GORSUCH, Circuit Judge.

As its name advertises, U.S. Magnesium produces magnesium, though in doing so it also generates various waste byproducts. This lawsuit concerns five of those wastes. The government says that U.S. Magnesium's handling of the wastes must but hasn't complied with regulations promulgated under Subtitle C of the Resource Conservation and Recovery Act of 1976 ("RCRA"). For its part, U.S. Magnesium challenges the premise of the government's suit, arguing that the Environmental Protection Agency ("EPA" or the "Agency") exempted the five wastes from Subtitle C's strictures in a prior interpretation of its own regulation. And, U.S. Magnesium says, the Agency cannot change that interpretation now, at least not without first complying with the notice and comment procedures of the Administrative Procedure Act ("APA"). At summary judgment, the district court agreed with U.S. Magnesium and entered judgment in its favor.

We must vacate that judgment. Even if we assume with U.S. Magnesium that a definitive regulatory interpretation prohibits an agency from later changing course and issuing a new interpretation without first undergoing notice and comment, that's simply not our case. The only prior EPA interpretation U.S. Magnesium can point to is, at best, a tentative one. Because EPA never previously adopted a definitive interpretation, it remained free, even under the legal precedents on which U.S. Magnesium seeks to rely, to change its mind and issue a new interpretation of its own regulations without assuming notice and comment obligations.

I

U.S. Magnesium mines and processes magnesium at its plant in Rowley, Utah, on the western shore of the Great Salt Lake. The Rowley facility, as it's known, extracts primary magnesium using what U.S. Magnesium refers to as the "anhydrous" process. That process — intricate, sophisticated, and well known to the parties — yields a variety of dangerous waste products. To make those waste products less dangerous, the facility uses a number of pollution-control measures. And many of these measures in turn generate other wastes, some of which are dangerous in their own right. This case concerns the interaction between five such "pollution-control wastes" and RCRA. By way of background, we first outline the relevant RCRA regulatory history (Section LA) and the history of this lawsuit (Section LB), before turning to our analysis of the appeal now before us (Section II).

A

In Subtitle C of RCRA, Congress required EPA to promulgate regulations establishing a comprehensive regulatory scheme for the transportation, treatment, and disposal of hazardous wastes. See 42 U.S.C. §§ 6921- 6939f. Meanwhile, Subtitle D of the statute addressed the regulation of nonhazardous solid wastes and authorized EPA to prepare regulations subjecting these wastes to less rigorous requirements. See id. §§ 6941-6949a.

After RCRA's enactment, EPA in 1978 proposed regulations implementing Subtitle C for notice and comment. Under that proposal, wastes related to the processing of ores or minerals generally were to be subject to Subtitle C, rather than Subtitle D. At the same time, mineral processing wastes produced in "very large volumes" but that were believed to pose "relatively low" health risks — a category apparently anticipated to include at least some of the wastes produced at Rowley — were to benefit from less stringent Subtitle C regulations than other hazardous wastes. See 43 Fed.Reg. 58, 946, at 58,991-92 (1978). After proposing its rule and receiving public comment, however, EPA changed its mind on this particular point, and the final Subtitle C regulations the Agency issued in 1980 treated large volume, low risk mineral processing wastes as hazardous wastes subject to the same stringent Subtitle C requirements as other such wastes. See 45 Fed.Reg. 33,154, at 33, 173-75 (1980).

Just before EPA's final Subtitle C regulations were to take effect, however, Congress reentered the picture. Apparently unsatisfied with the Agency's final decision to subject all hazardous mineral processing wastes to more stringent Subtitle C regulations, Congress enacted the so-called Bevill Amendment, named for its principal legislative sponsor. See Pub.L. No. 96-482, 94 Stat. 2334, codified at 42 U.S.C. § 6921(b)(3)(A)(ii). The Bevill Amendment essentially sent EPA back to the drawing board when it came to wastes generated in connection with the processing of ores and minerals. Congress required the Agency to conduct a "comprehensive study on the adverse effects on human health and the environment, if any, of the disposal and utilization of solid waste from the extraction, beneficiation, and processing of ores and minerals," and to produce in three years time a report for Congress's consideration. 42 U.S.C. § 6982(p); see also id. § 6982(f). To fore-stall any interstitial regulation, the Amendment required EPA to postpone the application of Subtitle C regulations to all mineral processing wastes until at least six months after the submission of the congressional report. Id. § 6921(b)(3)(A). In addition, Congress required EPA to determine — or redetermine, on the basis of "information developed or accumulated pursuant to such study, public hearings, and comment" — whether it should regulate ore and mineral processing wastes under Subtitle C or under a less stringent regime, such as Subtitle D. Id. § 6921(b)(3)(C). See generally Envt'l Def. Fund v. EPA, 852 F.2d 1316, 1318-20 (D.C. Cir. 1988).

