An agency’s construction of a statute it is charged with enforcing is entitled to deference if it is reasonable and not in conflict with the expressed intent of Congress. Chemical Manufacturers Assn. v. Natural Resources Defense Council, Inc., 470 U.S. 116, 125 (1985); Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837, 842-845 (1984). Accordingly, our review is limited to the question whether it is reasonable, in light of the language, policies, and legislative history of the Act[,] for the Corps to exercise jurisdiction over wetlands adjacent to but not regularly flooded by rivers, streams, and other hydrographic features more conventionally identifiable as "waters." [FN8]


On a purely linguistic level, it may appear unreasonable to classify "lands," wet or otherwise, as "waters." Such a simplistic response, however, does justice neither to the problem faced by the Corps in defining the scope of its authority under 404(a) nor to the realities of the problem of water pollution that the Clean Water Act was intended to combat. In determining the limits of its power to regulate discharges under the Act, the Corps must necessarily choose some point at which water ends and land begins. Our common experience tells us that this is often no easy task: the transition from water to solid ground is not necessarily or even typically an abrupt one. Rather, between open waters and dry land may lie shallows, marshes, mudflats, swamps, bogs—in short, a huge array of areas that are not wholly aquatic but nevertheless fall far short of being dry land. Where on this continuum to find the limit of "waters" is far from obvious.

Faced with such a problem of defining the bounds of its regulatory authority, an agency may appropriately look to the legislative history and underlying policies of its statutory grants of authority. Neither of these sources provides unambiguous guidance for the Corps in this case, but together they do support the reasonableness of the Corps’ approach of defining adjacent wetlands as "waters" within the meaning of 404(a). Section 404 originated as part of the Federal Water Pollution Control Act Amendments of 1972, which constituted a comprehensive legislative attempt "to restore and maintain the chemical, physical, and biological integrity of the Nation’s waters." CWA 101, 33 U.S.C. 1251. This objective incorporated a broad, systemic view of the goal of maintaining and improving water quality: as the House Report on the legislation put it, "the word ‘integrity’ ... refers to a condition in which the natural structure and function of ecosystems is [are] maintained." H.R.Rep. No. 92-911, p. 76 (1972). Protection of aquatic ecosystems, Congress recognized, *133 demanded broad federal authority to control pollution, for "[w]ater moves in hydrologic cycles and it is essential that discharge of pollutants be controlled at the source." S.Rep. No. 92-414, p. 77 (1972), U.S.Code Cong. & Admin.News 1972, pp. 3668, 3742.

In keeping with these views, Congress chose to define the waters covered by the Act broadly. Although the Act prohibits discharges into "navigable waters," see CWA 301(a), 404(a), 502(12), 33 U.S.C. 1311(a), 1344(a), 1362(12), the Act’s definition of "navigable waters" as "the waters of the United States" makes it clear that the term "navigable" as used in the Act is of limited import. In adopting this definition of "navigable waters," Congress evidently intended to repudiate limits that had been placed on federal regulation by earlier water pollution control statutes and to exercise its powers under the Commerce Clause to regulate at least some waters that would not be deemed "navigable" under the classical understanding of that term. See S.Conf.Rep. No. 92-1236, p. 144 (1972); 118 Cong.Rec. 33756- 33757 (1972) (statement of Rep. Dingell).

Of course, it is one thing to recognize that Congress intended to allow regulation of waters that might not satisfy traditional tests of navigability; it is another to assert that Congress intended to abandon traditional notions of "waters" and include in that term "wetlands" as well. Nonetheless, the evident breadth of congressional concern for protection of water quality and aquatic ecosystems suggests that it is reasonable for the Corps to interpret the term "waters" to encompass wetlands adjacent to waters as more conventionally defined. Following the lead of the Environmental Protection Agency, see 38 Fed.Reg. 10834 (1973), the Corps has determined that wetlands adjacent to navigable waters do as a general matter play a key role in protecting and enhancing water quality:

"The regulation of activities that cause water pollution cannot rely on ... artificial lines ... but must focus on all waters that together form the entire aquatic system. *134 Water moves in hydrologic cycles, and the pollution of this part of the aquatic system, regardless of whether it is above or below an ordinary high water mark, or mean high tide line, will affect the water quality of the other waters within that aquatic system.

