Environ. Sci. Technol. XXXX, xxx, 000–000
What is a stream?
M A R T I N W . D O Y L E * ,†
EMILY S. BERNHARDT‡
Department of Geography, University of North Carolina-Chapel
Hill, Chapel Hill, North Carolina, United States and
Department of Biology, Duke University, Durham,
North Carolina, United States
1. Introduction: From the U.S. Constitution to Rapanos
The Clean Water Act (CWA) is the primary instrument for
regulating “waters of the United States” with a goal “to restore
and maintain the chemical, physical, and biological integrity
of the Nation’s waters.” After almost four decades, the precise
definition of “the Nation’s waters” or “waters of the United
States” remains unclear, and the geographic scope of the
CWA shifts in response to legal challenges and rulings.
Federal authority for the CWA is rooted in the Commerce
Clause of the Constitution. The U.S. Army Corps of Engineers
(Corps), the primary Federal agency designated by Congress
to regulate navigable waterways, has defined via regulation
the extent of navigable waters. In 1899 Congress expanded
Corps jurisdiction to include tributaries of navigable waters
(for historical review see ref 1). Any hydrologic feature, once
classified as a navigable water (or tributary) by the Corps, is
subject to any requirements Congress chooses to impose.
Through the 20th century, the Corps began regulating
activities for ecological protection in addition to maintenance
of navigation and courts upheld this expanded role (2). With
the passage of the CWA and its 1977 revisions Congress
* Corresponding author e-mail: [email protected].
†
University of North Carolina.
‡
Duke University.
10.1021/es101273f
 XXXX American Chemical Society
defined “navigable waters” as “the waters of the United
States” (because they fall under federal jurisdiction, we refer
to these as “jurisdictional waters”). The CWA allows pollutants
in jurisdictional waters only after federal permitting. Specifically, Section 402 allows the Environmental Protection
Agency (EPA) to issue permits for the discharge of chemical
pollutants from a point source (e.g., pipe, ditch) into
jurisdictional waters, while Section 404 allows the Corps to
permit activities where jurisdictional waters are filled,
dredged, or physically altered.
If a land developer wishes to physically modify the
landscape in preparation for construction, the presence of
jurisdictional waters can increase the costs and time for a
project substantially (3); defining jurisdictional waters is thus
highly contested. At some point on the landscape a line must
be drawn, literally and figuratively, where water ends and
land begins. Federal jurisdiction becomes increasingly
controversial when landowners interpret water quality
protection as local land use regulation rather than protection
of interstate commerce (4). Moreover, land use regulation
has been a right strongly asserted by the states, and so the
line between waters and land also marks where the federal
regulation ends and state regulation begins.
Interpretation of what hydrologic features merit federal
regulation has moved gradually upstream. The Supreme
Court has supported federal jurisdiction over navigable
waters, tributaries of navigable waters, and wetlands adjacent
to navigable waters, but not necessarily jurisdiction over
hydrologically “isolated” wetlands (1). Until 2006, Corps
regulations designated a feature a “tributary,” and thus a
jurisdictional water, if it fed into a navigable water (or a
tributary thereof) and possessed an ordinary high-water mark,
defined as a “line on the shore established by the fluctuations
of water and indicated by [certain] physical characteristics”
(5).
In 2006 the Supreme Court took up the question of
tributaries and jurisdictional waters in the consolidated cases
of Rapanos v US and Carabell v US (hereafter Rapanos). The
Court issued five decisions, with no single opinion commanding a majority. Justice Scalia, writing for the plurality
of himself and three other justices, argued that jurisdictional
waters extend beyond navigable waters to include “relatively
permanent, standing or flowing bodies of water.” Justice
Kennedy’s concurring opinion represents the holding of the
Court, and while he also found the existing regulatory
definition of tributary was unacceptably broad, he left
tributary undefined. Rather, Justice Kennedy concluded that
if a hydrologic feature had a demonstrable “significant nexus”
with downstream navigable waters, then federal jurisdiction
applied to that feature (the legal literature reviewing Rapanos
is vast; see special issue Nat. Resour. Environ., Vol. 22 (Issue
1)).