So it is that EPA set out to determine the proper scope of the so-called Bevill exemption from RCRA Subtitle C. Eventually, the Agency published a proposed rule laying out various criteria to identify which mineral processing wastes should be held exempt from Subtitle C in light of the Bevill Amendment and subject instead to less onerous regulations, the exact details of which the Agency had yet to specify. 53 Fed.Reg. 41,288 (1988). In response to EPA's call for comments, the then-owner of the Rowley facility nominated various of the wastes it produced for exemption under these criteria. Aple. Supp.App. at 11, 15.

After various regulatory investigations and following more notice and comment, EPA issued a new rule in September 1989. 54 Fed.Reg. 36,592 (1989). This rule finalized the criteria a waste must meet to qualify for exemption from Subtitle C. Among other things, the criteria required a candidate waste to be "uniquely associated with mineral industry operations" and produced in "high volumes" and with "low hazard" levels — thus closely tracking and elaborating EPA's initial proposal from back in 1978. See id. at 36,628-31. Applying the criteria it had announced, EPA proceeded to opine that several wastes, including "[p]rocess wastewater from primary magnesium production by the anhydrous process" — the category of wastes at issue in this ease — were likely candidates for exemption, subject to further data collection and study, the required report to Congress, and. a final Agency determination. See id, at 36,631 36,642. See generally Solite Corp. v. EPA, 952 F.2d 473, 480-82 (D.C. Cir. 1991) (per curiam).

In July 1990, EPA submitted its Report to Congress on Special Wastes from Mineral Processing. See 55 Fed.Reg. 32,135 (1990) (announcing availability of report). The Report contained a detailed study of each of the various wastes previously proposed for exemption and recommended the exemption of many, including "[p]rocess wastewater from primary magnesium processing by the anhydrous process," id. at 32,136, though in doing so the Agency repeatedly noted that its "findings" on this score remained "tentative," id. at 32,135. Accordingly, EPA encouraged interested parties to review the Report and offer comments, which the Agency said it would use "in conjunction with the Report . . . to make the final regulatory determination." Id.

After publishing its Report to Congress and considering the public comments it had invited, EPA promulgated a "`[f]inal regulatory determination and final rule" in June 1991. This rule sought to apply the criteria for exemption announced in the Agency's 1989 rule to certain candidate wastes. 56 Fed.Reg. 27,300, at 27,300 (1991). In so doing, EPA confirmed that "[p]rocess wastewater from primary magnesium processing by the anhydrous process," among many other candidate wastes, now definitively qualified for exemption from Subtitle C and should be subject to less onerous regulatory terms, mostly under Subtitle D. Id. at 27,307. In this final rule, however, EPA did not purport to interpret the phrase "[p]rocess wastewater from primary magnesium processing by the anhydrous process." Id. at 27,306-07.

Beginning in 1991, EPA, the operators of the Rowley facility, and the State of Utah engaged in a series of discussions, wrote letters, and debated what this phrase encompasses and what it does not. Ultimately, the parties reached logger-heads. U.S. Magnesium took the view that the 1991 final rule exempted from Subtitle C all of the Rowley facility's pollution-control wastes. The Agency disagreed, arguing that its rule only exempted some such wastes and that others remained subject to Subtitle C disposal requirements. This lawsuit followed in 2001.

