PATRICK E. HIGGINBOTHAM, Circuit Judge:

The Clean Water Act establishes a statutory scheme to protect and improve the quality of the country's waters. The administration of the Act depends on complicated interactions of three actors: the states, with lead responsibility for protecting waters within their borders; the EPA, which steps in when the state-led efforts are inadequate; and the federal courts, which enforce Congressional mandates against state and federal regulators.

Not every state or EPA action taken under the Act is judicially cognizable; some are committed to agency discretion and are unreviewable. Under the statute, the EPA Administrator is obligated to issue new water quality standards in any case where she "determines that a revised or new standard is necessary to meet the requirements of" the Act. Here, the Administrator denied a petition for rulemaking, declining to make a so-called "necessity determination." The petitioners challenged this decision in federal court. The EPA countered that the denial was an unreviewable discretionary act.

This case poses two questions. First, do we have subject matter jurisdiction to review the EPA's decision not to make a necessity determination. We hold that we do. Second, was the EPA required to make such a determination. We hold that it was not.

I.

A.

Congress passed the Clean Water Act1 "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters."2 The Act bans "the discharge of any pollutant by any person," unless affirmatively allowed by law.3 In regulating discharge, the Act "anticipates a partnership between the States and the Federal Government,"4 with both sovereigns sharing regulatory responsibilities for water protection.5

One area where both states and the federal government play a role is in the setting and administration of water quality standards. These regulations "define[] the water quality goals of a water body... by designating the use or uses to be made of the water and by setting criteria necessary to protect the uses."6 The states are the primary player in this process; they are "responsible for reviewing, establishing, and revising water quality standards."7 The federal government plays a secondary role, with important backstop responsibilities. State standards must be submitted to the EPA, the agency tasked with reviewing and approving these standards, to ensure that they are sufficient to "protect the public health or welfare, enhance the quality of water and serve the purposes of this [Act]."8 If the state's standards do not pass muster, the EPA specifies changes required for approval.9

The EPA may also directly set water quality standards through its own regulations under the two circumstances set out in 33 U.S.C. § 1313(c)(4)(A) and (B) ("section 1313(c)(4)").

In other words, in order to regulate pursuant to its section 1313(c)(4)(B) powers, the EPA must make what is called a "necessity determination." If the agency sets water quality standards, it acts through a rulemaking process, and "is subject to the same policies, procedures, analyses, and public participation requirements established for States in these regulations."11

B.

This case began when a group of environmental organizations petitioned the EPA12 to "use its powers [pursuant to section 1313(c)(4)(B)] to control nitrogen and phosphorous pollution" within the Mississippi River Basin and the Northern Gulf of Mexico.

The EPA declined to do so. While the agency agreed that nitrogen and phosphorous pollution "is a significant water quality problem," it did "not believe that the comprehensive use of federal rulemaking authority is the most effective or practical means of addressing these concerns at this time." Instead, the EPA said that, because its "long-standing policy, consistent with the CWA, has been that states should develop and adopt standards in the first instance," and in light of the fact that the states had been "quite active" in addressing water pollution issues, it was appropriate to let the states take the primary role in issuing new standards. In denying the petition, the EPA was explicit that it was "not determining that [new standards] are not necessary to meet CWA requirements," but rather was "exercising its discretion to allocate its resources in a manner that supports targeted regional and state activities to accomplish our mutual goals of reducing [nitrogen and phosphorous] pollution and accelerating the development and adoption of state approaches to controlling [nitrogen and phosphorous]."

The petitioners filed suit, positing that the EPA had violated the Administrative Procedure Act13 and the CWA by declining to make a necessity determination. The EPA moved to dismiss the case on subject matter jurisdiction grounds, arguing that the decision whether to make a necessity determination was a discretionary act that the court lacked authority to review. The parties also cross-moved for summary judgment on the merits.

The district court ruled that it had jurisdiction to review the EPA's decision not to make a necessity determination.14 It then went one step further. Pursuant to the Supreme Court's decision in Massachusetts v. EPA,15 it held that the "EPA could not simply decline to make a necessity determination in response to ... [the] petition for rulemaking."16 It remanded the case to the agency with orders to conduct a necessity determination.17 In doing so, the district court declined to issue specific guidance on "the types of factors that EPA can or cannot consider when actually making the necessity determination."18

This timely appeal followed.

