This term is applied to the flow or movement of the water in rivers, creeks, and other streams.
In a legal sense, property In a water course is comprehended under the general name of land; so that a grant of land conveys to the grantee not only fields, meadows, and the like, but also all the rivers and streams, which naturally pass over the surface of the land.
Those who own land bounding upon a water course, are denominated by the civilians riparian proprietors, and this convenient term has been adopted by judges and writers on the common law.
Every proprietor of lands on the banks of a river has naturally an equal right to the use of the water which flows in the stream adjacent to his lands, as it was wont to run (currere solebat) without diminution or alteration.
No proprietor has a right to use the water to the prejudice of other proprietors, above or below him, unless he has a prior right to divert it, or a title to some exclusive enjoyment. He has no property in the water itself, but a simple usufruct as it passes along. Agua currit et debet currere, is the language of the law.
Though he may use the water while it runs over his lands, he cannot unreasonably detain it or give it another direction, and he must return it to its ordinary channel when it leaves his estate. Without the consent of the adjoining proprietors, he cannot divert or diminish the quantity of the water, which would otherwise descend to the proprietor below, nor throw the water back upon the proprietor above, without a grant, or an uninterrupted enjoyment of twenty years, which is evidence of it.
When there are two opposite riparian proprietors, each owns that portion of the bed of the river which is adjoining his land usque ad filum aquae; or, in other words, to the thread or central line of the stream and if hydraulic works be erected on both banks, each is entitled to an equal share of the water.
The water can only be used by each as an entire stream, in its natural channel; for of the property in the water there can be no severance.
But it seems that when an island is on the side of a river, so as to give the riparian owner on that side one-fourth of the water, the other is entitled to the whole of the three-fourths of the river.