B

In its 2001 complaint, the government sought injunctive relief and civil penalties for various alleged violations of RCRA and its implementing regulations. In 2005, the government filed an additional, related complaint — eventually consolidated with its RCRA complaint — alleging violations of a different statute, the Toxic Substances Control Act ("TSCA"). See U.S.App. Vol. I at 145-46. But the heart of the lawsuit always was and remains the status of five wastes, appropriately dubbed "the. Complaint wastes." Before the district court, the government argued that these wastes don't qualify for exemption from Subtitle C because they're not "[p]rocess wastewater from primary magnesium processing by the anhydrous process," as required by EPA's 1991 rule. See Second Am. Compl. ¶ 62, U.S.App. Vol. I at 109. This argument splits into two parts: according to EPA, most of the Complaint wastes aren't — in the words of the rule — "from primary magnesium processing," while the final remaining Complaint waste doesn't qualify as a "wastewater." Some explanation is in order. The company's magnesium production process generates large amounts of chlorine gas, which the company then processes to produce hydrochloric acid and chlorine for its own use and for sale. And the company's hydrochloric acid and chlorine production processes themselves yield various wastes. The government argued to the district court that these wastes — four of the five Complaint wastes — don't qualify for exemption because they're not "process wastewater from primary magnesium processing." 56 Fed.Reg. at 27,307 (emphasis added). Rather, they're process wastewaters from the processing of something else. See, e.g., United States' Combined Summary Judgment Memorandum at 41 ("The wastewaters that are the subject of the Complaint are wastewaters from the processing of chlorine and hydrogen chloride gasses [sic], not from the processing of the mineral magnesium."). Likewise, EPA took the position that, even if the fifth Complaint waste — dry anode dust — is the direct result of magnesium processing, it still isn't a " wastewater" within the meaning of the 1991 rule, but rather a non-exempt waste solid. See id. at 38. Thus, to the Agency's view, none of the five Complaint wastes fits under the umbrella of wastes exempted from regulation under Subtitle C.

In reply, U.S. Magnesium rested heavily on EPA's 1990 Report to Congress. In the company's view, certain language and a diagram in the Report suggested strongly that EPA had, at least then, interpreted the phrase "[p]rocess wastewater from primary magnesium processing by the anhydrous process" to mean that all pollution-control wastes produced at the Rowley facility, including each of the five Complaint wastes, were exempt from regulation under Subtitle C. In the company's view, EPA's lawsuit now sought to enforce a different, narrower interpretation of the term "[p]rocess wastewater from primary magnesium processing by anhydrous process." And under principles of administrative law, U.S. Magnesium submitted, this EPA could not do. U.S. Magnesium submitted that an agency may not interpret its own regulations in a way that conflicts with its own prior interpretation — at least not without first engaging in notice and comment, a process EPA admittedly had not undertaken.

Following extensive proceedings over several years that culminated in briefing at summary judgment, the district court eventually agreed with U.S. Magnesium. The court seemed to consider EPA's current interpretation of its 1991 rule — that the five Complaint wastes are not "[p]rocess wastewater from primary magnesium processing by the anhydrous process" — to be "plausible" as an initial matter. See D. Ct. Op. at 25 (holding that "each of the parties' interpretations of the exemption for primary magnesium processing wastewaters," the language used in the 1991 final rule, "[is] plausible"). But, the court held, the Agency's current interpretation was different from the interpretation it previously adopted In its 1990 Report to Congress and elsewhere. And, the court seemed to believe, EPA could not now change its interpretation of its own 1991 rule without first providing an opportunity for notice and comment. See Answer Br. at 29 (U.S. Magnesium characterizing the district court opinion as having "rejected the government's current interpretation because it was inconsistent with the original interpretation"). Given all this, the district court held that partial summary judgment for U.S. Magnesium on the Complaint waste claims was required as a matter of law.

Of course, a partial summary judgment does not a final judgment make, and the jurisdiction of the federal circuit courts ordinarily extends only to the final judgments of the federal district courts. See, e.g., Van Cauwenberghe v. Biard, 486 U.S. 517, 521, 108 S.Ct. 1945, 100 L.Ed.2d 517 (1988) (citing 28 U.S.C. § 1291). As a result, and to manufacture a final judgment out of the district court's partial summary judgment for U.S. Magnesium, the parties stipulated to the dismissal with prejudice of "[a]ll remaining claims pending in this action." Final Judgment, U.S.App. Vol. I at 89. The district court granted the motion, thus making the "summary judgment ruling . . . a final and appealable judgment." Id. It is this judgment EPA now appeals to us.

II

We review appeals from a district court's decision to grant summary judgment de novo, and will affirm only if, viewing the facts in the light most favorable to the non-movant, we discern no genuine dispute of material fact in need of resolution by a factfinder and conclude that the movant is entitled to judgment as a matter of law. Fed.R.Civ.P. 56(c). As it happens, this case presents a pure question about the legal consequences of undisputed facts. The precise contours of that question, though, require some explanation. That is where we begin (Section II.A) before turning to the parties' arguments about that question (Section II.B) and finally our disposition of it (Section II.C).

