The tug Aline B, towing a barge, headed northbound on a navigable stretch of the Atchafalaya River, outside a channel maintained by the Corps of Engineers. The barge struck an unmarked wreck and discharged oil. After the owner of the barge refused responsibility for the discharge, the United States removed the oil and sued the owner under 33 U.S.C. § 1321(f)(1) of the Federal Water Pollution Control Act for the cost of cleanup. The district court held the defendants liable, finding that the tug’s non-negligent decision to navigate outside the channel was a contributing cause of the oil discharge and that the defendants had therefore failed to prove that the discharge was caused solely by the act or omission of a third party. We affirm the judgment of the district court, holding that section 1321(f)(1) is causation-based, as opposed to fault-based, and that the district court’s finding that the tug’s choice of navigation, although non-negligent, was a contributing cause of the discharge was not clearly erroneous.

I.

On March 5, 1985, the M/V Aline B (“Aline B” or the “tug”), towing the T/B Hollywood 2006 (“Hollywood” or the “barge”), departed Morgan City, Louisiana, destined for Krotz Springs, Louisiana. The Hollywood carried a cargo of oil. Allen Boudreaux (“Boudreaux”), the captain of the Aline B, testified that at approximately 4:15 a.m. on the day of departure, the Aline B cleared the locks and that by 5:10 a.m. *1194the Aline B was heading northbound on the Atchafalaya River (the “river”).

Boudreaux stated that the Aline B proceeded upriver beyond Stouts Pass and entered a portion of the river called Six Mile Lake, which is located northwest of Morgan City. Within Six Mile Lake is a channel which the United States Army Corps of Engineers (“Corps of Engineers”) maintains.1 Although dotted lines mark the channel on a map, the parties stipulated that the actual channel in the river is not marked. Jorge Cano (“Cano”), a pilot employed by the Corps of Engineers, testified that in Six Mile Lake, the channel is closer to the left bank than to the right bank and that the water near the right bank is shallower than the water near the left bank. Boudreaux testified that he navigated the Aline B along the right bank to avoid the current and the downbound traffic. Trial testimony established that Boudreaux was navigating the tug outside the channel but within a navigable waterway.

At approximately 6:45 a.m. on March 5, 1985, the Hollywood struck an unmarked wreck, and as a result of this collision, the Hollywood discharged oil into the river. After the collision, the Coast Guard informed Hollywood Marine, Incorporated (“Hollywood Marine”), the owner of the Hollywood, that it was responsible for the cleanup of the oil, pursuant to section 1321 of the Federal Water Polution Control Act (“FWPCA”), 33 U.S.C. § 1321 (1986 and Supp. 1988). Hollywood Marine refused to clean up the oil, and the Coast Guard arranged for the removal of the oil at a cost of $89,189.25. Hollywood Marine was then billed for that amount on August 2, 1985. Hollywood Marine, again, denied responsibility. Meanwhile, the Corps of Engineers discovered the unmarked wreck between seventy-five and one hundred fifty feet off the right bank and removed it.

Because of Hollywood Marine’s refusal to pay for the oil cleanup, on December 5, 1986, the United States brought suit in federal district court against Hollywood Marine and West of England,2 which provided proof of financial responsibility for the Hollywood pursuant to 33 U.S.C. § 1321(p) (1986). The United States moved for summary judgment on April 27, 1987, which the district court denied without prejudice to the government’s right to reurge the motion after discovery. On September 9, 1987, the United States reurged its summary judgment motion, essentially arguing that section 1321(f)(1) provides for strict liability. The United States argued further that since the barge’s or the tug’s actions, though non-negligent, contributed to the oil spill, the liability exception under the statute was inapplicable because the act or omission of a third party was not the sole cause of the discharge. The district court denied the United States’ summary judgment motion, agreeing with the United States’ interpretation of the statute but finding that a genuine issue of fact existed as to whether the actions of the barge or the tug contributed to the oil spill.

