Waters of the United States, Navigable Waters, and Questions Concerning Wetland Inventories


by Thomas J. Slabe

Waters of the United States
The conterminous U.S. receives approximately 2,500 cubic kilometers of renewable freshwater per year (5.6 percent of global supply) supplied via the hydrologic cycle from oceans to the land (Gleick, 1988). An equal amount eventually flows as runoff back to the oceans. The amount of renewable fresh water supply is roughly equal to a yearly average of 30 centimeters (about 12 inches) evenly distributed across the 48 states. However, precipitation across this diverse region is geographically and temporally variable. For example, average annual precipitation ranges from as much as 348 centimeters (137 inches) in northwestern Washington State to as little as 6.7 cm (2.6 inches) in the Mojave Desert. Western coastal areas experience Mediterranean climates whereas most regions tend towards monsoonal climates, and every region is disposed to droughts and floods. Approximately 46 percent of total precipitation is lost from the land to the atmosphere via evapotranspiration. Moisture that is returned to the atmosphere is not considered a component of the renewable freshwater supply, yet is essential for ecosystems, silviculture, non-irrigated agriculture, and non-irrigated vegetative cover for domestic and public lawns, parks, golf courses, etc. (Postal, 1996). Renewable fresh water supply is responsible for sustaining aquatic systems – riverine (rivers and streams), lacustrine (lakes and ponds), palustrine (wetland meadows, prairie potholes, vernal pools, playa lakes, swamps, bogs, and fens) – that compose the freshwater components of the waters of the United States.

Historically, rivers and lakes were critical to national commerce for the transport of people and property and for the development of North America. Perhaps it is because of this historical context that Congress defined the term “navigable waters” as “waters of the United States” in Section 502 of the Federal Water Pollution Control Act of 1972. Today, this outdated definition muddies our notion of what indeed are waters of the United States (Squillace, 2006-2007). Where is the line of demarcation between waters of the United States and other waters? One would generally characterize navigable waters as those larger bodies of water at lower elevations that accommodate larger watercraft. Because water flows downhill, is it reasonable to consider water flowing downstream into “navigable waters” from higher elevation water bodies (and those water bodies themselves) as waters of the United States? If so, how far upstream is reasonable and what are the implications of classifying such aquatic resources as waters of the United States?

In contrast to rivers and lakes, wetlands historically were an obstacle to commercial expansion. Yet they are an extremely important component of the nations’ aquatic resources. Wetlands distribute freshwater supplies across a large proportion of the land surface and delay the eventual return of surface water to the sea. However, according to Section 2 of the North American Wetlands Conservation Act, today “more than 50 per centum of the original wetlands in the United States alone have been lost.” Wetlands are reported to originally comprise more than 11 percent (345,312 square miles) of the land area of the lower 48 states. A recent estimate indicates they comprise only about 5.39 percent (160,938 square miles; Dahl and Allord, 1999), a loss of more than 53 percent since European settlement of North America. These are only estimates. Inventories of wetlands do not specifically indicate that beaver-created wetlands are included in such estimates, perhaps because of their ephemeral nature or because most beaver-created wetlands in the conterminous U.S. were lost by the mid 1800s. In addition, very small wetlands (less than one acre in area) that fall outside the classical, agreed-upon definition of wetlands, or because of their small size are not resolved in aerial photographs used for wetland inventories, are largely absent from such inventories. Beaver-created and very small wetlands constitute a significant yet largely overlooked component of the nation’s wetland resources.

Questions Concerning Wetland Inventories
According to one U.S. Fish and Wildlife Service (USFWS, 2005) report on studies of the status and trends of the Nation’s wetlands, the definition of wetlands is:

Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification wetlands must have one or more of the following three attributes: (1) at least periodically, the land supports predominantly hydrophytes, (2) the substrate is predominantly undrained hydric soil, and (3) the substrate is non-soil and is saturated with water or covered by shallow water at some time during the growing season of each year.

Modern remote sensing techniques were employed with extensive ground observations (truthing) in these studies. However, “the minimum target delineation unit for a wetland was one acre (0.4 hectare)” and only sporadically were smaller (as low as 0.002 hectare) wetlands delineated. Thus, most wetlands under the size of one acre were likely not included in these studies. Very small wetlands, which seem to escape notice for regulatory purposes and for inventories, are vulnerable to loss and degradation, yet they are crucial for many sensitive species such as the boreal toad (Bufo boreas boreas). Unfortunately, substantive information on the status of the nation’s small wetlands was not found during this study. Because of serious disagreements over methods used for delineating wetlands, the U.S. Army Corps of Engineers Wetlands Delineation Manual (USACE, 1987) is controversial (Heimlich et al., 1998). A number of attempts have been made to supersede this work to virtually no effect. Controversies surrounding wetlands in combination with the confusion embodied in defining “navigable waters” as “waters of the United States” has significantly hampered the development of a unified approach to preserving the nations’ wetland resources. This is attested to by various U.S. Supreme Court decisions, e.g., United States v. Riverside Bayview Homes, Inc., 474 U.S. 121, 1985; Solid Waste Agency of N. Cook County v. United States Army Corps of Engineers, 531, U.S. 159 (2001; SWANCC); Rapanos v. United States, 126, S. Ct. 2209 (2006) (Squillace, 2006-2007). It is apparent that streams and lakes that support a form of navigation are “waters of the United States” and therefore are subject to federal statutes. This is not necessarily the case with wetlands, especially for small wetlands and for those wetlands distant from a “navigable water.”