A

In identifying what is in dispute in this case, it is important to start by delineating what isn't. While it appears much may have once been disputed in the district court, see supra notes 5 6, the case as it has been briefed to us has narrowed considerably. Before us, the parties don't dispute that EPA's 1991 final rule is "ambiguous." See Answer Br. at 34. As U.S. Magnesium argues and EPA doesn't contest, the 1991 rule — exempting from regulation under Subtitle C "process wastewater from primary magnesium processing by the anhydrous process" — is anything but self-defining. See id. Next, the parties also agree that the Agency's current interpretation of that language — an interpretation that excludes all of the five Complaint wastes — is, as the district court noted, a "plausible" one. See id. at 32. Finally, the parties all acknowledge that, usually at least, an agency's interpretation of its own ambiguous regulation is entitled to deference from the courts under Auer v. Robbins, 519 U.S. 452, 461, 117 S.Ct. 905, 137 L.Ed.2d 79 (1997). See Answer Br. at 30 ("Nor do the parties dispute that where a regulation is ambiguous, an agency's interpretation of its own regulation must be given substantial deference.").

With all this agreed, or at least not disputed, we are in the end asked to decide just one question of law: whether EPA is precluded from pursuing its current and concededly plausible interpretation of its ambiguous 1991 regulation, under which the five Complaint wastes do not qualify as "process wastewater from primary magnesium processing by the anhydrous process," because the Agency previously — especially in its 1990 Report to Congress — offered a different and inconsistent interpretation of that language. As U.S. Magnesium puts it, "[t]he sole issue presented in this appeal is whether EPA is bound by its original, contemporaneous interpretation of an ambiguous regulation". Stated otherwise, can EPA change its original interpretation of the regulation without following the [notice and comment] procedural requirements of the Administrative Procedure[] Act ("APA")"? Answer Br. at 2.

U.S. Magnesium's brief sometimes hints that EPA's current interpretation of the language in its 1991 final rule might be challenged as arbitrary and capricious or otherwise unlawful in its own right, apart from any purported inconsistency with EPA's own prior interpretation of the rule's terms. See, e.g., Answer Br. at 55-60; see also Opening Br. at 27-31, 37-42 (defending reasonableness of Agency's current view). But the company doesn't develop any such argument before us. So, for example, U.S. Magnesium's appellate brief never cites § 706 of the APA or any other legal authority necessary to advance an argument of that sort. Instead, each time the company might be perceived as raising some other complaint, it recurs to the theme that the real problem is the inconsistency between the Agency's current interpretation of the language in its 1991 rule and the prior interpretation EPA gave that same language, especially in its Report to Congress. See, e.g., Answer Br. at 2 (telling us this inconsistency is the "sole issue" in this case); id. at 30 ("The dispute centers on whether any deference must be given to an agency's later reinterpretation of a regulation that substantially changes its original interpretation."); id. at 33 ("[T]he only remaining issue is whether EPA's contemporaneous interpretation of the Bevill Amendment exemptions is inconsistent with EPA's current interpretation."). And all of the case law the company cites to us concerns only whether and under what circumstances an agency may amend, or abandon its interpretation of a previously adopted substantive (or legislative) rule. See Answer Br. at 30-33.

B

Having isolated the sole issue presented for our decision, what do the parties have to say about it?

According to U.S. Magnesium, EPA first interpreted the ambiguous language of its 1991 rule in its 1990 Report to Congress. Of course, that leads one to wonder how an agency might in 1990 interpret a rule that didn't come into existence until 1991. But to this the company has a ready reply. The 1991 regulation used the same ambiguous language as the 1989 rule: "[p]rocess wastewater from primary magnesium processing by the anhydrous process." 56 Fed.Reg. at 27,307. And the 1990 Report to Congress interpreted this language. So, as a practical matter, U.S. Magnesium says, EPA's Report to Congress discussing this language from the 1989 rule also amounts to an interpretation of the 1991 rule. See U.S. Magnesium's Memorandum in Support of Partial Summary Judgment at 48. And in the 1990 Report, the company adds, EPA effectively took the view that all of the Rowley facility's pollution-control wastes, including the five Complaint wastes, were exempt from the strictures of Subtitle C. Having espoused this interpretation of its own regulation, U.S. Magnesium argues, the Agency must abide it unless and until it adopts a new interpretation through notice and comment rulemaking. As the company sees it, EPA's current enforcement action represents the Agency's impermissible attempt to short-circuit that required procedure.

