federal jurisdic-
tion to small streams and isolated wetlands by virtue
of their direct hydrologic (regardless of form and rate)
and biologica l linkages to interstate or navigable
waters (Downing et al., 2003). Given the complexity
of hydrologic connections, it is essential that political
and legal determinations of thresholds of connect ivity
(for purposes of Clean Water Act jurisdiction) be
informed by scientific understanding of headwater
stream effects on ecological functions at larger scales.
A compelling example of how important it is to
consider the large-scale effects of altered hydrologic
connectivity concerns alterations in the biogeochemi-
cal transport and cycling of silica as a result of the
cumulative effects of dams. Rivers supply over 80% of
the total silicate input to oceans (Treguer et al.,
1995). Silicate stimulates production of diatoms,
which fuel food webs and play a critical role in CO
2
uptake (Smetacek, 1998). Increasing evidence links
dam construction to decreased silicate transport and
alterations in coastal food web structure (Conley
et al., 2000). Moreo ver, reduced riverine inputs of
FIGURE 1. The Hydrologic Pathways Connecting the Landscape to Streams and Rivers. When soils are undisturbed by grading, compaction,
and paving, most rainfall reaches streams and wetlands by subsurface pathways. Streams and wetlands can be considered the low parts of
the landscape into which ground water leaks from the uplands. Ground-water levels rise and fall in response to recharge from infiltrated
rainfall and leakage to streams and wetlands. Redrawn from Jackson (in press).
HYDROLOGIC CONNECTIVITY AND THE CONTRIBUTION OF STREAM HEADWATERS TO ECOLOGICAL INTEGRITY AT REGIONAL SCALES
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 7 JAWRA
other elements such as iron, may have far-reaching
effects beyond coastal ecosystem s (Hutchins and Bru-
land, 1998). Iron availability has been linked to pat-
terns of silicate uptake. Therefore, reductions of
riverine-transported iron (as a result of hydrological
alterations) might also affect silicate uptake in nutri-
ent-rich upwelling zones far from the coasts (Ittekkot
et al., 2000). Further declines in the delivery of sedi-
ments, dissolved silicate, and other elements to estu-
aries and coastal oceans can be expected as new
dams are constructed, with consequences to coast al
food webs and wild life.
Environmental effects of altered nutrient transport
in regula ted rivers have emerged within the last two
decades. This and other examples (e.g., mobilization
of methylmercury in reservoirs) suggest that the cur-
rent extent and magnitude of hydrologic alterations
and pollutant loading will result in new, perhaps
unexpected, environmental problems, and raise que s-
tions of the larger scale effects of other alterations in
hydrologic connectivity (Pringle, 2003c).
REGIONAL ECOSYSTEM RESPONSE TO
HEADWATER STREAM ALTERATION
Channelization, diversion through pipes (‘‘piping’’),
impoundment and burial of headwater streams una-
voidably impact stream systems by altering runoff
patterns, fluxes to downstr eam segments, and by
eliminating distinctive habitats. Managers and regu-
lators require information on the size and extent of
these effects, and also on how increasing cumulative
headwater alterat ion may affect ecosystem integrity,
locally and at larger scales. Headwater alteration
affects ecological function at larger scales through
the loss of unique functions and in relation to the
importance of headwater connectivity to downstream
and upland systems. We discuss three examples of
realized or potential large-sca le consequences of head-
water loss and alteration. Each of these examples,
coastal eutrophication, lowered riverine productivity,
and reduced viability of riverine biota, reflect the pre-
dominance and position of headwater streams as
riverine capillaries into the upland landscape.
Additionally, hydrologic alteration of headwater
streams is generally accompanied by water quality
impacts. Human activities commonly associated with
headwater stream modification include land develop-
ment, road construction, mining, agricultural drain-
age, and reservoir creation. Each activity entails
significant water quality changes beyond those
caused by the physical alteration of the headwater
channels. Stream piping to create additional space for
buildings, roads, or parking lots is accompanied by
elevated streamflow, nutrients, pesticides, fecal coli-
forms, and pharmaceuticals that are associated with
pavement, compacted soils, landscape management,
domestic animal waste, and sewer leaks (Paul and
Meyer, 2001). Strip mining and hilltop mining exca-
vate some headwater streams and bury others in
mine tailings. The downstream receiving waters are
affected not only by the loss of the streams, but
potentially by acidic ground water and streamflow
created by the exposur e of an enormous combined
surface area of unweathered rock and the resulting
oxidation of sulfides and pyrites. Stream systems
altered by ditching to improve drainage from agricul-
tural fields also receive high nutrient and sediment
concentrations because of fertilizer or manure appli-
cation and soil erosion. Small streams are often
impounded to create ‘‘farm ponds,’’ or increasingly, to
create ‘‘amenities’’ in residential developments; in
both cases, the downstream drainage is influenced
not only by replacement of the stream e cosystem with
a reservoir but also by nutrient and sediment runoff
from the landscape.
Finally, most U.S. (and other Holarctic) river sys-
tems are hydrologically altered by dams (Dynesius
and Nilsson, 1994), an important fact for considering
the emerging consequences of headwater disturbance.
In effect, river systems are being squeezed from both
ends – downstream by dams and levees that frag-
ment mainstems and isolate channels from their
floodplains, and upstream by disturbance and loss of
headwater streams. The free-flowing, mid-sized river
segments caught between downstream dams and
impoundments and upst ream headwater disturbance
are frequently essential to sustaining aquatic biodi-
versity (see, e.g. Freeman et al., 2005).
It is thus important to evaluate the ecological
effects of headwater stream alteration with respect to
additional water quality changes associated with
stream disturbance and in the context of downstream
channel modifications. Toward this end, we provide
examples that illustrate a range of ecosystem effects
associated with headwater alteration. Specifically,
we examine linkages between headwater modifica-
tion and: (1) coastal eutrophication and hypoxia,
(2) diminished secondary productivity in rivers, and
(3) reduced viability of stream biota.
Coastal Eutrophication ⁄ Hypoxia
Loss of nutrient processing in headwaters (Meyer
and Wallace, 2001; Triska et al., this issue), accom-
panied by increased nutrient runoff with landscape
disturbance, can cause downstream nutrient loading
and contribute to coastal eutrophication and hypoxia.
F
REEMAN,PRINGLE, AND JACKSON
JAWRA 8 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