Castor canadensis, the American beaver, a wetland ecosystem engineer.

Wetlands throughout most of North America are highly correlated with the American beaver (Castor canadensis), which before European settlement numbered between 60 – 400 million (Seton, 1929). Today beavers number at around 6 – 12 million (Naiman et al., 1988). Butler and Malanson (2005) reported estimated pre-European North America numbers of beaver ponds to range from a low of 15 – 100 million to a high of 37.5 – 250 million and post-European numbers of beaver ponds to range from a low of 1.5 – 3.0 million to a high of 3.75 – 7.7 million. Averages derived from estimated numbers of pre-European beaver and of beaver ponds suggest that beaver wetlands in the conterminous U.S. could easily have covered more than 85,000 square miles, a combined area equal to Lakes Superior, Huron, Michigan, and Ontario. Clearly, most beaver wetlands were permanently lost in concert with the rapid decline of the beaver population in the 18th and 19th centuries.

Importance of Beaver-Created Wetlands
Beaver prefer small streams (4th order or lower) with a stream channel gradient below 6%. However, they inhabit streams with channel gradients as high as 12% (Retzer, 1955). They also prefer valley bottoms that exceed the channel width. Indeed, the activity of beavers apparently aggrades valley bottoms as the cycle of pond creation, sedimentation, and meadow formation are repeated over the millennia (Ruedemann and Schoonmaker, 1938). Ives (1942) commented that such aggradation, which occurs at an average rate of approximately 0.04 to 0.012 inches per year in the study area in Colorado’s Grand County, produced a “false senility” of those mountain streams that resemble filled glacial lakes.

Ives (1942) presents this graphic narrative of beaver activity as is likely to occur over time in Grand County, Colorado:

“Whenever a stream is dammed by beaver, the local water table is elevated, and many of the spruce and fir trees of the montane forest are killed. Within a few years, these dead trees fall, and aspen trees, more tolerant to moist subsoil conditions, replace some of them.During this time of active beaver occupancy, the shores and the delta beds of the pond become covered with dense brush, which is the normal pre-aspen stage in the regional plant succession. As soon as the pond is abandoned, local conditions are ideal for the rapid growth of a new aspen forest.

Because of this plant succession, the beaver, although he locally exhausts the supply of aspen trees, perpetuates his chosen forest environment, and in most instances, over a period of years, increases the extent of the valley-floor aspen forest. Thus, in a large system of valleys, such as the upper Colorado valley, the supporting power of the area for beaver tends to increase with time. Thus it should be apparent that the beaver farms, rather than mines, the forest resources of the valley he occupies.”

Beaver dams and wetlands are landscape features that attenuate surface water flow and contribute to landscape heterogeneity, species diversity, and regional plant succession regimes.

In addition to creating beaver ponds, beaver activity favors wetland creation in additional ways by reducing the kinetic energy of streams, raising the water table, creating canals, and generally increasing water storage capacity of watersheds (Collen and Gibson, 2001).

Although beaver prefer lowlands with wide valley bottoms and a low stream gradient, they in fact occupy an extremely wide array of habitats. “They have successfully colonized tundra and taiga in the far North, bottomland hardwood forests and marshes in the deep South, riparian areas in both cold and hot deserts, and elevations that vary from sea level to above 3,400 meters [11,155 feet].” (Baker and Hill, 2003) Very small wetlands and wetlands created by beavers play an important role in preserving landscape heterogeneity and biological diversity. However, for many reasons these types of wetlands are particularly vulnerable even today to loss and degradation.

From 1780 to 1980, wetlands loss in the conterminous U.S. occurred at a rate of 60 acres per hour (Squillase, 2006-2007). It is uncertain if this figure takes into account the loss of beaver wetlands, which eventually disappear from the landscape a couple of decades after beaver removal or abandonment (Neff, 1957). A report from the U.S. Fish and Wildlife Service (USFWS, 2005) indicates that the rate of loss of wetlands in the lower 48 states has reversed and a net gain of approximately 50 square miles was realized from 1998 to 2004. The Report indicates that only the physical extent of wetlands was measured rather than trends in the quality of wetlands. Moreover, although a net gain in palustrine wetlands was realized, a net loss would have occurred if not for an increase in total area of shallow artificial ponds, which do not provide an equivalent level of ecosystem services generally contributed by natural wetland types.

Contrary to the historical view that “wetlands presented obstacles to development, and that wetlands should be eliminated and the land reclaimed for other purposes” (Dahl and Allord, 1999), wetland ecosystems are increasingly valued by society. In 1997, Costanza et al. estimated the average annual value of ecosystem services provided by wetlands to be $14,785 per hectare ($5,986 per acre). By contrast, the reported value of ecosystem services is $8,498 ($3,440 per acre) and $969 per hectare ($392 per acre) for lakes and rivers and for forests, respectively. Although this may be a controversial and speculative measure of ecosystems services, it does indicate a positive trend toward appreciating the values attributed to wetlands. The USFWS (2005) Report likewise indicates a positive trend in the reduction in the rate of wetlands loss in the conterminous U.S. However, there remain serious unanswered questions. For example, what was the actual extent of wetlands before European settlement? What is the rate of loss of very small wetlands? Is the rate of wetland and riparian area loss and degradation outpacing protection, mitigation, and restoration efforts? Despite the historical losses of aquatic resources and the many remaining questions, there are some indications that the trends are reversing and that these resources are understood and valued more today than at any other time in history


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