In response, the Agency doesn't seem to dispute that its 1990 Report effectively interpreted its later 1991 rule by offering a view about the meaning of the phrase "process wastewater from primary magnesium processing by the anhydrous process." So, for purposes of this appeal, we will assume without deciding that to be the case. Instead, EPA focuses its fire on the argument that the initial interpretation it offered in the 1990 Report to Congress was a tentative one, and an agency, EPA says, need not undertake the rigors of notice and comment to change a merely tentative interpretation of its own rules.

C

On this score, we must agree with EPA. Even assuming the rule of administrative law that U.S. Magnesium urges us to adopt — that an agency may not interpret a substantive (or legislative) regulation one way and then later adopt a competing interpretation without undergoing notice and comment rulemaking — the initial interpretation is only binding if it is definitive. And, as we will explain, nothing in EPA's Report to Congress, or in its other communications or actions, qualifies as that. So, even if we accept the company's premise that EPA previously adopted an interpretation of its 1991 final rule, and that EPA now seeks to modify that interpretation, U.S. Magnesium's argument still suffers a fundamental flaw. It is on this flaw that we focus our attention.

1

In support of its claim that an agency may not abandon a prior interpretation of its own ambiguous regulation without first going through notice and comment, U.S. Magnesium directs our attention to certain cases from the D.C. Circuit, beginning with Alaska Professional Hunters Ass'n v. FAA, 177 F.3d 1030 (D.C. Cir. 1999). In that case, plaintiffs who worked as fishing and hunting guides in Alaska challenged a Federal Aviation Administration ("FAA") notice that required the guides to comply with commercial airline regulations. The notice broke dramatically with the practice of the FAA's Alaskan Region, which for decades had advised guides that they were exempt from restrictions on commercial pilots. The court of appeals declared the new notice invalid, holding that the agency couldn't "significantly revise[]" its previous "definitive interpretation" of its own regulations without first engaging in "notice and comment." Id. at 1034.

In reaching that holding, Alaska Hunters relied almost exclusively on dicta from an earlier D.C. Circuit case suggesting that "[o]nce an agency gives its regulation an interpretation, it can only change that interpretation as it would formally modify the regulation itself: through the process of notice and comment rulemaking." Paralyzed Veterans of America v. D.C. Arena L.P., 117 F.3d 579, 586 (D.C. Cir. 1997). And to reach that conclusion, Paralyzed Veterans in turn started with the well-known premise that the APA generally requires an agency to allow for notice and comment before it issues any new rules. See 5 U.S.C. § 553. Of course, § 553 specifically exempts interpretive rules from its notice and comment requirements, see 5 U.S.C. § 553(b)(A), but this exemption, Paralyzed Veterans suggested, doesn't apply when an agency amends a previous interpretative rule. This is so, the court said, because a different provision of the APA defines the term "rule making" to include the "agency process for . . . amending . . . a rule," id. § 551(5), so, ipso facto, amending a rule requires notice and comment. See Paralyzed Veterans, 117 F.3d at 586. The implicit reasoning appears to be this: if an agency amends its interpretation of a rule, it is effectively "amending . . . [the] rule" itself, 5 U.S.C. § 551(5), and the APA by its own terms defines this amendment as a kind of rulemaking, something the agency may not accomplish without notice and comment procedures. "To allow an agency to make a fundamental change in its interpretation of a substantive regulation without notice and comment," the court explained, would "undermine" the notice and comment provisions of the APA. Paralyzed Veterans, 117 F.3d at 586.

Paralyzed Veterans and Alaska Hunters have generated considerable debate. As the government points out, the issue whether an agency may alter its interpretation of its own regulation without notice and comment is the subject of a circuit split, with the Third, Fifth, and Sixth Circuits apparently adopting the D.C. Circuit's view and the First and Ninth Circuits seemingly taking the contrary position. Our circuit, as far as we can tell, hasn't yet had an opportunity to weigh in.

