Vulnerable waters refer to geographically isolated wetlands (GIWs) and to ephemeral and intermittent streams. Ephemeral and intermittent streams are seasonally flowing and are located in headwater position. They are the outer and smallest stems of hydrological networks. Isolated wetlands are located outside floodplain and show poor surface connection to tributaries or floodplains. Geographically isolated wetlands encompass saturated depressions that are the result of fluvial, aeolian, glacial and/or coastal geomorphological processes. They may be natural landforms or the result of human interventions. Vulnerable waters represent the major proportion of river networks.
These water bodies show vulnerability to natural and human disturbances because they are poorly hydrologically connected, and they are often located in the gray zone of countries and states' protected water regulatory frameworks.Womble, P., Kihslinger, R. L., McElfish Jr, J. M., & Sweeney, E. (2011). America's vulnerable waters: assessing the nation's portfolio of vulnerable aquatic resources since Rapanos v. United States. Environmental Law Institute report, 97 p. In the US, the protection status of GIWs and ephemeral/intermittent streams in regard to the Clean Water Act is being revised. In the context of European Union (EU) Water Framework Directive (WFD), small headwater streams are neglected, especially in agricultural setting.
These water bodies play an essential hydrological and ecological role at the local-to catchment scale. They control storage of water, sediment in the drainage network, increase sediment filtering and biochemical transformation.Capps, K. A., Rancatti, R., Tomczyk, N., Parr, T. B., Calhoun, A. J., & Hunter, M. (2014). Biogeochemical hotspots in forested landscapes: the role of vernal pools in denitrification and organic matter processing. Ecosystems, 17(8), 1455-1468. Furthermore, vulnerable waters contribute to increases in landscape biodiversity as they serve as refuge to endemic species Meyer, J. L., Strayer, D. L., Wallace, J. B., Eggert, S. L., Helfman, G. S., & Leonard, N. E. (2007). The contribution of headwater streams to biodiversity in river networks. JAWRA Journal of the American Water Resources Association, 43(1), 86-103. and conduits for migration. Headwater streams and isolated wetlands shows hydrological and ecological connectivity through intermittent surface processes and groundwater processes.
Definition, types and distribution of vulnerable waters
Ephemeral and intermittent headwater streams Headwater streams refer to the smallest channels of a river network, where streamflow begins. They are considered first- to third-order streams in the Strahler 's stream classification system.Alexander, L. C., Autrey, B., DeMeester, J., Fritz, K. M., Golden, H. E., Goodrich, D. C., ... & McManus, M. G. (2015). Connectivity of streams and wetlands to downstream waters: review and synthesis of the scientific evidence (Vol. 475). EPA/600/R-14. The designation of intermittent and ephemeral stream refers to the continuum of streamflow within a year. An ephemeral stream flows episodically, following a precipitation event, while an intermittent stream flow continuously during a portion of the year. In both cases, the drying of the channel results from the local water table declining below the bed surface. The majority of ephemeral and intermittent streams are in headwater positions, but in lowland settings, small tributaries along the river network can be ephemeral or intermittent.
Geographically-isolated wetlands (GIWs) Geographically isolated wetlands (GIWs) are wetlands entirely surrounded by uplands. GIWs receive water from adjacent uplands and precipitation. However, no stream of any type supplies water to GIWs. Despite GIWs having poor hydrological connectivity with stream networks, they can exhibit subsurface connectivity or even temporary surface-water outflows toward other wetlands or streams. GIWs lacking complete surface or subsurface hydrological connectivity with any water body will lose water mainly by evapotranspiration or to groundwater that is not connected to a stream network. Despite the absence of hydrological connectivity, they can exhibit biological and chemical connectivity with fluvial systems.
Non-floodplain wetlands GIWs that are hydrologically connected (by subsurface connection or temporary surface connection) can be considered non-floodplain wetlands. Non-floodplain wetlands are located outside floodplains and display a unidirectional hydrological connectivity with streams, meaning the water is flowing only toward streams located at lower elevations. Hydrological connectivity between non-floodplain wetlands and streams occurs through surface or subsurface processes. Surface connections can be ephemeral and intermittent streams.
Morphological classification In the US, the natural types of GIWS are: prairie pothole wetlands, playas, Nebraska's Rainwater Basin and Sandhills wetlands, West Coast vernal pools, sinkhole wetlands, Carolina bays, intradunal and interdunal wetlands, desert springs, endorheic basin in the Great Basin, and kettle-hole in glaciated regions.
Non-floodplain wetlands are classified in three categories which include GIWs: depressional wetlands, slope wetlands and flats wetlands. Depressional wetlands occur in topographic depressions with or without surface outlets.Brinson, M. M. 1993. A hydrogeomorphic classification for wetlands. Technical Report WRP-DE-4, U.S. Army Corps of Engineers, Waterways Experiment Station, Wetlands Research Program, Vicksburg, MS. Depressional wetlands include kettle holes, potholes, vernal pools, playas lake and Carolina bays. Slope wetlands are located along hillslopes and are mainly recharged by groundwater inputs. Fens are the usual type of slope wetlands. Flats wetlands occur on large flat areas like interfluve, dried lake bottoms or large floodplain terraces. Large playas are a type of mineral soil-dominated flats. Flats wetlands can also be formed from organic soils, like peatbogs.