After the parties stipulated to the source of the oil discharge, the cost of the cleanup, and the lack of negligence on the part of the tug, the barge, and the government,3 a bench trial was held on October 5, 1987, concerning the issue of causation. At the conclusion of the trial, the district court held that because the decision of where to navigate — namely, outside the channel— was a contributing cause of the oil discharge, the liability exception provided by section 1321(f)(1)(D) was unavailable to the defendants. Therefore, the district court *1195held that the defendants were jointly, severally, and in solido liable to the United States for the actual cost of the cleanup plus prejudgment interest at a rate of nine percent.4 The district court entered a final judgment on October 13,1987, and on October 23, 1987, the defendants filed a timely notice of appeal. On appeal, the defendants argue that the district court erred in holding: (1) that section 1321(f)(1)(D) is causation-based and not fault-based and (2) that because the choice of navigation was a contributing cause of the oil discharge, the liability exception of section 1321(f)(1)(D) does not apply in that the defendants cannot prove that the discharge was caused solely by the act or omission of a third party.5

II.

A. Fault or Causation?

We first address the defendants’ argument that the district court erred in holding that section 1321(f)(1)(D)6 is causation-based and not fault-based. Given that such a holding is a conclusion of law, we review it de novo. Byram v. United States, 705 F.2d 1418, 1421 (5th Cir. 1983).

The defendants argue that while the FWPCA is a strict liability statute, section 1321(f)(1) — consisting of four liability exceptions — relieves the discharger of strict liability. We agree that if a defendant can establish the existence of one of the four exceptions, it will be exempt from liability. See United States v. M/V Big Sam, 681 F.2d 432, 437 (5th Cir. 1982), cert. denied, 462 U.S. 1132, 103 S.Ct. 3112, 77 L.Ed.2d 1367 (1983). The defendants argue further that merely proving that the barge and tug were non-negligent7 satisfies section 1321(f)(1)(D). We disagree with this contention and affirm the district court on this issue for the following reasons.

“[T]he starting point for interpreting a statute is the language of the statute itself. Absent a clearly expressed legislative intention to the contrary, that language must ordinarily be regarded as conclusive.” Consumer Prod. Safety Comm’n v. GTE Sylvania, Inc., 447 U.S. 102, 108, 100 S.Ct. 2051, 2056, 64 L.Ed.2d 766 (1980); accord American Tobacco Co. v. Patterson, 456 U.S. 63, 68, 102 S.Ct. 1534, 1537, 71 L.Ed.2d 748 (1982).8 The language of section *11961321(f)(1)(D), see supra note 6, is couched in causation terms — an owner or operator must prove that the discharge was caused solely by the act or omission of a third party in order to escape liability. Moreover, the statute does not state that a showing of non-negligence on the part of the discharger will suffice to invoke the exception and absolve the owner and operator from liability. Congress expressly used the term “negligent” in other parts of section 1321 but did not use such a term in articulating the burden that an owner or operator must carry in order to satisfy the liability exceptions. Instead, Congress used the phrase “caused solely by” with no indication that fault, or the lack thereof, plays a role in establishing a section 1321 liability exception. See Reliance Ins. Co. v. United States, 677 F.2d 844, 848 (Ct.Cl. 1982) (proof of non-negligence does not mitigate liability under section 1321(i)(l)(D) because this section incorporates no standard of limited liability — section 1321(i)(l)(D) is the equivalent of section 1321(f)(1)(D) for onshore and offshore facilities). Finally, there is no indication in the legislative history that, contrary to its causation-based statutory language, section 1321 is fault-based.

The defendants argue that Congress expressed a clear intent that section 1321 be fault-based in the following statement in the Senate Committee Report:

The committee determined that while the owner or operator should not be liable if he could prove that a discharge was caused by one of these acts [the four liability exceptions in section 1321], it was also necessary that such exceptions be allowed only when the owner or operator proved the discharge to be solely the result of one of the exceptions. Any culpability on the part of the owner or operator would vitiate the exception.

Senate Comm, on Public Works, Federal Water Pollution Control Act [hereinafter the Senate Comm. Report], S.Rep. No. 351, 91st Cong., 1st Sess. 6 (1969) (emphasis added), reprinted in III EPA Compilation of Legal Authority [hereinafter EPA Compilation], Legislative History of the Federal Water Pollution Control Act at 1329 (1973). Although, in theory, we look to the legislative history only if the statutory language is unclear, Blum v. Stenson, 465 U.S. 886, 896, 104 S.Ct. 1541, 1547, 79 L.Ed.2d 891 (1984), in fact, the legislative history is often consulted where the language of the statute is clear (as it is here) for what might be called “further assurances.”