Commentators, however, have joined the fray. As a matter of statutory construction, some critics suggest, the text of the APA can't bear the weight foisted upon it by Alaska Hunters. The D.C. Circuit relied primarily on APA § 551(5) for its holding, but that section, these scholars observe, merely offers a definition of rulemaking. For a prescription of how to conduct rulemaking, we must look instead at § 553, which makes perfectly clear that the notice and comment procedures required for substantive (or legislative) rules just don't apply to "interpretative rules." 5 U.S.C. § 553(b)(A) ("Except when notice or hearing is required by statute, this subsection [requiring notice and comment] does not apply . . . to interpretative rules, general statements of policy, or rules of agency organization, procedure, or practice."). And so it doesn't matter whether an interpretive rule is the first or second or seventeenth in a series: on this view, none has to undergo notice and comment before taking effect. The Alaska Hunters regime misses this point, the argument goes, and in so doing flouts the APA's clear distinction between interpretive and substantive rules. See, e.g., Richard J. Pierce, Jr., Distinguishing Legislative Rules from Interpretive Rules, 52 Admin. L. Rev. 547, 567 (2000); Jon Connolly, Note, Alaska Hunters and the D.C. Circuit: A Defense of Flexible Interpretive Rulemaking, 101 Colum. L. Rev. 155,172-73 (2001).

Other scholars take a different view. One possible way to defend the Alaska Hunters doctrine, they suggest, may be that an interpretation of a substantive (or legislative) regulation essentially becomes part of that regulation itself. On this logic, a superseding interpretation would necessarily amend the substantive regulation and thus require notice and comment. See Richard W. Murphy, Hunters for Administrative Common Law, 58 Admin. L. Rev. 917, 923 (2006).

2

Though U.S. Magnesium spends considerable energy encouraging us to join the circuits that have adopted Alaska Hunters, and invites us to conclude that those circuits have the better view of administrative law, we have no need to wade into such deep waters to decide the appeal before us. This is because, even if we assume without deciding that Alaska Hunters's reading of the APA is the correct one (as U.S. Magnesium argues), the company still cannot prevail. By its terms, the Alaska Hunters doctrine applies only to definitive regulatory interpretations; even under Alaska Hunters, an agency remains free to disavow and amend a tentative interpretation of one of its rules without notice and comment. And the only prior interpretation U.S. Magnesium has identified in this case can fairly be characterized as no more than that — tentative.

Under the Alaska Hunters line of cases on which U.S. Magnesium relies, an agency commits itself to a particular interpretation of its own regulation only when it adopts that interpretation definitively, and "conditional or qualified statements, including statements that something `may be' permitted, do not establish definitive and authoritative interpretations." Met-West Inc. v. Sec'y of Labor, 560 F.3d 506, 509-10 (D.C. Cir. 2009); see also Devon Energy Corp. v. Kempthorne, 551 F.3d 1030, 1041 (D.C. Cir. 2008) (holding that "guidance documents [that] were far from conclusive in what they said" were insufficient to bind agency's future interpretation of regulation); Darrell Andrews Trucking, Inc. v. Fed. Motor Carrier Safety Admin., 296 F.3d 1120, 1126 (D.C. Cir. 2002) (holding that agency's "ambiguous" regulatory guidance did not "mark a definitive interpretation" and thus did not constrain its subsequent discretion); Ass'n of Am. R.R. v. Dep't of Transp., 198 F.3d 944, 948-50 (D.C. Cir. 1999) (holding agency "never adopted a definitive interpretation of [the regulation] that it could change only through notice and comment rulemaking" because "none of th[e] documents [cited by the regulated party] even comes close to the express, direct, and uniform interpretation" necessary to constrain agency's interpretive discretion); cf. Thomas Jefferson Univ. v. Shalala, 512 U.S. 504, 516, 114 S.Ct. 2881, 129 L.Ed.2d 405 (1994) (rejecting university's argument that agency had adopted, inconsistent interpretive positions because agency's initial "intermediary letter did not purport to be a comprehensive review of all conditions" that the ambiguous regulation might impose). Even under the Alaska Hunters doctrine, then, before an agency adopts a definitive interpretation of its own rule it remains free to hear new arguments, make adjustments, and change directions, all without having to undergo notice and comment.

