Opinion ID: 1196629
Heading Depth: 1
Heading Rank: 4

Heading: rights and liabilities of private property owners

Text: Defendants are both property owners and governmental entities. Their potential liability as governmental entities for damage caused by discharge of surface waters into a natural watercourse is no longer limited, as it was at the time of Archer, supra, 19 Cal.2d 19, to the liability a private party would incur. Additionally, since Archer we have made it clear that private parties do not enjoy the broad immunity recognized at the time of Archer for discharge of surface waters across lower properties. (3a) Therefore, we shall first consider the holding of the Court of Appeal that a private property owner has no liability for damage to downstream riparian owners caused by his discharge of surface waters into a natural watercourse.
At common law the common enemy doctrine gave an owner of land over which surface water flowed from a higher elevation the right to obstruct the flow of that water, turning it back or diverting it onto the land of another owner, without liability for any damage that might result. ( Keys v. Romley, supra, 64 Cal.2d 396, 400-401.) By contrast, the civil law rule adopted for California more than a century ago (see Ogburn v. Connor (1873) 46 Cal. 346) gave the owner of the higher land an easement or servitude over a lower parcel which allowed him to discharge surface waters as they naturally flow from his higher land onto the lower land of the servient owner. ( Los Angeles C. Assn. v. Los Angeles (1894) 103 Cal. 461, 466 [37 P. 375]; Gray v. McWilliams (1853) 98 Cal. 157, 165 [32 P. 976]; Ogburn v. Connor, supra, 46 Cal. 347, 352-353.) The lower owner had no right to obstruct that flow. In theory, the owner of the lower parcel accepted it with the burden of natural drainage. ( Keys v. Romley, supra, 64 Cal.2d 396, 402.) Nonetheless, the owner of the higher land was not permitted to gather the surface waters by artificial means and discharge them on to lower lying land in greater volume or in a different manner than they would naturally be discharged. ( San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 398.) [13]
The rule differed with respect to discharge of surface waters into a natural watercourse. As we noted in Archer ( supra, 19 Cal.2d at p. 26), an upper riparian owner had the right, for a reasonable purpose, to discharge surface waters, including those whose volume was increased as a result of development which altered both the absorption of waters by the soil and the drainage pattern, into a natural watercourse. It was immaterial that the watercourse was inadequate to accommodate the increased flow and flooded downstream property. The riparian owner also had the right to improve the channel even if the accelerated flow caused downstream damage. ( Bauer v. County of Ventura (1955) 45 Cal.2d 276, 283 [289 P.2d 1].) [14] Thus, a riparian owner might be the ultimate beneficiary of the civil law rule subjecting lower parcels of property to the burden of surface water runoff from parcels at a higher elevation. The owner had the right, in turn, however, to discharge the surface waters into a natural watercourse without liability for damage that the addition of these waters to the stream might do to downstream riparian property. The downstream riparian owner is also deemed to take the property subject to an easement or servitude, one burdening the downstream property with accepting the flow of whatever water is thereby carried onto or through it in a natural watercourse. Moreover, a riparian landowner has had the right to collect or gather surface waters and discharge them at a location or locations other than those where natural runoff would enter the watercourse. The owner could straighten the stream or improve its bed with paving, drains, or conduits, and, to protect the land, construct dikes or bulkheads even if the result was to increase the volume and velocity of the waters to the injury of lower owners. [A] riparian owner has no right to complain because the volume of water in the stream is increased by artificially draining surface waters into it above, provided