II.

We review de novo the district court's legal conclusions about its subject matter jurisdiction.19

A.

We begin with the elementary principle that "the United States, as sovereign, is immune from suit save as it consents to be sued."20 The petitioners have the burden of proving that Congress has consented to suit by affirmatively waiving sovereign immunity in the specific context at issue.21 In the Administrative Procedure Act, the statute governing federal agency operations generally, Congress provided a general waiver of sovereign immunity for "[a] person suffering legal wrong because of agency action, or adversely affected or aggrieved by agency action within the meaning of a relevant statute."22 In light of this language, federal courts must apply a general presumption that they have jurisdiction to review final agency actions.23 But this waiver is not absolute, and Congress has provided that the APA — and its concomitant grant of judicial review — does not apply in two circumstances: first, if the "statute[] preclude[s] judicial review," an exception not at issue in this case; and second, if "agency action is committed to agency discretion by law."24

In a quartet of cases, the Supreme Court provided two principles that guide our discretion analysis. The first is that the agency discretion clause "is a very narrow exception" to the principle of judicial review of administrative action.25 It applies only "in those rare instances where statutes are drawn in such broad terms that in a given case there is no law to apply."26 These are situations where "the statute is drawn so that a court would have no meaningful standard against which to judge the agency's exercise of discretion. In such a case, the statute (`law') can be taken to have `committed' the decisionmaking to the agency's judgment absolutely."27

In determining whether Congress has provided a "meaningful standard," the court conducts a "careful examination of the statute on which the claim of agency illegality is based."28 We look first to the statutory text, paying particular attention to the words Congress has chosen. For example, in Webster v. Doe, reviewing a statute that allowed the Central Intelligence Agency Director to terminate an employee, the Supreme Court highlighted the fact that the statute was drawn so that the Director could fire the employee whenever he "`shall deem such termination necessary or advisable in the interests of the United States,' not simply when the dismissal is necessary or advisable to those interests."29 This word choice, the Court concluded, "fairly exudes deference to the Director, and appears to us to foreclose the application of any meaningful judicial standard of review."30 The reviewing court must also look at the structure and purpose of the statute.31 Turning again to Webster, there, the Court found dispositive the fact that the CIA's "efficacy, and the Nation's security, depend in large measure on the reliability and trustworthiness of the Agency's employees."32 Judicial review of the termination decision, the Court implicitly concluded, would hinder the agency's effectiveness.

The second agency discretion principle is that different substantive types of agency decisions are subject to different presumptions of reviewability. In general, agency decisions to affirmatively do something are presumptively reviewable.33 The reviewability of agency decisions not to do something depends on the type of activity at issue. For "[r]efusals to take enforcement steps ... the presumption is that judicial review is not available."34 While Congress can trump this presumption, it must be explicit in doing so.35 In contrast, an agency's denial of a petition for rulemaking is "susceptible to judicial review" though, as a substantive matter, "such review is `extremely limited' and `highly deferential.'"36

We pause to resolve one doctrinal uncertainty: whether a denial of a rulemaking petition is categorically reviewable, or whether it is merely presumptively reviewable? The petitioners urge us to adopt the former construction. We cannot. While the Supreme Court's language in Massachusetts v. EPA could support such a holding,37 we conclude that the better reading is that these denials are presumptively reviewable, subject to Congressional language clearly to the contrary, a reading faithful to Webster's exhortation that we determine reviewability only after a "careful examination of the statute."38 It would accord with readings of Massachusetts v. EPA by some of our sister circuits,39 and our own court's long-standing conclusion that there is a "strong presumption," subject to Congressional language, that "action taken by a federal agency is reviewable in federal court."40 By "strong" we mean that this presumption is not easily overcome. Nonetheless, textual limits on agency action remain a prerequisite to our jurisdiction.

B.

Our inquiry proceeds in two steps: First, we determine whether the agency action is akin to a denial of a rulemaking petition or whether it is properly termed a refusal to engage in enforcement actions. If it is the former, we employ the presumption of reviewability, if it is the latter, the presumption is nonreviewability. Second, we look to the statutory provision at issue to see whether Congress has spoken sufficiently clearly as to override the appropriate presumption.