The prior interpretation on which U.S. Magnesium most prominently hangs its hat is the Report to Congress. The company argues strenuously and at length that EPA's current interpretation of its ambiguous 1991 final rule is inconsistent with the interpretation the Agency previously offered in its 1990 Report. And in that Report, the company says, "EPA interpreted its regulation . . . as covering the aggregated wastestreams from all of the [Rowley facility's] pollution-control operations." Answer Br. at 24 (emphasis added).

But even assuming (again without deciding) that the Report did suggest that all of the Rowley facility's pollution-control wastes qualify as "process wastewater from primary magnesium processing by the anhydrous process," and so are exempt from Subtitle C, U.S. Magnesium offers us no reason to think that this constituted anything more than a tentative view. To start, the Report itself repeatedly describes its conclusions as "tentative." And when it issued the Report, EPA expressly solicited public comments, which it said it planned to use "in conjunction with the Report to Congress to make the final regulatory determination[s]" that it would enshrine in what was, ultimately, the final 1991 regulation. See 55 Fed.Reg. at 32,135. More process was thus expressly contemplated and actually undertaken. It may be that an agency can offer a definitive interpretation of its own regulations in a variety of forms, perhaps even in a congressional report. And no doubt a report to Congress is a solemn thing. But U.S. Magnesium hasn't given us any reason to conclude that the Report at issue in this case was anything other than tentative.

To this, U.S. Magnesium replies that there's more than just the Report to Congress to suggest that EPA previously adopted an interpretation of its 1991 rule that is at odds with its current interpretation. The company also points to certain of EPA's actions and communications. By way of example, U.S. Magnesium highlights the fact that the Agency, in June 1989, tested an aggregated wastestream at the Rowley facility, rather than individual wastes from the facility's different pollution-control devices. This action, the company says, confirms that in its 1990 Report to Congress EPA intended to treat all of the facility's pollution-control wastes as a unified whole for purposes of any exemption from Subtitle C, rather than distinguish between different wastes as it seeks to do now. See Answer Br. at 37-39.

This allegation has sparked an extended dispute between the parties about what exactly transpired during the site visit. EPA's site visit director, Robert Hall, has offered a declaration explaining that his sampling team wanted to test "numerous wastestreams discharging into ditches from the ends of unmarked pipes," but that their access was "impeded" by steep embankments and piles of refuse and debris. Declaration of Robert Hall, U.S.App. Vol. III at 581. U.S. Magnesium, for its part, challenges Mr. Hall's account, arguing EPA could have safely tested individual wastestreams if it had really wished to do so.

But all this tussling over the facts overlooks the salient legal point. Even if the 1989 site visit evinced EPA's contemporaneous intention to aggregate all of the Rowley facility's pollution-control wastes for purposes of an exemption from Subtitle C, it still doesn't tell us whether EPA's commitment to that view of its regulations in its 1990 Report was definitive: We've operated in this opinion on the assumption that the 1990 Report itself demonstrated an intent to treat all of the Rowley facility's pollution-control wastes as a unified whole for purposes of the Subtitle C analysis; at best, the 1989 site visit seems to tell us no more than what we've already assumed. However viewed, the site visit simply offers no insight into whether the 1990 Report was itself a definitive or tentative interpretation.

Beyond the site visit, U.S. Magnesium directs us to certain correspondence as evidence of EPA's alleged prior inconsistent interpretation. And the company also points to the fact that, under EPA's current view disfavoring the aggregation of individual wastestreams, none of the waste from the Rowley facility would meet the high volume criteria set forth in the final 1989 rule for exemption from regulation under Subtitle C. See Answer Br. at 51-55; see also 54 Fed.Reg. at 36,629-31. These arguments once again lead the parties into thickly factual disputes. EPA responds, for example, that U.S. Magnesium has misread the parties' correspondence. It argues that any previous Agency error concerning the volume of the various pollution-control wastes was the result of bad information provided by the facility's then-owner. And it adds that any inaccuracy in its previous measurement of the wastes produced at Rowley doesn't make a difference anyway because it isn't seeking to hold U.S. Magnesium to Subtitle C's terms for any wastes other than the five Complaint wastes. But once again, none of this matters to the resolution of this appeal. What does matter is that none of these arguments — no more than those analyzed above — establishes that EPA in its Report to Congress, or elsewhere, adopted a definitive, rather than a tentative, interpretation that the five Complaint wastes were "process wastewater from primary magnesium processing by the anhydrous method."