only the stream is the natural drainage channel for the lands so drained. Furthermore, this rule is adopted regardless of whether the so-called common-law rule concerning surface waters prevails in the particular jurisdiction or, as here, the civil-law rule, which forbids the gathering together of surface waters and discharging them as a stream upon adjoining lands. If the surface waters are gathered and discharged into the stream which is their natural means of drainage, so that they come to the land below only as a part of the stream, it is held that no action lies because of their being added. ( San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 401-402.) If a riparian owner cannot complain if surface waters be actually added by artificial drainage above to the volume of the stream, it must certainly be that he cannot complain of a drainage improvement which adds no water to the stream but merely protects the adjoining lands against the water already in it. ( Id., at p. 402.) The immunity of the upper riparian owner for downstream damage caused by his discharge of surface water runoff into a natural watercourse through improvements was initially for improvements undertaken to drain and/or protect the upper riparian owner's land. [A]n improvement for the purposes of the drainage and protection of lands above does not give a lower riparian owner on the stream a cause of action merely because such improvement increases the volume of water in the stream as it comes to his land, even though the burden he is necessarily under of protecting his land against the stream is thereby increased and his land is injured because of his failure to meet such increased burden; and, further ... the rule is not subject to the limitation that the increased volume must not be such as to make the stream exceed the capacity of its channel. ( San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 406.) We again recognized that this immunity was for damage to downstream land caused by improvements made in the stream for the purpose of draining or protecting the land above in Archer. ( Supra, 19 Cal.2d 19, 24-25.) And, in Archer, since the drainage improvements in the creekbed were permissible, we declined to impose a requirement that the plan minimize downstream damage by provision for improving the outlet from the lagoon into which the watercourse drained. ( Id., at pp. 25-26.) The California rules applicable to runoff of surface waters onto adjacent property and into natural watercourses have accommodated progression from a rural, agricultural society to gradual urbanization. Although immunity of upper landowners was limited to natural runoff of surface waters, it was broad enough to encompass surface water runoff from fields cultivated in a natural way, even though cultivation altered the runoff from that which occurred from untilled fields. ( Coombs v. Reynolds (1919) 43 Cal. App. 656, 660 [185 P. 877]. See also Switzer v. Yunt (1935) 5 Cal. App.2d 71, 78 [41 P.2d 974].) But immunity did not extend to city lots where changes and alterations in the surface were essential to the enjoyment of such lots.... ( Los Angeles C. Assn. v. Los Angeles, supra, 103 Cal. 461, 467.) (4) As suggested above, the natural watercourse rule has two aspects. The first permits the riparian landowner to gather surface waters and discharge them into the watercourse at a location other than that at which natural drainage would occur. The second permits the owner to make improvements in the bed of the stream to improve drainage and to protect the land from erosion by constructing dikes or embankments even though the result may be increased flow and velocity which might damage the property of lower riparian owners. Both aspects of the rule have as their purpose facilitating the development of upstream properties. Not to permit an upper land owner to protect his land against the stream would be in many instances to destroy the possibility of making the land available for improvement or settlement and condemn it to sterility and vacancy. ( San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 401; see also, Archer, supra, 19 Cal.2d at p. 27.)