In response to the Court’s decision, the Corps and EPA issued
new guidance by which hydrologic features would be classified
and regulated (6). This new guidance allows agencies to assert
jurisdiction over features which flow year-round or have
continuous flow at least seasonally (“typically three months”),
to fulfill Scalia’s requirements. The agencies will also consider
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A
TABLE 1. Characteristics of Different Landscape Features from Which Streams Must Be Distinguished
a
Spatial variability in flow velocity and depth, substrate size, and food quality.
the significant nexus requirement within the context of
“hydrologic and ecological factors.” But jurisdiction will not be
asserted over purely erosional features (e.g., gullies), nor ditches
carrying relatively nonpermanent flow.
In the wake of Rapanos and as it had on two earlier
occasions Congress began considering the Clean Water
Restoration Act (CWRA) in April 2009. The CWRA is intended
to “clearly define the waters of the United States.” The CWRA
would apply federal jurisdiction to a range of features,
including “intrastate lakes, rivers, streams (including intermittent streams), mudflats, sandflats, wetlands, sloughs,
prairie potholes, wet meadows, playa lakes, or natural
ponds...tributaries of the aforementioned waters...and wetlands adjacent to the aforementioned waters.”
As indicated by the CWRA and the controversy in Rapanos,
streams and tributaries are critical to the operation of the
CWA, yet surprisingly problematic to define. As Chief Justice
Roberts said during the Rapanos oral arguments, “where a
tributary ends [confluence] is clear; but where it begins is a
problem.” Here we provide a brief overview of stream science,
attempting to distinguish streams within the continuum of
hydrologic features. We focus on streams because they (a)
provide functions that make them central to achieving goals
of the CWA, and (b) have received less regulatory attention
than other features (e.g., wetlands). Moreover, streams are
now explicitly covered by the federal 2008 Compensatory
Mitigation Rule (7); permitted impacts to streams must now
be mitigated through compensatory stream restoration or
preservation. This has increased the demand for precision
in stream policies and practices in the same way that earlier
wetland mitigation rules increased the demand for precision
of defining wetlands. Through this rule, streams are rapidly
becoming an increasing portion of the burgeoning industry
in restoration and ecosystem service markets (8).
Our focus is on distinguishing streams from other features,
particularly land, lentic systems (e.g., ponds, wetlands), and
simple hydrologic conveyances such as drains and pipes
B
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(Table 1). Following the goals of the CWA, we review relevant
stream science in terms of physical, chemical, and biological
characteristics and processes.
2. State of the Science
2.1. Physical: Hydrology and Geomorphology. Streams are
formed by the convergence of surface and/or groundwater
flow into the lowest topographic area of a valley. Flow
frequency and duration varies markedly with topography,
geology, and climate. Streams dominated by groundwater
have consistent baseflows, making streamflow origin locations consistent over time. Streams with flows derived
predominantly from precipitation, such as those in arid or
urban watersheds, have dynamic and inconsistent flow
origins (9), and the extent of headwater streams and the
expanse of the channel network itself can vary dramatically
(Figure 1).
Hydrologic metrics include measures of frequency, duration, and magnitude of flow, and each aspect of flow is an
important determinant of the chemical and biological
features and functions of stream ecosystems. The magnitude
of flow is important for the formation of a channel, transport
of sediment, and the flux of solutes (10). Flow duration is
critical to biological processes and communities, while the
timing of high and low flows exerts strong selection pressure
on organisms and structures biological communities (11).
Geomorphically, a channel is a feature caused by concentrated flow of water that preferentially erodes sediment
and material from the ground surface. Stream channel flow
is often several orders of magnitude faster than overland
flow (12) or flow through soils (13) where water would flow
if a channel were not eroded. The presence of a channel
concentrates flow, increases its speed and erosive power,
and thus more effectively transports water, solutes, and
sediment to downstream.
