Environ. Sci. Technol. 2011, 45, 354–359
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. 354
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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 10.1021/es101273f
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Published on Web 12/06/2010
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
(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 VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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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 356
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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
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
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 VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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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) 358
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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.
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