"For this reason, the landward limit of Federal jurisdiction under Section 404 must include any adjacent wetlands that form the border of or are in reasonable proximity to other waters of the United States, as these wetlands are part of this aquatic system." 42 Fed.Reg. 37128 (1977).

We cannot say that the Corps’ conclusion that adjacent wetlands are inseparably bound up with the "waters" of the United States—based as it is on the Corps’ and EPA’s technical expertise—is unreasonable. In view of the breadth of federal regulatory authority contemplated by the Act itself and the inherent difficulties of defining precise bounds to regulable waters, the Corps’ ecological judgment about the relationship between waters and their adjacent wetlands provides an adequate basis for a legal judgment that adjacent wetlands may be defined as waters under the Act.

This holds true even for wetlands that are not the result of flooding or permeation by water having its source in adjacent bodies of open water. The Corps has concluded that wetlands may affect the water quality of adjacent lakes, rivers, and streams even when the waters of those bodies do not actually inundate the wetlands. For example, wetlands that are not flooded by adjacent waters may still tend to drain into those waters. In such circumstances, the Corps has concluded that wetlands may serve to filter and purify water draining into adjacent bodies of water, see 33 CFR 320.4(b)(2)(vii) (1985), and to slow the flow of surface runoff into lakes, rivers, and streams and thus prevent flooding and erosion, see 320.4(b)(2)(iv) and (v). In addition, adjacent wetlands may "serve significant natural biological functions, including food chain production, general habitat, and nesting, *135 spawning, rearing and resting sites for aquatic ... species." 320.4(b)(2)(i). In short, the Corps has concluded that wetlands adjacent to lakes, rivers, streams, and other bodies of water may function as integral parts of the aquatic environment even when the moisture creating the wetlands does not find its source in the adjacent bodies of water. Again, we cannot say that the Corps’ judgment on these matters is unreasonable, and we therefore conclude that a definition of "waters of the United States" encompassing all wetlands adjacent to other bodies of water over which the Corps has jurisdiction is a permissible interpretation of the Act. Because respondent’s property is part of a wetland that actually abuts on a navigable waterway, respondent was required to have a permit in this case. [FN9]



Following promulgation of the Corps’ interim final regulations in 1975, the Corps’ assertion of authority under 404 over waters not actually navigable engendered some congressional opposition. The controversy came to a head during Congress’ consideration of the Clean Water Act of 1977, a major piece of legislation aimed at achieving "interim improvements within the existing framework" of the Clean Water Act. H.R.Rep. No. 95-139, pp. 1-2 (1977). In the *136 end, however, as we shall explain, Congress acquiesced in the administrative construction.

Critics of the Corps’ permit program attempted to insert limitations on the Corps’ 404 jurisdiction into the 1977 legislation: the House bill as reported out of committee proposed a redefinition of "navigable waters" that would have limited the Corps’ authority under 404 to waters navigable in fact and their adjacent wetlands (defined as wetlands periodically inundated by contiguous navigable waters). H.R. 3199, 95th Cong., 1st Sess., 16 (1977). The bill reported by the Senate Committee on Environment and Public Works, by contrast, contained no redefinition of the scope of the "navigable waters" covered by 404, and dealt with the perceived problem of overregulation by the Corps by exempting certain activities (primarily agricultural) from the permit requirement and by providing for assumption of some of the Corps’ regulatory duties by federally approved state programs. S.1952, 95th Cong., 1st Sess., 49(b) (1977). On the floor of the Senate, however, an amendment was proposed limiting the scope of "navigable waters" along the lines set forth in the House bill. 123 Cong.Rec. 26710-26711 (1977).

