In the last installment in this series we looked at how the English rule on escaping water in Rylands v Fletcher was rejected in British India due to perceived differences in the environments and needs of the two countries. Today we look at a classic issue of water law, complaints by riparian owners of upstream water uses that interfered with their own uses. As with the issue of escaping water, what was considered "natural" could turn, here too, on local natural conditions.
The traditional common law rule of conflicting riparian uses – that an upstream riparian may not divert a stream's water to the (unreasonable) detriment of downstream owners – was explained by the court of King's Bench in 1625 (Shury v Piggot, 81 English Reports 280) in terms of natural law: "a water-course […] doth begin ex jure naturæ, having taken this course naturally, and cannot be averted" and "the nature of this [water-course] is to be current [i.e. to flow]".
In the leading case of Evans v Merriweather in 1842, the Illinois Supreme Court advanced a distinction: An upper riparian might reasonably consume all the water in the stream if his use were a "natural" one, necessary for existence, but not were it to be considered "artificial", for the mere increase of "comfort and prosperity". The court then suggested that the application of this distinction would vary depending on natural conditions:The supply of man's artificial wants is not essential to his existence; it is not indispensable; he could live if water was not employed in irrigating lands, or in propelling his machinery. In countries differently situated from ours, with a hot and arid climate, water doubtless is absolutely indispensable to the cultivation of the soil, and in them, water for irrigation would be a natural want. Here [in Illinois] it might increase the products of the soil, but it is by no means essential, and can not, therefore, be considered a natural want of man.
What riparian uses were to be recognized as "natural", and therefore permitted, was thus dependent on the climate and environment. Irrigation would be considered an "artificial" use in the American Old Northwest, as it would be in England, but the court had legitimized changing the law if and when the common law spread to arid lands.
This way of thinking was crucial for the development of water law in the arid western states and territories of the United States, in which the traditional common law of riparian rights was rejected in favor of a new system. The new law, known as "prior appropriation", opened up water resources to appropriation by all, not just riparian owners, and allowed them to take as much of the water as they could put to beneficial use, regardless of the effect on later appropriators. An early case from Colorado discussed whether an appropriator could dig a ditch across another's land without permission, an invasion of property that would clearly not be allowed under the traditional common law. Along the way it advanced a general theory of the adaptation of property law to environmental conditions:

