(5), (3b) The modern rule governing landowner liability for surface water runoff and drainage is no longer simply a rule of property law dependent upon the existence of rights, servitudes, or easements. The civil law rule was modified more than a quarter of a century ago by the landmark decision in Keys v. Romley, supra, 64 Cal.2d 396. There we recognized the tendency of the civil law rule limiting immunity for damages caused by surface water runoff onto an adjacent property to inhibit development of land, since any change in the upper property would affect the natural runoff. ( Id., at p. 402.) Today a landowner's conduct in using or altering the property in a manner which affects the discharge of surface waters onto adjacent property is subject to a test of reasonableness. It is ... incumbent upon every person to take reasonable care in using his property to avoid injury to adjacent property through the flow of surface waters. Failure to exercise reasonable care may result in liability by an upper to a lower landowner. It is equally the duty of any person threatened with injury to his property by the flow of surface waters to take reasonable precautions to avoid or reduce any actual or potential injury. If the actions of both the upper and lower landowners are reasonable, necessary, and generally in accord with the foregoing, then the injury must necessarily be borne by the upper landowner who changes a natural system of drainage, in accordance with our traditional civil law rule. ( Keys v. Romley, supra, 64 Cal.2d 396, 409.) At least with respect to surface water runoff onto adjacent lands, the California rule is that stated in Keys v. Romley, supra, 64 Cal.2d 396, 409: No party, whether an upper or a lower landowner, may act arbitrarily and unreasonably in his relations with other landowners and still be immunized from all liability. It has been suggested that with the adoption of the reasonable use test for surface waters there is no longer any valid reason for distinguishing between surface waters and those that flow through a natural watercourse with respect to the rights and obligations of the respective property owners. (5 Miller & Starr, Cal. Real Estate (2d ed. 1989) § 14:24, p. 357; see also Hall v. Wood (Miss. 1983) 443 So.2d 834, 838 [As recently as 1978 this Court intimated that there might be a difference in principle between the two types of cases. [Citation.] Upon reflection, that difference escapes us.].) (6a) Defendants argue that the natural watercourse rule does not include a reasonableness element and should remain the law. They contend that the rationale on which the rule is based remains valid, and that application of the Keys v. Romley rule of reasonableness would create a virtual strict liability for public entities owning streets, storm drains, or other water-impervious improvements. Public entities, they claim, would bear extraordinary liability solely because storm waters falling on their thoroughfares ultimately reach a natural watercourse. The natural watercourse rule, they argue, properly permits the natural and intended usage of the creeks and waterways as a means of discharging the waters which would normally be conveyed therein. The argument both misstates the issue and exaggerates the potential liability. Draining surface waters from impermeable surfaces and channeling the flow into a waterway in culverts and storm drains is not the manner in which surface water would naturally be discharged into a waterway. Both the volume and the velocity of the discharge are abnormal, and it is the damage which may be caused by that unnatural method of drainage that is in issue. Our past decisions do not hold that immunity exists under the natural watercourse rule or its analogs for conduct which alters natural drainage and thereby creates a danger to downstream property owners if that danger is unreasonable in light of the purpose of the upstream action, the manner in which it is carried out, and the alternatives that might avoid or mitigate the potential damage. Nor is the reasonable use rule one of strict liability. It requires consideration of all of the relevant circumstances, and anticipates that both the upstream riparian owner and the downstream owners will act reasonably. It does not, however, give defendants what they ask  an unqualified right to discharge surface water runoff in a manner that will cause downstream damage, and even destroy downstream property, without attempting reasonable measures to prevent or minimize downstream damage. [15] Defendants offer no justification for a rule that would distinguish between the discharge of surface waters directly onto another owner's property and the discharge into a natural waterway that ultimately has the same injurious effect. They seek instead absolute immunity for the discharge of surface water runoff into a natural watercourse whether or not they have reduced or eliminated the capacity of the ground to absorb normal