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Ecoregions and Watersheds
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Ecoregions are intended to provide a spatial framework for ecosystem assessment, research, inventory, monitoring, and management. These regions delimit large areas within which local ecosystems reoccur more or less throughout the region in a predictable pattern. By observing the behavior of the different kinds of systems within a region it is possible to predict the behavior of an unvisited one. This affords the extrapolation mechanism for identifying areas from which site specific knowledge on ecosystem behavior can be applied. As such, they also suggest areas within which similar responses and management strategies are applicable (Bailey, 1987). Ecoregions should be thought of as multi-purpose regions, designed to show areas within which the aggregate of all terrestrial and aquatic ecosystem components is different from or less variant than that in other areas.
Quite simply, watersheds are topographic areas within which apparent surface water runoff drains to a specific point on a stream or to a waterbody such as a lake. There is an infinite number of points from which topographic watersheds can be delineated, although regarding streams, confluences are normally used. Large watersheds are commonly termed basins (e.g., the Colorado River Basin or the Susquehanna River Basin). The hierarchical classification of hydrologic units as mapped by the U.S. Geological Survey (Seaber et al., 1987) is made up of watersheds or segments of watersheds often with adjacent interstices (areas in between). However, at each level of classification, the majority of these hydrologic units are not true topographic watersheds.
Misunderstanding Watersheds
Much of the apparent usefulness of watersheds as study units arises from the general understanding that the quantity and quality of water at a point on a stream reflects the aggregate of the characteristics of the topographic area upgradient from that point. However, the conclusion that because of this a framework of watersheds, basins, or hydrologic units is ideally suited for spatially organizing ecosystem management, or even water quality management, is flawed for at least three major reasons.
First, and most important, the areas within which there is similarity in the aggregate of geographic characteristics related to the quality and quantity of environmental resources seldom if ever correspond to patterns in topographic watersheds. Second, in many xeric regions of the country where watersheds can be defined and "influent" streams predominate (where streams feed the groundwater, as compared to "effluent," where the groundwater feeds the streams), topographic watersheds do not encompass the same integrating processes as in mesic and hydric areas. Third, in many areas watersheds are difficult or impossible to define (Hughes and Omernik, 1981). These types of areas comprise roughly a third of the conterminous United States (Figure 1). Regions of continental glaciation, deep sand, karst topography, flat plains, and xeric climates all fall into this category. More than one of these conditions occur in many areas making the problem more complex.
Many parts of the country that have been affected by continental glaciation are pocked with lakes, potholes, swamps, and marshes where surface water does not drain directly into streams. Although most of the Midwest, much of the Northeast, and the northern fringe of the western United States contain these characteristics, they are most common in North Dakota, Minnesota, Wisconsin, and Michigan. Delimiting the watershed boundaries of large rivers can be approximated fairly accurately in much of this region, but that is not the case with many of the smaller streams. Other parts of the country where watershed delimitation is particularly problematic include the nearly level, karst and sand dominated state of Florida, the Sand Hills of Nebraska, the semiarid karst and playa lands of west Texas and Oklahoma and eastern New Mexico, and the deserts of western and southwestern United States.
Watersheds and Hydrologic Units
There is also a common misunderstanding that hydrologic units do in fact comprise watersheds. For example, in a well meaning effort to implement a spatial and more integrated approach to environmental resource management, particularly regarding water quality programs, the EPA has recently advocated a "watershed approach" (USEPA, 1995, 1997). This approach emphasizes "managing by watersheds" and recommends use of U.S. Geological Survey hydrologic units (Seaber et al., 1987) which are aimed to "provide a common national framework for delineating watersheds and their boundaries at a number of different geographic scales" (USEPA, 1995). Another EPA Office of Water publication, accompanied by a set of maps, is directed toward characterizing "the aquatic condition and vulnerability of each of the 2,150 watersheds in the continental United States" (USEPA, 1997). Watersheds in this publication are defined as the eight digit cataloging units of the U.S. Geological Survey hydrologic unit system.
Aside from providing additional examples of misuses of watersheds, the problem here is that it is not possible to divide the country, any state, or as far as we know, any county, into a finite number of topographic watersheds. For instance, in Tennessee only four of the eleven "accounting level" hydrologic units covering the state are true topographic watersheds (Seaber et al., 1987)(Figure 2). Most are segments of watersheds with adjacent interstices. Less than half (26 of 54) of the "cataloging units," the next more detailed hierarchical level of hydrologic units, that are partly or completely in Tennessee, are true topographic watersheds (U.S. Department of the Interior, Geological Survey, 1994).
One reason given by the USFWS for their decision to use what they term "watershed based units" was that "watersheds are discrete physical units that provide widely recognized and generally well-defined boundaries" (USFWS 1995). Another reason given was "watersheds provide a vehicle to consider the critical linkages between upstream and downstream effects." The implication here is that the hydrologic units they adapted and the "watershed based units" are at least similar to true pathways for interactions of the ecosystem components. However, of the 41 USFWS "watershed based units" defined for the conterminous United States, only 17 percent are actually topographic watersheds. One of these units, the Mississippi Headwaters/Tallgrass Prairie straddles a continental divide; the northwestern portion drains north to the Hudson Bay, whereas the southeastern part is in the Mississippi drainage system.