There is often a distinct point where channels physically
beginsthe channel headswhere the sediment transport
FIGURE 1. When should the stream presence/absence be measured? River channel networks can be highly dynamic, expanding and
contracting in time, particularly in arid regions, like this example from Sycamore Creek, AZ (adapted from 9, with permission from
American Institute of Biological Sciences).
regime becomes dominated by advective processes, carried
downstream by the force of flowing water (14). In steep,
mountainous landscapes channels are often initiated by
shallow landslides via geotechnical and groundwater processes. In landscapes with less topographic relief, however,
channel initiation and formation occurs via saturation of
soil during intense storms followed by overland flow and
erosion of a channel into the ground surface often via
headcutting (15).
In landscapes with little topographic relief, as would be
the case in prairies or coastal plains, slope may be insufficient
to physically scour a channel. In these cases, the application
of traditional geomorphic approaches to discern channels
and to understand stream processes becomes problematic
or inappropriate because headwaters lack a distinct channel
head and discernible channel banks. More often the headwater is just an inundated swale with insufficient advective
flow to form a distinct channel and the physical differences
between channels and uplands or wetlands are blurred.
2.2. Chemical. Streams are dominated and defined by
unidrectional flow and in this sense are similar to simple
conveyances (pipes, gutters, ditches) that are designed to
efficiently transport water downstream. But unlike conveyances, streams can significantly alter the magnitude, timing,
and form of chemical delivery to downstream waters (16).
Stream channels are physically more complex than conveyances and thus trap, delay, and attenuate water, chemicals,
and sediment pulses delivered from upslope (17). The
communities of organisms within stream ecosystems largely
survive by consuming and transforming terrestrial materials
before passing them to the atmosphere or downstream, thus
further altering the timing and quantity of chemical exports
(18).
Although materials do accumulate in floodplain or instream depositional areas, streams are not net aggrading
systems and are less retentive of chemicals and solutes in
comparison to hillslopes and wetlands. Because of the
comparatively high velocities of flow in channels (i.e., limited
residence time), chemicals and solutes that reach streams
are routed much more rapidly to downstream receiving
waters than would occur via flowpaths in subsurface or
overland flows. For elements that have no gaseous form (e.g.,
the limiting nutrient phosphorus, most trace metals), the
only possible fate once introduced to streams is transport to
downstream floodplains, lakes, reservoirs or coastal zones,
although this transport may require anywhere from days to
centuries.
For elements with a gaseous form at ambient conditions,
substantial conversion and thus permanent export from
streams to the atmosphere can occur in streams. In particular,
denitrification can convert ∼16-50% of the nitrate (NO3-)
that enters streams to N2 (19) and more than 50% of the fixed
carbon that enters streams and rivers is respired as CO2 (20).
Because streams provide ideal conditions for a variety of
metabolic processes with high water availability and the
intersection of oxic and anoxic waters, streams typically have
disproportionately high rates of nutrient transformations and
decomposition (mass per unit area) compared to adjacent
surrounding soils (21). Anoxic habitats within streams and
riparian zones are often the primary watershed locations
where significant denitrification and methanogenesis occur
within temperate and arid landscapes.
2.3. Biological. The biological communities in streams
are distinctly different from those found in drains, ditches,
and other features. Small, headwater streams are the most
distal segments within continuous river channel networks,
which allows streams to be home to organisms that migrate
from distant, hydrologically connected downstream water
bodies. For many iconic or commercially important anadromous species (such as salmon, cutthroat trout, river herring,
lamprey, and American eels) as well as the majority of riverine
fish, headwater coastal streams provide critical nursery
habitats where young fish can escape predation (22).