3

Having held that EPA was in this case free under Alaska Hunters doctrine to change its interpretation of its 1991 rule without undertaking notice and comment, one might worry that administrative law has simply abandoned regulated parties to the whims of an agency's arbitrary interpretive reversals. What about the reasonable and settled expectations of the regulated public? As it happens, however, there is no reason for undue alarm; at least two other layers of protection exist, even without the added aegis of Alaska Hunters.

First, the APA itself empowers courts to review "agency action, findings, and conclusions" if they are "arbitrary and capricious, an abuse of direction, or otherwise not in accordance with law." 5 U.S.C. § 706(2)(A). This requirement means, among other things, that an agency "must cogently explain why it has exercised its discretion in a given manner." Motor Vehicle Mfrs. Ass'n of the U.S. v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 48, 103 S.Ct. 2856, 77 L.Ed.2d 443 (1983); see also Fabi Constr. Co. v. Sec'y of Labor, 508 F.3d 1077, 1089 (D.C. Cir. 2007). It is hard to see how this obligation could be any less salient when an agency seeks to abandon a prior interpretation in favor of a new one. See Manning, supra, at 936.

Second, even if Congress repealed the APA tomorrow, the Due Process Clauses of the Fifth and Fourteenth Amendments would still prohibit the imposition of penalties without fair notice. See U.S. Const. amend. V; id. amend. XIV, § 1. Due process, after all, requires at the least that "laws give the person of ordinary intelligence a reasonable opportunity to know what is prohibited." Grayned v. City of Rockford, 408 U.S. 104, 108, 92 S.Ct. 2294, 33 L.Ed.2d 222 (1972). This principle applies to civil as well as criminal penalties, albeit in slightly different form. See Village of Hoffman Estates v. Flipside, Hoffman Estates, Inc., 455 U.S. 489, 498-99, 102 S.Ct. 1186, 71 L.Ed.2d 362 (1982). And it pertains when an agency advances a novel interpretation of its own regulation in the course of a civil enforcement action. See Walker Stone Co. v. Sec'y of Labor, 156 F.3d 1076, 1083-84 (10th Cir. 1998) ("In order to satisfy constitutional due process requirements, regulations must be sufficiently specific to give regulated parties adequate notice of the conduct they require or prohibit." (quoting Freeman United Coal Mining Co. v. FMSHRC, 108 F.3d 358, 362 (D.C. Cir. 1997))); see also General Elec. Co. v. EPA 53 F.3d 1324, 1328-34 (D.C. Cir. 1995); cf. Long Island Care at Home, Ltd. v. Coke, 551 U.S. 158, 170-71, 127 S.Ct. 2339, 168 L.Ed.2d 54 (2007) ("[I]nterpretive changes [to regulations must] create no unfair surprise."). If an agency could punish a regulated party for following the agency's own interpretation of its own ambiguous regulations, after all, "the practice of administrative law would come to resemble `Russian Roulette.'" Satellite Broad. Co. v. FCC, 824 F.2d 1, 4 (D.C. Cir. 1987).

One might wonder about the potential application of these doctrines to U.S. Magnesium's cause. Interesting as these questions may be, though, they are not ones that we have to or may answer in this case. At no point in the proceedings before this court has the company raised any argument or sought decision of any issue arising under either § 706(2)(A) of the APA or the Due Process Clause. Accordingly, these arguments are waived. See Rollins Envtl. Servs. (NJ) Inc. v. EPA 937 F.2d 649, 652 n. 2 (D.C. Cir. 1991) (finding waiver of due process notice argument in agency enforcement action involving novel interpretation of ambiguous regulation); see also Montes v. Vail Clinic, Inc., 497 F.3d 1160, 1172 n. 17 (10th Cir. 2007).

* * *

For purposes of summary, we hold that EPA hasn't previously adopted a definitive interpretation of its 1991 rule. Even under the case law U.S. Magnesium asks us to follow, the Agency is at liberty to adopt without notice and comment a reasonable interpretation of that ambiguous regulation. At least before us, U.S. Magnesium does not dispute that EPA has done so with this litigation. For this reason, we vacate the entry of summary judgment in U.S. Magnesium's favor and remand this matter to the district court. We do not prejudge what, if any, further proceedings may be appropriate in that court in light of and consistent with this opinion.