rainfall, channeled the runoff into a single destructive outlet, or otherwise altered the volume and velocity of the waters discharged into the watercourse, and regardless of whether the watercourse is capable of carrying the increased flow of waters. In short, they seek to avoid the conclusion of Keys v. Romley ( supra, 64 Cal.2d 396) that both upper and lower landowners must act reasonably with respect to one another. The upper riparian owner, they argue, has a right to engage in unreasonable conduct without regard to the impact of the action on downstream property. Defendants, who assume that Keys v. Romley, supra, 64 Cal.2d 396, does not apply to discharge of surface water into a natural watercourse or to improvements in a watercourse, offer no justification, other than the fact that they might incur liability, for recognizing a distinction between the duty of a riparian property owner to avoid injury to downstream property owners, and the duty of an uphill owner to downhill owners. We do not share the assumption that either a private or public property owner may disregard the impact of its conduct on other properties whether those properties are downstream or downslope. This court has restated the natural watercourse rule in several cases since Keys v. Romley , each of which involved an action against a municipal corporation or other governmental entity. In those cases, however, we have not considered whether that rule, as applied in this state, does include an element of reasonableness, or whether the rule of Keys v. Romley, supra, 64 Cal.2d 396, which expressly holds that the upper owner's conduct be reasonable, applies to the manner in which a riparian owner discharges surface waters into a natural watercourse. [16] Although this court has not considered the latter question, the Court of Appeal has done so in a series of decisions in which the court either assumed that the rule of reasonableness is applicable or expressly held it to be applicable to discharges into natural watercourses or flood control improvements in a watercourse. In Ektelon v. City of San Diego (1988) 200 Cal. App.3d 804 [246 Cal. Rptr. 483], nonsuit for a private developer was reversed. The Court of Appeal held that Keys v. Romley, supra, 64 Cal.2d 396, created a broad rule of reasonableness to be applied to all factual situations, ... (200 Cal. App.3d at p. 808.) Therefore, ordinary negligence principles governed the manner in which flood control structures were constructed because [a]n upstream landowner has no absolute right to protect his land from floodwaters by constructing structures which increase the downstream flow of water into its natural watercourse, but is instead governed by the ordinary principles of negligence. ( Id., at p. 810.) In Martinson v. Hughey (1988) 199 Cal. App.3d 318 [244 Cal. Rptr. 795], the court assumed that the rule of reasonableness applied to the discharge of irrigation waters and surface waters into a natural watercourse. There a lower owner had blocked a natural watercourse with debris which backed water up onto the upper land. Applying the rule to irrigation waters, the court concluded: The rule we deduce ... is that the upper owner has the right to discharge reasonable and noninjurious amounts of irrigation water through natural areas of flow onto the lower owner's property. The lower owner has a co-equal burden to receive reasonable and noninjurious amounts of irrigation water through natural flowage channels. (199 Cal. App.3d 318, 328.) In Weaver v. Bishop (1988) 206 Cal. App.3d 1351 [254 Cal. Rptr. 425], the question was liability for damages for improvements in a natural watercourse constructed for the purpose of protecting the defendants' property. The court held that the reasonable use doctrine articulated in Keys v. Romley, supra, 64 Cal.2d 396, 408-409, was properly applied in an action predicated on damage caused by the riparian owner's installation of riprap (boulders) along the stream bank to protect the land from erosion. The riprap altered the flow of the stream sufficiently that erosion occurred on the opposite bank, the owners of which sued. Given an instruction that liability depended on the reasonableness of each party's conduct, the jury found by special verdict that defendants' conduct was reasonable, while that of plaintiffs was not. The Court of Appeal reasoned in Weaver v. Bishop that neither the rule which gave a riparian owner absolute immunity for alteration in stream flow to protect his property, nor the common enemy doctrine which permits an owner to protect himself against floodwaters, even by turning them onto another's land, should apply. The common enemy doctrine is one form of the `right to inflict damage,' which was traditionally referred to under the [rubric] `damnum absque injuria' (harm without legal injury). This notion, peculiar to water law, rested on the `generally perceived reasonableness' of actions taken to protect one's property and on a policy of encouraging