Many stream insect taxa have specifically adapted to spend
all or part of their life cycle in flowing waters, developing
characteristics or behaviors that make them distinct from
those in terrestrial or lentic systems. For instance, many
stream insect taxa rely on the continuous supply of high
VOL. xxx, NO. xx, XXXX / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
C
FIGURE 2. Variation in parameters across a climatic and
topographic range with characteristic landscapes (National
Parks) for reference. As topographic relief increases the utility
and strength of geomorphic indicators of streams increases;
greater precipitation increases strength of hydrologic indicators; and the combination of both increases utility of biological
indicators. Most regulatory definitions of streams were developed for humid, moderate or high relief areas.
dissolved oxygen to supply their metabolic demands. Likewise, flowing water carries particulate matter, and filter
feeders like the net spinning caddisflies (Trichoptera) and
sessile blackflies (Simuliidae)soften dominant members of
stream communitiessdepend on this continuous supply of
particulate material for food.
The biological communities found in the uppermost
reaches of river networks are generally similar in composition
to but less diverse than the communities found lower in the
network. The ephemeral or intermittent hydrographs of many
headwater streams reduce the total species diversity because
long-lived, less mobile aquatic organisms are unable to
complete their life cycle under these conditions. At the same
time, the lack of permanent water and their geographic
isolation makes some headwaters ideal locations for sensitive
larval fish and amphibians or salamanders to thrive for
periods of time because they are too small and transient to
support the large piscivorous fish that dominate downstream
food webs (23). Because small streams constitute more than
half of the total stream habitat in most watersheds and
because communities are highly variable among headwaters,
small streams collectively contribute disproportionately to
landscape scale biodiversity (23).
2.4. Variability Across Climate, Topography, and Land
Use. The distinctiveness of stream processes can vary
dramatically across climatic and topographic gradients
(Figure 2). Advective sediment transport is greater in regions
with high topography, and geomorphic features develop more
clearly in such areas even when precipitation is infrequent.
For example, arroyos or desert washes have strong geomorphic characteristic of streams but flow only rarely and do not
support characteristic stream flora or fauna.
Chemical processes in streams also vary across these
gradients; high topography areas where flows are more
channeled and rapid will have less opportunity for transformations. Streams draining regions with low topography
will behave chemically similarly to lakes and ponds, with
high rates of retention and transformation of incoming
chemicals. Climatic drivers are also important for chemical
characteristics. When the bulk of the annual water export
occurs during brief, extreme floods, stream ecosystems have
little opportunity to transform solutes, making streams more
D
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comparable to drainage ditches and pipes. If the bulk of
annual flow occurs as baseflow (i.e., perennial, steady flow
regime) then streams can be effective transformers of
chemical loads (24).
Biological communities are more distinct from lentic
communities in the streams of humid, high relief landscapes.
Channeled, persistent flows allow the development of entire
communities of organisms that are dependent on flowing
water. As topography is reduced, flow velocities and dissolved
oxygen concentrations decrease, and communities can
become more similar to lentic systems. Biota in regions of
infrequent flow will be largely absent, or composed of
characteristic short-lived organisms or taxa with life history
strategies that allow them to resist or escape desiccation
(25).
Anthropogenic changes can significantly alter the characteristics and distribution of stream processes on the
landscape (26). Removal of vegetation or increase in watershed imperviousness decreases long-term sediment loads
and increases runoff, both of which can cause channels to
erode and channel heads to migrate upstream (27). Urban
areas can also be impacted by the complete burial and piping
of streams (28). The removal of water by groundwater
extraction or upstream flow diversion can also drastically
alter flow regimes and commensurate stream features and
processes, and changes to flow regimes by shifts in climate
will inevitably change the current patterns of stream form
and process (29).