GIWs and non-floodplain wetlands can emerge from one or a combination of geomorphological processes: aeolian (potholes, playas, Rainwater basin, Carolina Bays, interdunal wetlands), (peri-)glacial (kettle, fens), karstic (sinkholes) and lacustrine (Carolina Bays, endorheic basin).
Relative abundance of vulnerable waters Ephemeral and intermittent headwater streams and GIWs display the shortest drainage area and stream length, but together they can represent the major proportion of river networks and watersheds.
In the U.S., headwater streams represent more than 60% of the river network lengthOlsen, A. R., & Peck, D. V. (2008). Survey design and extent estimates for the Wadeable Streams Assessment. Journal of the North American Benthological Society, 27(4), 822-836. and geographically isolated wetlands encompass about 16% of freshwater resources.Lane, C. R., & D'Amico, E. (2016). Identification of putative geographically isolated wetlands of the conterminous United States. JAWRA Journal of the American Water Resources Association, 52(3), 705-722. In 17 states, there are streams with an intermittent-stream-length-over-total-length's ratio higher than 82%. North Dakota, South Dakota and Minnesota are the three states with the most hectares of geographically isolated wetlands. Many studies report that actual maps of U.S. hydrographic network underestimate the distribution of headwater streams Hansen, W. F. (2001). Identifying stream types and management implications. Forest Ecology and Management, 143(1-3), 39-46.
The approximate length of first to third order streams in the world is respectively 45 660 000, 22 061 000 and 10 660 100 km, and they represent the dominant Strahler' order of streams in the world.Downing, J. A., Cole, J. J., Duarte, C. M., Middelburg, J. J., Melack, J. M., Prairie, Y. T., ... & Tranvik, L. J. (2012). Global abundance and size distribution of streams and rivers. Inland waters, 2(4), 229-236.
Vulnerable water legal status The legal status of ephemeral and intermittent headwater streams and GIWs differ from one legislation to another.
United States In the US, Environmental Protection Agency (EPA) has, since 1972, the responsibility to regulate the waters of the United States, under the Clean Water Act (CWA). The Clean Water Act, introduced by president Richard Nixon, made clear that continental waters should be "swimmable and fishable" for the American public. That was a great step toward protection of riverine habitats and improvement of water quality.
Because of the U.S Constitution, the federal government can only protect interstate waters that is used for navigation, which is defined as the "waters of the United States" (WOTUS). From 1972 to 2015, the EPA defined WOTUS as:
"`[...] traditional navigable waters, interstate waters (including interstate wetlands), all other waters that could affect interstate or foreign commerce, impoundments of waters of the United States, tributaries, the territorial seas, and adjacent wetlands" (CWA, (33 CFR 328.3; 40 CFR 122.2)
The CWA defined wetlands as:
"[...] areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas." (CWA, section 404).
The definition of WOTUS was challenged in court several times, especially regarding the integration of isolated wetlands, but until the years 2000, the U.S. Federal Court stood by the initial definition saying that floodplain wetlands are bound to the streams they are adjacent to. Then, In 2001, a judgement ruled that isolated wetlands are not comprised in the WOTUS definition because they do not show "significant nexus" with navigable streams. In 2006, in the Rapanos vs. United States case, further confusion was created concerning the WOTUS definition. No majority decision was obtained, as two main opinions divided the judges. The first one, defended by Justice Antonin Scalia, was that the scope of the CWA only included permanent streams and wetlands with a significant surface connections with navigable streams. The second opinion, led by Justice Anthony Kennedy, was that waters have a significant nexus with navigable waters if they are significantly affecting their chemical, physical, and biological integrity.
The definition of WOTUS is actually under revision. The United States President, Donald Trump, signed the executive Order 13778 (82 FR 12495, March 3, 2017), asking the EPA and United States Army Corps of Engineers Corps to review the definition of WOTUS in a manner that is consistent with Justice Antonin Scalia's opinion. This opinion calls for protection of permanent waters and wetlands showing surface connections to relatively permanent waters, which exclude GIWS, and ephemeral/intermittent streams.
European Union In the European Union (EU), since 2000, the Water Framework Directive (WFD) aims to achieve a number of objectives to improve water quality of lakes, rivers and coastal areas. Key objectives are general protection of the aquatic ecosystem, protection of drinking water resources, and protection of swimmable waters. These objectives are realized through river management measures at basin scale. The WFD requires the delineation of water bodies that will be targeted for specific diagnosis and management measures. The smallest area range considered to identify those water bodies is 10 to 100 km square. Due to their small size, headwater streams are not usually identified as one of the water bodies targeted by the WFD and, therefore, become more vulnerable to human activities.
Ecological functions