the preservation of land resources. [Citation.] However the nearly unanimous trend has been away from per se rules based on categorical judgments of `generally perceived reasonableness,' and toward fact-based determinations of reasonableness in the particular circumstances of each case. ( Weaver v. Bishop, supra, 206 Cal. App.3d 1351, 1357, fn. omitted.) [A]s Keys acknowledges and illustrates, the general trend in waterdamage cases is to replace the rigidities of property law with the more flexible, conduct-oriented principles of tort. (See 64 Cal.2d at p. 408.) (7)(See fn. 17.) Under the latter as expressed in the Second Restatement of Torts, defendants' liability would depend on a balancing of reasonableness, either by analogy to the rules concerning interference with water use, or under the rules of nuisance and trespass. (206 Cal. App.3d at p. 1358.) [17] (6b) The Court of Appeal in this case reasoned that important policy reasons had initially supported the immunity doctrine and saw no compelling reason to reject it, noting that it was bound in any case to follow the precedent established in Archer. The court observed that the Archer rule had been followed in Deckert v. County of Riverside (1981) 115 Cal. App.3d 885, 895, 896 [171 Cal. Rptr. 865], and elected to join the Deckert court, rather than the courts which had concluded that the rule of absolute immunity had been replaced by a rule of reasonableness. The Deckert opinion followed what that court believed to be the rule established in Archer and our subsequent decision in Bauer v. County of Ventura, supra, 45 Cal.2d 276, without mention of Keys v. Romley, supra, 64 Cal.2d 396, however. It does not support a conclusion that the Archer rule does not include a requirement of reasonableness or that, if the Archer rule does not, it survived Keys v. Romley . By contrast, Weaver v. Bishop, supra, 206 Cal. App.3d 1351, Ektelon v. City of San Diego, supra, 200 Cal. App.3d 804, and Martinson v. Hughey, supra, 199 Cal. App.3d 318, and, of course, Keys v. Romley, supra, 64 Cal.2d 396, reflect the nationwide trend toward merger of the rules governing diffused surface water and those governing watercourses. (See Tarlock, Law of Water Rights and Resources (1993) § 305[1], pp. 3-14 to 3-15.) We need not decide whether the natural watercourse rule applicable at the time of Archer, supra, 19 Cal.2d 19, included an element of reasonableness because we agree with those courts which have held that Keys v. Romley states a rule that is applicable to all conduct by landowners in their disposition of surface water runoff whether the waters are discharged onto the land of an adjoining owner or into a natural watercourse, as well as to the conduct of upper and lower riparian owners who construct improvements in the creek itself. Although Keys v. Romley was decided in the context of damage caused to adjacent land by the discharge of surface waters, the reasoning of the court has broader applicability. The decision rests on the broad principle that a landowner may not act arbitrarily and unreasonably in his relations with other landowners and still be immunized from all liability. [¶] It is therefore incumbent upon every person to take reasonable care in using his property to avoid injury to [other] property.... ( Keys v. Romley, supra, 64 Cal.2d at p. 409.) While the court spoke in terms of the responsibilities of adjacent landowners with respect to surface waters, we did not intend thereby to imply that the obligation to take reasonable care was not one imposed also on upper and lower riparian owners. There is no exception from the rule of reasonableness for riparians. No logic would support such a distinction and we decline to recognize one. Defendants' argument that Keys v. Romley is not and should not be applicable to discharge of surface waters into a natural watercourse overlooks the authority on which Keys v. Romley relied for the rule of reasonableness that it enunciated. That rule is derived from Armstrong v. Francis Corp. (1956) 20 N.J. 320 [120 A.2d 4, 59 A.L.R.2d 413]. ( Keys v. Romley, supra, 64 Cal.2d at p. 410.) Armstrong is a natural watercourse case. The facts which gave rise to the New Jersey Supreme Court's decision to abandon the common enemy/immunity rule in Armstrong v. Francis Corp., supra, 20 N.J. 320 [120 A.2d 4], are not unlike the facts in the case before this court. A housing developer stripped his tract, covered a stream that was a natural watercourse, and built a drainage system which conveyed not only the surface water runoff, but also percolating waters under the surface, into a pipe which discharged these waters into the stream above plaintiffs' property. As a result, the increased volume of water and its accelerated speed tore into the banks of the stream and, at the time of the lawsuit had carried away 10 feet of the plaintiff's creekside property with no end to the erosion in sight. Adopting the rule of reasonableness for that state, the