In evaluating the quoted language from the Senate Committee Report, we note that the final sentence does state that if an owner or operator is at fault, then it may not successfully claim a liability exception. The language does not state, or even imply, the reverse: that an owner or operator establishes the existence of a section 1321 exception, absolving itself from liability, once it proves that it was non-negligent. Moreover, in the preceding sentence, the Committee used causation language in explaining section 1321(f)(1) by stating that the exceptions will be allowed only when the discharge is solely the result of one of the exceptions.

Other language in the FWPCA’s legislative history reinforces our conclusion that an owner’s or operator’s proof of non-negligence alone does not satisfy the section 1321 liability exceptions. As we recognized in United States v. Dixie Carriers, Inc., 627 F.2d 736, 739 (5th Cir. 1980), the FWPCA represents a compromise between two proposed statutes, the House bill and the Senate bill. The proposed House bill prohibited the discharge of oil in navigable waters but imposed civil penalties and cleanup liability only on an owner or operator who willfully or negligently discharged the oil. The House bill also provided for a reverse burden of proof — the discharge established a prima facie case of liability *1197against the owner or operator which it could rebut. Apparently, it could rebut the prima facie case by showing that his actions were not willful or negligent. House Comm, on Public Works, Federal Water Pollution Control Act, H.R.Rep. No. 127, 91st Cong., 1st Sess. 10-11 (1969), reprinted in III EPA Compilation at 1258-60 (1973). The Conference Committee, H.R. Rep. No. 940, 91st Cong., 2d Sess. 5 (1970), reprinted in III EPA Compilation 1475-76 (1973), however, rejected the negligence approach with the reverse burden of proof and adopted the Senate bill approach of strict liability with limitations. See Senate Comm. Report, S.Rep. No. 351, 91st Cong., 1st Sess. (1969).

The Senate Committee Report, discussing the decision, stated:

The committee determined ... that some form of absolute liability should be imposed....

... [Marine] [insurance ... has been designed to protect people who either work for, use, own or operate a vessel. Were this the case with oil pollution, the imposition of liability based on negligence would not be questioned. However, the discharge of oil can and usually does affect the general public, and persons and property wholly unrelated to the vessel, who have no control over it, and who have no interest in it.

The public interest, it can be argued, can be completely protected only by absolute and unlimited liability; negligent and limited liability should protect only private interests....

Under absolute liability with limits, a vessel owner would be absolutely liable regardless of fault, but the injured party would be limited in the amount of the damages which could be collected. This approach avoids the difficult, if not impossible, task of proving negligence or rebutting the case for non-negligence made by the vessel owner. It also places the risk on the responsible party, not on the general public.

Id. at 5 (emphasis added).

The Senate Committee Report then discussed the limitations of the liability, stating the four liability exceptions that section 1321(f)(1) lists. The Senate’s approach of strict liability with limitations, as opposed to the House’s approach of negligence liability with a reverse burden of proof, was the one ultimately enacted.

Given its statutory language and legislative history, it is clear that section 1321(f)(1) is causation-based and not fault-based. Therefore, we reject the defendants’ argument that proof of non-negligence alone is enough to satisfy section 1321(f)(1)(D) and exonerate them from liability.

B. Sole Cause

The defendants next argue that even if section 1321(f)(1)(D) is causation-based and not fault-based, the oil discharge was caused solely by an act or omission of a third party.9 The defendants assert that the district court erred in holding that the choice of navigation was a contributing cause10 of the oil discharge and that, therefore, the defendants were liable.

Causation is generally a question of fact, Employers Ins. v. Suwannee River Spa Lines, Inc., 866 F.2d 752, 768 (5th Cir. 1989); Southern Nat’l Bank v. Crateo, Inc., 458 F.2d 688, 697 (5th Cir. 1972), and a *1198finding of fact will not be disturbed on appeal unless it is clearly erroneous. Anderson v. Bessemer City, 470 U.S. 564, 573, 105 S.Ct. 1504, 1511, 84 L.Ed.2d 518 (1985).

