The English water law first adopted in the United States was premised on the natural flow doctrine, pursuant to which a riparian owner has the right to a natural water flow of undiminished quantity and unimpaired quality.
In general, the right of a riparian owner to appropriate the water is limited to his use for such purposes, to such an extent and in such a way as will not be inconsistent with a similar use by the lower owners. Since the effect upon the lower riparian proprietor is made the test, it would make little difference whether the water was taken for use on riparian or non-riparian lands, just so long as it did not essentially interfere with the exercise of the common right by the other riparian owners.If the use is lawful and beneficial, it must be deemed to be reasonable, and not an infringement of the right of other riparian owners to whom it occasions no actual and perceptible damages either as to the present or future use of the riparian land. The question in such a case is not whether the diversion being for a legitimate use and in quantity such as is reasonable, having regard to all the circumstances, but only whether it causes actual damage to the person complaining.
The doctrine of equitable apportionment is oriented toward satisfying equally the interests of all riparians. The doctrine of equitable apportionment alone, however, establishes only the equal right of each riparian to an equitable share in the benefits of the river. The total benefits of the river have to be shared equitably by all the riparians is apt to engender the incentive for enlarging the opportunities to be shared through joining in a regional project.