New Jersey Supreme Court concluded: Social progress and the common wellbeing are in actuality better served by a just and right balancing of the competing interests according to the general principles of fairness and common sense which attend the application of the rule of reason. (120 A.2d at p. 10.) New Jersey is not alone in abandoning the common enemy/immunity doctrine. A majority of jurisdictions now apply a rule of reasonableness to the discharge of surface waters into a natural watercourse when the discharge will overtax the capacity of the watercourse or drainage channel and cause damage to downstream property. (5 Waters and Water Rights, supra, § 59.02(b)(1), p. 503; see, e.g., Heins Implement Co. v. Hwy. & Transp. Com'n (Mo. 1993) 859 S.W.2d 681; Hansen v. Gary Naugle Const. Co. (Mo. 1990) 801 S.W.2d 71; Johnson v. NM Farms Bartlett, Inc. (1987) 226 Neb. 680 [414 N.W.2d 256]; Martin v. Weckerly (N.D. 1985) 364 N.W.2d 93; Peterson v. Town of Oxford (1983) 189 Conn. 740 [459 A.2d 100]; County of Clark v. Powers (1980) 96 Nev. 497 [611 P.2d 1072].) The suggestion that the court would find the reasoning of Armstrong v. Francis Corp., supra, 120 A.2d 4, persuasive, but only insofar as surface water runoff onto adjacent property is concerned and not in the context in which that case was decided, is further undermined by recognition that other cases on which the court relied in Keys v. Romley, supra, 64 Cal.2d 396, also stated rules of reasonableness applicable to both runoff onto adjacent property and into a natural watercourse. In Bassett v. Salisbury Manufacturing Company (1862) 43 N.H. 569, 576, the court discussed both types of drainage and stated: [S]o far as a similarity of benefits and injuries exists, there should be a similarity in the rules of law applied. The rights of each land-owner being similar, and his enjoyment dependent [ sic ] upon the action of the other land-owners, these rights must be valueless unless exercised with reference to each other, and are correlative. The maxim, `Sic utere,' &c., [ Sic utere tuo ut alienum non laedas  use your own property in such a manner as not to injure that of another] therefore applies, and, as in many other cases, restricts each to a reasonable exercise of his own right, a reasonable use of his own property, in view of the similar rights of others. Instances of its similar application in cases of water-courses ... are too numerous and familiar to need more special mention. As in these cases of the water-course, so in the drainage, a man may exercise his own right on his own land as he pleases, provided he does not interfere with the rights of others. ( Id., at p. 577.) In Swett v. Cutts (1870) 50 N.H. 439, 446, also relied on by the court in Keys v. Romley ( supra, 64 Cal.2d at p. 404), the same theme of reasonable conduct regardless of the nature of the water right being exercised was again expressed: The doctrine which we maintain adapts itself to the ever varying circumstances of each particular case,  from that which makes a near approach to a natural water-course, down by imperceptible gradations to the case of mere percolation, giving to each land owner, while in the reasonable use and improvement of his land, the right to make reasonable modifications of the flow of such water in and upon his land. [¶] In determining this question all the circumstances of the case would of course be considered; and among them the nature and importance of the improvements sought to be made, the extent of the interference with the water, and the amount of injury done to the other land owners as compared with the value of such improvements, and also whether such injury could or could not have been reasonably foreseen. (50 N.H. at p. 446.) Most recently, the Missouri Supreme Court has brought all types of water within the rule of reasonableness. In Heins Implement Co. v. Hwy. & Transp. Com'n, supra, 859 S.W.2d 681, 691, the court observed: The standard we sanction today is in harmony with the most basic tenets of our law. `Reasonableness is the vital principle of the common law.' [Citation.] Reasonable use concepts already govern the rights of users of our watercourses, subterranean streams, and subterranean percolating waters. [Citations.] To some extent, they have also applied to upper land owners through the modified common enemy doctrine. Their extension to the management of all diffuse surface waters finally `bring[s] into one classification all waters over the use of which controversy may arise.' Defendants have offered no persuasive reason to limit the requirement that landowners act reasonably with regard to one another to their treatment of surface water discharge onto adjacent property while permitting unreasonable conduct when the waters are discharged into a natural watercourse. Indeed, defendants appear to overlook the impact on their own interests of a rule which would afford them no recourse if a private developer discharged surface water runoff