3. Discussion
3.1. Designating Streams. Agencies are now faced with
codifying which factors distinguish streams from land, and
streams from ditches and hydrologic conveyances. Such
efforts must also address regional variations associated with
topography and climate. Some classifications already exist,
having been developed by local and state agencies. For
instance, the Ohio Tiered Aquatic Life Uses classification
designates streams using a range of scales to distinguish
regional physiographic influences, as well as historic and
contemporary land use impacts (30, 31). This and similar
approaches (e.g., 32) use a suite of hydrologic, geomorphic,
and biotic indices that can be rapidly measured or assessed
and, while some subjectivity will inevitably be needed in
their implementation, the inclusion of biotic indicators (flora
and fauna) together with topographic/hydrologic metrics
provides substantial advantages for identifying ecologically
and hydrologically important features that are functioning
as streams. Some of these classification approaches have
already been used to distinguish between perennialintermittent streams and intermittent-ephemeral streams.
Refining these classifications or devising metrics that apply
to the upstream limits of the stream continuum as based on
basic science of stream-land interfaces will need to be made
more explicit for future policies.
A second critical policy question remains what to do with
water quality impacts; not all features that affect downstream
waters are streams. Pipes, culverts, ditches, and streams that
have been converted into pipes or channelized ditches often
have disproportionately large impacts on downstream water
quality. Indeed, Solicitor General Clement argued in Rapanos
that storm drains merit jurisdictional waters status for this
chemical impact reason. One approach to regulating such
features is to designate them as low quality or heavily
degraded streams. Regulation of impacts to such features
might be more lenient, with greatest consideration on how
pollutants derived from development are transferred downstream rather than on how the development impacts the
feature itself. In this approach, the feature itself, despite
degradation, could remain classified as a stream, although
with acknowledgment that it lacks some functions or
characteristics typically associated with streams that may be
able to be recovered.
Alternatively, conveyances which lack sufficient characteristics to be considered streams could be regulated as point
sources, as suggested by Justice Scalia in Rapanos, who doubted
the validity of “storm drains under anybody’s concept be[ing]
water of the United States.” Such conveyances would fall under
the NPDES program of the CWA rather than as jurisdictional
waters. If this approach were adopted, a feature could potentially
lose its status as a stream, and thus potentially as a jurisdictional
water, through degradation. If physical, chemical, and biological
aspects all must exist for a feature to be designated as a stream,
then increased pollution alone could cause the “loss” of a stream;
e.g., if water quality pollution were so toxic that normal stream
flora and fauna ceased to exist (33). Conversely, habitat
restoration or upstream water quality control measures might
be sufficient to improve a conveyance to the extent that it could
be reclassified as a streamsconversion of a point source to a
jurisdictional water through restoration. Restoration of stream
status could become a source of lucrative mitigation credits for
documented improvements in water quality and biota (8).
The central policy question that remains is whether serving
as a conduit aloneswhich would be a significant nexus to
downstream waters through its transfer of pollutantssis
sufficient to merit being a jurisdictional water, or must the
feature have additional physical, chemical, or biological
characteristics? That is, is the feature itself important, or is
it the connection to downstream that is important? This
distinction underlies the divergent holding positions of
Justices Kennedy and Scalia, and is likely to be debated in
agencies and courts in the coming years.
3.2. Science Needed. In addition to improving the science
of stream classifications and indicators, there is a great need
for basic studies of characteristics that distinguish streams
mechanistically and biologically from saturated hillslopes,
lentic features, and simple conveyances. Identifying fundamental shifts in processes which are likely to occur at the
land-stream interface (34) will be critically important to
justify alternative classification metrics. Such developments
will also be important for understanding the potential impacts
of changing climate and land use.
Studies are also needed to assess the implications of
alternative policy and regulation scenarios. For instance, a
series of empirical studies is needed in different climatotopographic regions to quantify the “movement” of stream
starting points. As geomorphologists and hydrologists have
examined the location and movement of channel and stream
heads through time (14), some basic empirical information
is needed on whether and how the biological or chemical
starting points of streams migrate over time under different
topographic, geologic, and climatic conditions. It is unclear
how physically, chemically, and biologically distinct stream
“beginnings” are from their surroundings, and how consistent
these differences are over time.