More important, from a standpoint of suitability for framing ecosystem or water quality assessment and management, this "watershed based unit" covers distinctly different ecological regions, which are based on entirely different criteria. The unit encompasses a large part of a region of nutrient poor soils and high quality lakes and forests in northwestern Wisconsin and northeastern and central Minnesota, as well as sections of several formerly prairie but now largely agricultural regions - the Corn Belt in southern Minnesota, the flat Lake Agassiz Plain (Red River Valley), and the semi-arid prairie pothole country of North Dakota. Hence, not only do hydrologic units lack spatial correspondence to areas within which there is similarity in the quality and quantity of surface waters and the factors associated with spatial differences in quality and quantity, most are not true topographic watersheds.
Now What?
State, regional, and national level management strategies, particularly those involving ecosystem management, require a spatial framework that considers the regional tolerances and capacities of the landscape. Ecoregions were designed to fill that need and identify areas with similarity in the combination of geographic phenomena that cause and reflect regional differences in the quality of ecosystems and ecosystem components (Omernik, 1995; Bailey, 1995a). Watersheds and basins do not correspond to these patterns. However, we stress that basins and watersheds are the appropriate spatial unit for resource management agencies to assess the relative contribution of human activities to the quality and quantity of water at specific points on streams and on particular water bodies. Because they integrate the surface and subsurface flow of water upgradient from the point at which measurements are made, watersheds allow drainage basin-specific accountings to be made of factors such as point and nonpoint source pollutants, whose transport is associated with the movement of water. Watersheds are essential for these purposes.
Although the two frameworks have very different purposes, they can be used together to effectively help assess and manage environmental resources. In areas where watersheds are relevant and can be defined, both ecoregions and watersheds are necessary for developing a system of regional reference sites enabling an understanding of the attainable quality, integrity, and health of ecosystems and their components. A true ecosystem approach for reaching these objectives requires consideration of the mosaic of biotic and abiotic components in both the aquatic and terrestrial environment. Watersheds, in part, facilitate this meshing. For those areas where watersheds cannot be defined, where they are of little relevance (e.g., in xeric areas), and where there are few if any streams (e.g., parts of the Prairie Pothole region and the western part of the Nebraska Sand Hills), relevant representative "areas" rather than watersheds must be chosen. For the bulk of the country, however, ecoregional reference sites will comprise watersheds. We therefore define these reference sites as watersheds or areas that are representative of the ecoregions they occupy, but that are relatively unimpacted (Hughes, 1995; Omernik, 1995; Hughes et al., 1986).
This material was taken directly fromOmernik and Bailey (1997). Learn more about ecoregions and watersheds by downloading a PDF file of Omernik and Bailey (PDF) (1997) (15pp, 690 K,About PDF) and Omernik (1995) (PDF) (19pp, 552K,About PDF).
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References
Omernik and Bailey 1997. Distinguishing between watersheds and ecoregions. Journal of the American Water Resources Association. 33(5): 935-949.
Bailey, R. G., 1987. Mapping Ecoregions to Manage Land. In: 1987 Yearbook of Agriculture. U.S. Department of Agriculture. Wash- ington, D.C., pp 82-85.
Bailey, R. G., 1995a. Ecosystem Geography. Springer-Verlag, New York, New York.
Hughes, R. M., 1995. Defining Acceptable Biological Status by Comparing with Reference Conditions. In: Biological Assessment and Criteria: Tools for Water Resource Planning and Decision Making, W. Davis and T. Simon (Editors). Lewis Publishers, Boca Raton, Florida, pp. 31-47.
Hughes, R. M., D. P Larsen, and J. M. Omernik, 1986. Regional Reference Sites: A Method for Assessing Stream Potentials. Environmental Management 10(5):629-635.
Hughes, R. M. and J. M. Omernik, 1981. Use and Misuse of the Terms Watershed and Stream Order. American Fisheries Society, Warmwater Stream Symposium, Bethesda, Maryland, pp. 320-326.
Omernik, J. M., 1995. Ecoregions: A Spatial Framework for Envi ronmental Management. Jo: Biological Assessment and Criteria: Tools for Water Resource Planning and Decision Making, W. Davis and T. Simon (Editors). Lewis Publishers, Boca Raton, Florida, pp.49-62.
Seaber, P.R., F. P Kapinos, and G. L. Knapp, 1987. Hydrologic Unit Maps. U.S. Geological Survey Water-Supply Paper 2294, United States Department of the Interior, Geological Survey, Denver, Colorado.
U S. Environmental Protection Agency, 1995. Watershed Protection: A Statewide Approach. EPA841-R-95-004, U. S. Environmental Protection Agency, Office of Water, Washington, D.C.
U. S. Environmental Protection Agency, 1997. The Index of Watershed Indicators. EPA841-R7-010, U. S. Environmental Protection Agency, Office of Water, Washington, D.C.
U. S. Department of the Interior, Fish and Wildlife Service, 1995. An Ecosystem Approach to Fish and Wildlife Conservation. Conceptual Document. U.S. Fish and Wildlife Service, Washington, D.C.
U. S. Department of Interior, Geological Survey, 1994. Hydrologic Unit Map - 1994: State of Tennessee. U.S. Geological Survey, Reston, Virginia.