into a natural watercourse adjoining publicly owned property in a manner which undercut and washed away a portion of that property. We agree with plaintiffs, therefore, that the rule of Keys v. Romley applies to this dispute. (8) Under that rule: The issue of reasonableness becomes a question of fact to be determined in each case upon a consideration of all the relevant circumstances, including such factors as the amount of harm caused, the foreseeability of the harm which results, the purpose or motive with which the possessor acted, and all other relevant matter. ( Armstrong v. Francis Corp. (1956) supra, 20 N.J. 320.) It is properly a consideration in land development problems whether the utility of the possessor's use of his land outweighs the gravity of the harm which results from his alteration of the flow of surface waters. [Citation.] The gravity of harm is its seriousness from an objective viewpoint, while the utility of conduct is meritoriousness from the same viewpoint. (Rest., Torts, § 826.) If the weight is on the side of him who alters the natural watercourse, then he has acted reasonably and without liability; if the harm to the lower landowner is unreasonably severe, then the economic costs incident to the expulsion of surface waters must be borne by the upper owner whose development caused the damage. If the facts should indicate both parties conducted themselves reasonably, then courts are bound by our well-settled civil law rule. (64 Cal.2d at p. 410.) As we have shown above, however, the well-settled civil law rule dictates a different result for riparian owners than that applicable to upland owners. Under the Keys v. Romley rule, if both parties act reasonably with respect to draining surface waters onto adjacent property the upper owner will be liable for damage caused by the alteration of the natural flow of the water. The result will differ in disputes between riparian owners, each of whom acts reasonably. The civil law rule with respect to natural watercourses, unlike that applicable to draining surface waters onto adjacent property, immunizes the upper riparian owner for damage caused by the alteration of the natural discharge of surface water into a watercourse and by improvements in the stream bed. Therefore, if the upper owner acts reasonably, or if the lower owner has not acted reasonably to protect the property, the lower riparian owner must continue to accept the burden of damage caused by the stream water. (9) As we noted earlier, however, the reasonableness of a landowner's action in discharging surface water runoff into a natural watercourse or in altering the watercourse itself cannot be determined in isolation. An owner in the lower reaches of a natural watercourse whose conduct has a relatively minor impact on the stream flow in comparison with the combined effect of actions by owners in the upper reaches of the watercourse may not be held liable for any damage caused by the stream flow beyond the proportion attributable to such conduct. If the rule were otherwise, owners at the lowest reaches of a watercourse could preclude development of upstream property by imposing on a single upstream owner the cost of all damage caused by the addition of surface water runoff if that addition combined with the existing stream flow damaged the lowest properties. The purpose of both the civil law rule creating immunity for damage caused by surface water runoff onto adjacent property and the natural watercourse rule which imposed the burden of damage caused by upstream development on the downstream owner was to ensure that development of property would not be foreclosed by imposition of liability for damage caused by changes in the treatment of surface water occasioned by that development. Keys v. Romley and the application of the rule of reasonableness to natural watercourses further that purpose. The rules applicable to surface water runoff onto adjacent property or into a natural watercourse have been modified only by limiting the immunity created by the civil and common law rules to conduct that is reasonable. (3c) The trial court and the Court of Appeal thus erred in concluding that the natural watercourse rule immunized defendants from tort liability as landowners for damage caused by their discharge of surface water runoff into Reliez Creek regardless of the reasonableness of their conduct. We shall nonetheless affirm the judgment of the Court of Appeal. Even were we to assume that the evidence would support a finding that increased surface water runoff or altered discharge of surface water runoff caused by improvements on any defendant's property was a substantial cause of the damage to plaintiffs' properties, plaintiffs have not established that defendants acted unreasonably in the construction of improvements or alteration of the method of discharge of the runoff; nor have plaintiffs established that they acted reasonably to protect their properties from stream-caused damage.