1.

We begin by determining whether the EPA's denial of the plaintiffs' request for the adoption of water quality standards is properly classified as a denial of a rulemaking petition or is better termed a refusal to engage in enforcement activities. While we recognize that the line between enforcement and rulemaking is not always clear,41 we conclude that the EPA's action was akin to a denial of a rulemaking petition and is presumptively reviewable.

In classifying a petition, we look not to the title of the plaintiffs' filing but to the substance of their request.42 In their petition, the plaintiffs proposed that:

On their face, the wide scope of these requests, which would require the adoption of water quality standards across many different states, resembles the type of "broadly applicable ... policy" that is generally considered a hallmark of rulemaking.43 The standards, if adopted, would also "grant rights, impose obligations, or produce other significant effects on private interests," and would "effect a change in existing law or policy," both of which are considered essential features of substantive rules.44 Moreover, the mechanism by which the EPA would implement the new water quality standards would be by "prepar[ing] and publish[ing] proposed regulations"45 pursuant to "the same policies, procedures, analyses, and public participation requirements" that bind the states when they issue their own standards.46 This implementation process sounds in rulemaking, not enforcement.

In arguing that the denial of the water quality standards petition is an unreviewable nonenforcement decision, the EPA relies heavily on our decision in Public Citizen, Inc. v. United States Environmental Protection Agency.47 There, the petitioner challenged the EPA's decisions not to issue a Notice of Deficiency48 to the state of Texas for failing to comply with certain regulatory requirements set out in Title V of the Clean Air Act.49 We concluded that the decision not to issue a NOD was essentially a "decision not to invoke an enforcement mechanism," and was presumptively unreviewable.50 The language of the statute, which stated that the EPA must "issue an NOD when it determines a program is being inadequately administered," was not sufficiently specific to constrain EPA's discretion and overcome the presumption against judicial review.51

The EPA argues that Public Citizen controls. We disagree. Given the factual differences between the NOD process under the CAA and the necessity determination mechanism under the CWA, our earlier decision is inapposite. First, a NOD determination is explicitly premised on the determination by the EPA that the state in question is not "adequately administering and enforcing" its Title V permitting program.52 Agency action, then, depends on a conclusion that the state is failing to meet its statutory requirements, a finding that fits comfortably within the ambit of an enforcement action.53 By contrast, section 1313(c)(4)(B) of the CWA requires the EPA to issue new water quality standards "in any case where the Administrator determines that a revised or new standard is necessary to meet the requirements of this chapter."54 Under a plain reading of this provision, the state need not do anything wrong for the EPA to take action. Further buttressing that conclusion is that the immediately preceding clause, section 1313(c)(4)(A), requires the EPA to issue a new standard "if a ... water quality standard submitted by such State ... for such waters is determined by the Administrator not to be consistent with the applicable requirements of this chapter."55 Here, the EPA must determine that the state's standards do not meet the federal requirements. An action to correct that inadequacy could be termed an enforcement mechanism. But the two sections are set off by the disjunctive "or," which suggests that section 1313(c)(4)(B) does not require a finding of inadequacy, a feature more in line with rulemaking.

Second, the consequences of noncompliance with the EPA's actions differ between these regulatory processes. After issuing a NOD, the EPA "is authorized to sanction the state if the deficiencies are not corrected within eighteen months.... Possible sanctions include the loss of federal highway funds and the application of strict emissions offset requirements for new sources in certain areas within the state."56 These sanctions are essentially punitive in nature, a marking of enforcement. By contrast, the CWA authorizes no financial consequences for noncompliance.

Finally, the procedures by which the agency actions occur are different. With the CAA, after making a NOD determination, the agency must "provide notice to the State" before imposing sanctions,57 akin to a due process requirement prior to punishment. With a CWA water quality standard, by contrast, the EPA must "promptly prepare and publish proposed regulations," without any explicit requirement that it inform the affected states.58 This general notification process is a feature characteristically found in rulemaking.59 We conclude that the EPA has denied a rulemaking petition, an action presumptively subject to judicial review.

2.

With this presumption in place, we turn to whether section 1313(c)(4)(B) provides "no meaningful" or "no substantive" standards to apply.60 We hold that Congress has given sufficient guidance for judicial review of the agency's actions under the statute, and we have subject matter jurisdiction.

a.