New scientific research that examines the individual and
cumulative influence of headwater streams at landscape and
network scales is needed to analyze the sensitivity of
downstream waters to alternative regulations. The implications of new regulatory frameworks need to be evaluated.
For example, how many stream miles have flow for “typically
three months” and how does this change under slightly
altered climate or land use conditions? Similarly, available
watershed hydrology models should be used to examine the
water quality and quantity impacts of different regulatory
scenarios on the streams themselves and on downstream
navigable waters. A simple sensitivity analysis might be to
take the most upstream 10% of the channel network and
convert it from natural streams to inert pipes (via experimental manipulation or, more likely, modeling experiments)
to quantify the potential impact to downstream water quality
or flow regimes.
Finally, the processes and characteristics of certain types
of streams are poorly understood. Most of what we know
about hydrology and geomorphology of streams comes from
research done in high relief and humid environments;
relatively little research has been done on low-relief streams.
Because low-relief areas make up regions under considerable
development pressure (e.g., coastal plain of Southeastern
U.S.), it is imperative that they receive greater scientific
attention from which to base future regulations and management decisions.
3.3. Conclusions. Streams are not just important; they
are different from other landscape features. While other
ecosystem types provide some similar chemical and biological
functions (Table 1), it is the combination of these functions
together with unidirectional flow and tight connections to
downstream waters which make streams so critical. Recognizing that streams and stream-like features vary in their
hydrologic and chemical connectedness, and biological
“uniqueness,” and quantifying that variation can provide
better guidance about the types and locations of land use
change that are likely to be most and least detrimental to the
overall health of navigable waters.
Existing policies give regulators two choices in dealing with
features that are not clearly streams either because they are
very small or very degraded: jurisdictional waters regulation or
point source regulation. Designation of a feature as a stream
(and thus jurisdictional water) may be best reserved for
functioning systems that merit or are in need of protection
themselves, with point source designation used to manage
stream-like conveyances. The ability of a feature to lose or gain
its status as a stream via degradation or restoration, respectively,
may provide a useful additional incentive to restrain pollution
and encourage pollution mitigation. It is unclear which trajectory of policy (jurisdictional water or point source) will prevail
in agency regulation or in legal interpretation. Because of this,
and in the face of unrelenting land development pressure,
streams in which multiple functions are present should be given
particular preservation protection.
The crux of the issue is that environmental policy and law
depends on clearly defined boundaries that science cannot
easily provide. Increasing the understanding of basic,
underlying science is necessary to ensure well-grounded
policies and appropriate interpretations by courts and
agencies. It is the interaction of both environmental scientists
and policy makers that is imperative to realize the goal of
restoring and maintaining the integrity of the waters of the
United States.
Martin Doyle is a river scientist specializing in how geomorphology
and hydrology affect ecology. His research focuses on river restoration,
water policy, and on emerging ecosystem service markets. He has also
worked on opportunistic decommissioning of aging and obsolete
infrastructure including dams and levees. Emily Bernhardt is a river
scientist specializing in the structure and function of freshwater
ecosystems. Her research focuses on how climate and land use change
affect ecosystem nutrient cycling and energy balance. Current work
includes evaluating the effectiveness of river restoration efforts and
the impacts of chemical contaminants in river ecosystems.
Acknowledgments
We appreciate many discussions with state and federal agency
personnel. Mark Sudol, Mike Paul, Adam Riggsbee, and Jeff
Muehlbauer gave detailed and helpful reviews. M.W.D. and
E.S.B. were both supported by NSF Early Career Awards.
While writing this paper, M.W.D. was supported by the FJ
Clarke Visiting Scholar at the U.S. Army Corps of Engineers.
The views expressed here do not necessarily represent those
of the Corps or NSF.
VOL. xxx, NO. xx, XXXX / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
E
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