In both Chambers, debate on the proposals to narrow the definition of navigable waters centered largely on the issue of wetlands preservation. See id., at 10426-10432 (House debate); id., at 26710-26729 (Senate debate). Proponents of a more limited 404 jurisdiction contended that the Corps’ assertion of jurisdiction over wetlands and other nonnavigable "waters" had far exceeded what Congress had intended in enacting 404. Opponents of the proposed changes argued that a narrower definition of "navigable waters" for purposes of 404 would exclude vast stretches of crucial wetlands from the Corps’ jurisdiction, with detrimental effects on wetlands ecosystems, water quality, and the aquatic environment generally. The debate, particularly in the Senate, was lengthy. In the House, the debate ended with the adoption of a narrowed definition of "waters"; but in the Senate the limiting *137 amendment was defeated and the old definition retained. The Conference Committee adopted the Senate’s approach: efforts to narrow the definition of "waters" were abandoned; the legislation as ultimately passed, in the words of Senator Baker, "retain[ed] the comprehensive jurisdiction over the Nation’s waters exercised in the 1972 Federal Water Pollution Control Act." [FN10]

The significance of Congress’ treatment of the Corps’ 404 jurisdiction in its consideration of the Clean Water Act of 1977 is twofold. First, the scope of the Corps’ asserted jurisdiction over wetlands was specifically brought to Congress’ attention, and Congress rejected measures designed to curb the Corps’ jurisdiction in large part because of its concern that protection of wetlands would be unduly hampered by a narrowed definition of "navigable waters." Although we are chary of attributing significance to Congress’ failure to act, a refusal by Congress to overrule an agency’s construction of legislation is at least some evidence of the reasonableness of that construction, particularly where the administrative construction has been brought to Congress’ attention through legislation specifically designed to supplant it. See Bob Jones University v. United States, 461 U.S. 574, 599-601 (1983); United States v. Rutherford, 442 U.S. 544, 554, and n. 10 (1979).

Second, it is notable that even those who would have restricted the reach of the Corps’ jurisdiction would have done so not by removing wetlands altogether from the definition of "waters of the United States," but only by restricting the scope of "navigable waters" under 404 to waters navigable in fact and their adjacent wetlands. In amending the definition of "navigable waters" for purposes of 404 only, the backers of the House bill would have left intact the existing definition of "navigable waters" for purposes of 301 of the *138 Act, which generally prohibits discharges of pollutants into navigable waters. As the House Report explained: " ‘Navigable waters’ as used in section 301 includes all of the waters of the United States including their adjacent wetlands." H.R.Rep. No. 95-139, p. 24 (1977). Thus, even those who thought that the Corps’ existing authority under 404 was too broad recognized (1) that the definition of "navigable waters" then in force for both 301 and 404 was reasonably interpreted to include adjacent wetlands, (2) that the water quality concerns of the Clean Water Act demanded regulation of at least some discharges into wetlands, and (3) that whatever jurisdiction the Corps would retain over discharges of fill material after passage of the 1977 legislation should extend to discharges into wetlands adjacent to any waters over which the Corps retained jurisdiction. These views provide additional support for a conclusion that Congress in 1977 acquiesced in the Corps’ definition of waters as including adjacent wetlands.

Two features actually included in the legislation that Congress enacted in 1977 also support the view that the Act authorizes the Corps to regulate discharges into wetlands. First, in amending 404 to allow federally approved state permit programs to supplant regulation by the Corps of certain discharges of fill material, Congress provided that the States would not be permitted to supersede the Corps’ jurisdiction to regulate discharges into actually navigable waters and waters subject to the ebb and flow of the tide, "including wetlands adjacent thereto." CWA 404(g)(1), 33 U.S.C. 1344(g)(1). Here, then, Congress expressly stated that the term "waters" included adjacent wetlands. [FN11] Second, the *139 1977 Act authorized an appropriation of $6 million for completion by the Department of Interior of a "National Wetlands Inventory" to assist the States "in the development and operation of programs under this Act." CWA 208(i)(2), 33 U.S.C. 1288(i)(2). The enactment of this provision reflects congressional recognition that wetlands are a concern of the Clean Water Act and supports the conclusion that in defining the waters covered by the Act to include wetlands, the Corps is "implementing congressional policy rather than embarking on a frolic of its own." Red Lion Broadcasting Co. v. FCC, 395 U.S. 367, 375 (1969).



We are thus persuaded that the language, policies, and history of the Clean Water Act compel a finding that the Corps has acted reasonably in interpreting the Act to require permits for the discharge of fill material into wetlands adjacent to the "waters of the United States." The regulation in which the Corps has embodied this interpretation by its terms includes the wetlands on respondent’s property within the class of waters that may not be filled without a permit; and, as we have seen, there is no reason to interpret the regulation more narrowly than its terms would indicate. Accordingly, the judgment of the Court of Appeals is


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