An important qualification: our task is not to determine whether there are adequate statutory standards to judge the EPA's decision that new water quality standards are or are not necessary. Rather, we must decide whether Congress has placed sufficient guideposts around the EPA's prerequisite decision not to make a necessity determination.61 These two inquiries are related, however, and Massachusetts v. EPA provides insight as to how.

There, the Court clarified the type of permissible response the EPA could give after receiving a petition asking it to make a "judgment" that greenhouse gases "cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare."62 The Court held that the EPA was not obligated to make a judgment that such gases do or do not contribute to climate change if "it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they do."63 That explanation, in turn, must be "ground[ed] ... in the statute."64 The Court was not precise in specifying how tight the connection must be between the underlying statute and the agency decision to decline to exercise its discretion to make a prerequisite determination that it would or would not take action under that statute. It did, however, reject as inadequate several explanations posited by the EPA, which provide us some useful guidance.

First, the Court rejected the EPA's arguments that it could decline to make a determination based on certain "policy judgments," which included (1) the presence of "voluntary Executive Branch programs [that] already provide an effective response to the threat of global warming," (2) the potential impact of a determination on the President's negotiations with foreign powers, and (3) the fact that regulating automobiles would be "an inefficient, piecemeal approach" to climate change.65 Whatever the merits of these arguments, the Court concluded, "they ha[d] nothing to do with whether greenhouse gas emissions contribute to climate change."66 Second, the Court recognized that scientific uncertainty could be an acceptable explanation for refusing to make a threshold judgment.67 If the agency wanted to rely on this explanation, however, it had to be explicit about why it lacked "sufficient information... to make an endangerment finding" — it could not merely "not[e] the uncertainty surrounding various features of climate change."68 These examples suggest that the court was looking for a close and specific linkage between the decision not to make a threshold determination and the statutory provision setting out the underlying choice. The agency cannot rely on alternative policy grounds, even if reasonable, if those explanations do not find clear textual support. Nor can it resort to general claims of scientific uncertainty — if it justifies its refusal to make a threshold determination on that basis, it must be explicit about what uncertainty is present.

Justice Scalia's dissent comports with this understanding. He criticized the majority for its narrow definition of an acceptable "reasonable explanation." He focused on the distinction between the reasons the agency can rely on when it makes such a judgment about air pollutants, and those it can depend on when refusing to make a judgment — and concluded that the latter category was much broader:

Justice Scalia, then, would have allowed the agency to put forward reasonable explanations for not making threshold determinations that are not inconsistent with the statute, rather than insisting upon an explicit textual connection. That the majority rejected this reading suggests a tighter linkage is required.70

Informed by this precedent, we conclude that the EPA's reasons for declining to make a necessity determination must be rooted in the words of section 1313(c)(4)(B). And because the agency can only justify its decision not to make a necessity determination based on factors identified in the language of the statute, we look to those words to decide whether the statute is sufficiently specific to allow judicial review.

b.

We turn back to the Clean Water Act and hold that the EPA has not overcome the statutory presumption that we have subject matter jurisdiction to review its denial of the plaintiffs' rulemaking petition.

We begin with the text. The EPA is required to publish new water quality standards "in any case where the Administrator determines that a revised or new standard is necessary to meet the requirements of [chapter 26 of title 33 of the United States Code.]"71 Those statutory requirements are further defined in the statute; for example, section 1313(c)(2)(A) defines the necessary features of a water quality standard:

The EPA expanded upon these requirements in regulations issued pursuant to the CWA.73 While broadly drawn, these requirements provide guidance for the types of considerations the EPA must take into account in deciding the necessity of regulation. And, by Massachusetts v. EPA, these are the same factors that must be considered when the EPA declines to make a necessity determination. As general factors are still reviewable factors, we cannot conclude that there are no standards to judge the EPA's decision to elect not to make a necessity determination.74