[A] finding is “clearly erroneous” when, although there is evidence to support it, the reviewing court on the entire evidence is left with the definite and firm conviction that a mistake has been com-mitted_ If the district court’s account of the evidence is plausible in light of the record viewed in its entirety, [we] may not reverse even though convinced that had [we] been sitting as the trier of fact, [we] would have weighted the evidence differently. Where there are two permissible views of the evidence, the factfinder’s choice between them cannot be clearly erroneous.... When findings are based on determinations regarding the credibility of witnesses, Rule 52(a) demands even greater deference to the trial court’s findings.

Id. at 573-75, 105 S.Ct. at 1511-12. For the reasons set forth below, we find no clear error in the district court’s finding of fact that the decision to navigate outside the channel was a contributing cause of the oil discharge. We recognize, however, that the challenge levelled by the defendants to the district court’s finding may also be viewed as a challenge to the legal sufficiency of the decision to navigate outside the channel as a “cause” of the discharge and to the resulting denial of the exception to liability provided by section 1321(f)(1)(D). As such, we will also review it as a question of law, de novo, and we hold that the navigation decision was a “cause” of the discharge within the meaning of section 1321(f)(1)(D).

Liability exceptions under section 1321(f)(1) must be narrowly construed to effectuate Congress’ strict liability scheme. LeBeouf, 621 F.2d at 789; Burgess v. M/V Tamano, 564 F.2d 964, 982 (1st Cir. 1977), cert. denied, 435 U.S. 941, 98 S.Ct. 1520, 55 L.Ed.2d 537 (1978); 115 Cong.Rec. 28957, reprinted in IV EPA Compilation at 1771 (1973) (Senator Boggs stated, “[S]uch exemptions have the effect of protecting the public in nearly every case, while safeguarding private interests at rare times of great disaster”) (emphasis added). As we previously stated, for a discharger to avoid liability, the act or omission of a third party must be the sole cause of the discharge. The FWPCA, however, does not define the terms “solely” and “caused.” As for the term “solely,” we will assume its common definition: “without an associate: singly, alone.” Webster’s Third New International Dictionary 2168 (1976); see City of Pawtucket v. United States, 211 Ct.Cl. 324, 546 F.2d 430 (1976) (assuming the common definition of “solely” contained in section 1321(i)(l)). If the tug’s choice of navigation was a proximate cause of the oil spill, then it necessarily follows that the act or omission of a third party was not the sole cause. See Seaboard Air Line R. Co. v. Deese, 185 F.2d 290, 291 (5th Cir. 1950). Concerning the term “caused,” we turn to legislative history, tort law,11 and cases which have previously interpreted causation under section 1321.

It is axiomatic that cause in fact is essential to liability. W. Prosser & P. Keeton, Prosser and Keeton on Torts § 41 (5th ed. 1984) [hereinafter Prosser & Keeton]. Under the infamous “but for” test, the tug’s decision to navigate outside the maintained channel is a cause in fact of the oil spill. “[Although causation [cause in fact] is essential to liability, it does not determine it.” Id. More than “but for” causation is necessary for the defendants to be liable.12 *1199The question whether the tug’s decision to navigate outside the channel was a proximate or contributing cause of the discharge is essentially a question of “whether the policy of the law will extend the responsibility for the conduct to the consequences which have in fact occurred.” Id. at § 42. In determining proximate cause, the trier of fact must decide whether the oil discharge was, at least to some degree, see infra, foreseeable,13 given Boudreaux’s decision to navigate outside the channel. United States v. Tex-Tow, Inc., 589 F.2d 1310, 1314 (7th Cir. 1978). If the oil discharge was thus foreseeable, then Bou-dreaux’s choice of navigation was a proximate cause of the discharge, and the act or omission of a third party cannot be the sole cause of the discharge.

In Tex-Tow, the Seventh Circuit held a discharger liable for civil penalties under section 1321(b)(6) (1986). Although section 1321(b)(6) does not include a third party defense, we find Tex-Tow helpful in ascertaining the boundaries of proximate cause. In Tex-Tow, an oil barge was being loaded with gasoline, and as it was loaded, it sank deeper into the water and punctured its hull on an underwater steel piling that was part of the dock structure. The Seventh Circuit recognized that neither the owner nor the operator was negligent even though the presence of the barge was the cause in fact of the spill. Furthermore, the court stated that the mere presence of the barge was not enough to satisfy proximate cause. Nevertheless, the court held that the defendant’s actions were the proximate cause of the spill because actual pollution occurred and the possibility of such pollution as a result of the heavy loading of the barge was foreseeable. Tex-Tow, 589 F.2d at 1312-14.