The structure of section 1313(c)(4)(B), which employs mandatory language, also suggests reviewability. There, Congress required regulation if the EPA Administrator makes a "determin[ation]" that new standards are necessary. In section 7521(a)(1), found reviewable by Massachusetts v. EPA, the EPA Administrator had to regulate if she made a "judgment" that the emission of greenhouse gases by motor vehicles causes or contributes to air pollution.75 Both statutes are structured the same way: the agency has a mandatory obligation to take regulatory action if it makes a judgment (or determination) that regulation is required. This is in contrast to provisions that other courts have found unreviewable that use exclusively discretionary language, stating only that the agency "may" regulate, but need not do so.76

Nor does the overall structure of the Clean Water Act call this conclusion into question. Both parties emphasize the fact that the CWA is a cooperative federalism regime. The EPA argues that the CWA is a "carefully crafted scheme of cooperative federalism" that would be "placed at risk" if the courts were "to second-guess every EPA decision not to interfere with duly promulgated State water quality standards." The petitioners, in turn, focus on the backstop role the federal government plays in setting standards when state action is not enough, and argue that the Congressional intent of maintaining federal involvement would be frustrated if there was no judicial review. While both positions have merit, by the light of the required presumption of reviewability, we conclude that petitioners' argument carries more weight. This statutory scheme is defined by federal action: as Justice White noted in a different context, even though the CWA is a state-federal partnership, "the Federal Government maintains an extraordinary level of involvement" in administering the act.77

Finally, the subject matter of the CWA is also consistent with judicial review. Federal courts regularly hear cases addressing environmental regulations, including those implicating federalism issues.78 This case does not bring the sensitive national security issues of the genus that the Supreme Court has held supports a determination that the actions taken are not judicially cognizable.79 Indeed, federal courts have reviewed or held reviewable EPA decisions not to propose new or revised water quality standards under section 1313(c)(4)(B).80 While these reviews have been deferential, by reviewing at all, those courts implicitly concluded that they had subject matter jurisdiction.81

Given the text, structure, and subject matter of section 1313(c)(4), we hold that the agency has not overcome the presumption in favor of reviewability of agency action, and that we have jurisdiction to review the EPA's decision not to make a necessity determination.

III.

We now turn to whether the EPA had discretion to decide not to make a necessity determination. The district court concluded that the agency lacked such authority.82 We do not agree.

In Massachusetts v. EPA, the Court is explicit that the EPA could avoid making a threshold determination (in that case, that greenhouse gases do not contribute to climate change) "if it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they do."83 In dissent, Justice Scalia explicitly recognized that the majority held that the EPA could decline to make a prerequisite determination:

We recognize that the language of the CWA and that of the CAA is not identical. However, the CAA section at issue in Massachusetts and the CWA provision at issue here have the same structure: (1) a mandatory clause requiring the EPA Administrator to issue regulations on a certain topic, (2) if she makes a specific threshold determination, using her bounded discretion, (3) that a substantive standard has been satisfied.85 We hold that the Massachusetts v. EPA "reasonable explanation" rule applies to section 1313(c)(4)(B), and that the EPA may decline to make a necessity determination if it provides an adequate explanation, grounded in the statute, for why it has elected not to do so.86

The district court ordered the "EPA to conduct a necessity determination in response to Plaintiffs' rulemaking petition."87 Because the agency had the option of declining to make a necessity determination, this order was error. We remand this case to the district court to decide in the first instance whether the EPA's explanation for why it declined to make a necessity determination was legally sufficient.

In doing so, the district court must bear in mind several principles. First, the court applies the arbitrary and capricious standard of review set out in the APA.88 "As applied to refusals to initiate rulemakings, this standard is `at the high end of the range' of deference,"89 and "such review is `extremely limited' and `highly deferential.'"90 Second, in deciding whether the EPA appropriately declined to make a necessity decision, the district court's review is limited to determining whether the EPA has "provide[d] some reasonable explanation as to why it cannot or will not exercise its discretion" to make a necessity determination.91 That explanation must be grounded in the statute.92

In light of this highly deferential standard of review, the agency's burden is slight. That is particularly true when the statute is as broadly written as section 1313(c)(4)(B). Moreover, when a statute sets out competing considerations, agencies are generally given discretion to choose how to best give effect to those mandates.93 Nonetheless, we leave it to the capable hands of the district court to determine in the first instance the propriety of the EPA's actions.

IV.

We VACATE the order of the district court requiring the EPA to make a necessity determination and REMAND this case for proceedings consistent with this opinion.