As in Tex-Tow, the oil discharge in the present case was sufficiently foreseeable to result in liability under section 1321(f)(1), even if not sufficiently foreseeable to result in liability for negligence. Cano, whose testimony the district court found to be “quite credible,”14 testified that the water where the Corps of Engineers found the wreck was much shallower than the water in the channel. Further, Cano stated that obstructions are common in the general area of the river where the wreck was found. Cano stated: “About a mile downstream of where this wreck was they have several fishing boats, large size fishing boats, just abandoned, scattered all over.... Fishermen generally put nets, big hook nets, along the bank as they catch their fish.” More important to the foreseeability inquiry than the actual condition of the river outside the channel was the risk inherent in choosing to navigate outside the channel which the Corps of Engineers maintains by conducting soundings to locate obstructions and making that information available to the public. In making that decision, Boudreaux clearly increased the risk that the tug and tow would encounter *1200unmarked hazards. The possibility of a collision resulting in an oil discharge in that portion of the river was sufficiently foreseeable to give rise to liability under the statute, and after reviewing the entirety of the evidence, we are not left with the definite and firm conviction that the district court made a mistake in finding that Bou-dreaux’s choice of navigation, though non-negligent, was a proximate or contributing cause of the oil discharge.15

The defendants argue, however, that such a holding is tantamount to holding that a vessel underway will always be liable because its choice to navigate will always be a contributing cause. We cannot agree. The government concedes and we recognize that underway status alone is not an automatic proximate or contributing cause to an oil spill. Merely choosing to navigate, however, is not the only factor involved in the present case. The choice made was not simply a choice to navigate, but it was a choice to navigate outside a maintained, marked channel. The non-negligence of such a choice does not insulate the choice from being a proximate or contributing cause under the statute.

The defendants finally argue that an act or omission of the owner or operator of the unmarked wreck was the proximate cause of the oil spill, and they define the proximate cause as “the dominant and efficient cause of the loss and not a merely incidental cause which may be nearer in time to the casualty.” The defendants in effect urge that we equate “the proximate cause” with “the sole cause,” but the defendants point to nothing in either the statute or the legislative history to support the proposition that Congress intended to equate the term “the sole cause” or “caused solely by” with the term “the proximate cause.” Indeed, our reading of the statute and the legislative history suggests that when there are multiple causes of a discharge and one cause is the dominant or proximate cause, to view that dominant or proximate cause as being the sole cause of the discharge would not be faithful to Congressional intent. See also Southern Nat’l Bank, 458 F.2d at 697 (the terms proximate cause and sole cause do not have the same legal meaning); Phoenix Indem. Co. v. Givens, 263 F.2d 858, 861 (5th Cir. 1959) (same).

Congress’ strong concern for the nation’s water resources is apparent in the language 16 and the legislative history of the FWPCA. See Senate Comm. Report, S.Rep. No. 351, 91st Cong., 1st Sess. 2-3 (1969), reprinted in III EPA Compilation at 1325-26 (1973). The legislative history of section 1321 clearly shows that Congress desired a comprehensive solution to the nation’s oil spill problem. Id. at 4, 7, reprinted in III EPA Compilation at 1326, 1330 (1973). Congress was concerned not only with negligent and careless oil discharges but also with accidental oil discharges. Id. at 7, reprinted in III EPA Compilation at 1330 (1973). After much consideration, Congress endorsed an absolute liability system with limited exceptions, which are to be narrowly construed. In the view of Congress, such a system best protected the public and placed the risk on the responsible party, not on the general public. Id. at 5, reprinted in III EPA Compilation at 1328 (1973). Our decision is in harmony with Congressional intent, and we hold that the district court did not err as a matter of fact or law.

III.

For the foregoing reasons, we hold that section 1321(f)(1) is causation-based and that simply proving non-negligence on the part of a discharger will not satisfy the liability exceptions listed in section *12011321(f)(1). Furthermore, we hold that the district court’s finding that the choice of navigation was a proximate or contributing cause of the oil discharge was neither an error of fact nor law. We therefore AFFIRM the district court.