What is River?

Legal Definition
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases a river flows into the ground and becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to using names such as stream, creek, brook, rivulet, and rill. There are no official definitions for the generic term river as applied to geographic features, although in some countries or communities a stream is defined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and northeast England, and "beck" in northern England. Sometimes a river is defined as being larger than a creek, but not always: the language is vague.

Rivers are part of the hydrological cycle. Water generally collects in a river from precipitation through a drainage basin from surface runoff and other sources such as groundwater recharge, springs, and the release of stored water in natural ice and snowpacks (e.g. from glaciers). Potamology is the scientific study of rivers while limnology is the study of inland waters in general.

Extraterrestrial rivers of liquid hydrocarbons have recently been found on Titan. Channels may indicate past rivers on other planets, specifically outflow channels on Mars and rivers are theorised to exist on planets and moons in habitable zones of stars.
-- Wikipedia
Legal Definition
A natural collection of waters, arising from springs or fountains, which flow in a bed or canal of considerable width and length, towards the sea.

2. Rivers may be considered as public or private.

3. Public rivers are those in which the public have an interest.

4. They are either navigable, which, technically understood, signifies such rivers in which the tide flows; or not navigable. The soil or bed of such a navigable river, understood in this sense, belongs not to the riparian proprietor, but to the public. 3 Caines' Rep. 307; 10 John. R. 236; 17 John. R. 151; 20 John. R. 90; 5 Wend. R. 423; 6 Cowen, R. 518; 14 Serg. & Rawle, 9; 1 Rand. Rep. 417; 3 Rand. R. 33; 3 Greenl. R. 269; 2 Conn. R. 481; 5 Pick. 199.

5. Public rivers, not navigable, are those which belong to the people in general, as public highways. The soil of these rivers belongs generally, to the riparian owner, but the public have the use of the stream, and the authors of nuisances and impediments over such a stream are indictable. Ang. on Water Courses, 202; Davies' Rep. 152; Callis on Sewers, 78; 4 Burr. 2162.

6. By the ordinance of 1787, art. 4, relating to the north-western territory, it is provided that the navigable waters, leading into the Mississippi and St. Lawrence, and the carrying places between the same, shall be common highways, and forever free. 3 Story, L. U. S. 2077.

7. A private river, is one so naturally obstructed, that there is no passage for boats; for if it be capable of being so navigated, the public may use its waters. 1 M'Cord's Rep. 580. The soil in general belongs to the riparian proprietors. (q. v.) A river, then, may be considered, 1st. As private, in the case of shallow and obstructed streams. 2d. As private property, but subject to public use, when it can be navigated; and, 3d. As public, both with regard to its use and property. Some rivers possess all these qualities. The Hudson is mentioned as an instance; in one part it is entirely private property; in another the public have the use of it; and it is public property from the mouth as high up as the tide flows. Ang. Wat. Co. 205, 6.

8. In Pennsylvania, it has been held that the great rivers of that state, as the Susquehanna, belong to the public, and that the riparian proprietor does not own the bed or canal. 2 Binn. R. 75; 14 Serg. & Rawle, 71. Vide, generally, Civ. Code of Lo. 444; Bac. Ab. Prerogatives, B 3; 7 Com. Dig. 291; 1 Bro. Civ. Law, 170; Merl. Repert, h. t.; Jacobsen's Sea Laws, 417; 2 Hill. Abr. c. 13; 2 Fairf. R. 278 3 Ohio Rep. 496; 6 Mass. R. 435; 15 John. R. 447; 1 Pet. C. C. Rep. 64; 1 Paige's Rep. 448; 3 Dane's R. 4; 7 Mass. Rep. 496; 17 Mass. Rep. 289; 5 Greenl. R. 69; 10 Wend. R. 260; Kames, Eq. 38; 6 Watts & Serg. 101. As to the boundaries of rivers, see Metc. & Perk. Dig. Boundaries, IV.; as to the grant of a river, see 5 Cowen, 216; Co. Litt. 4 b; Com. Dig. Grant, E 5.
-- Bouviers Law Dictionary
Legal Definition
A natural stream of water, of greater volume than a creek or rfvulet, flowing in a more or less permanent bed or channel, between defined banks or walls, with a current which may either be continuous in one direction or affected by the ebb and flow of the tide. See Howard v. Ingersoll, 13 How. 391,14 In Ed. 189; Alabama v. Georgia, 23 How. 513, 16 L. Ed. 556; The Garden City (D. C.) 26 Fed. 772; Berlin Mllls Co. v. Wentworth's Location, 60 N. H. 156; Dud-den v. Guardians of Clutton Union, 1 Hurl. & N. 627; Chamberlain v. Hemingway, 63 Conn. 1, 27 Atl. 239, 22 In R. A. 45, 38 Am. St. Rep. 330. Rivers are public or private; and of public rivers some are navigable and others not. The common-law distinction is that navigable rivers are those only wherein the tide ebbs and flows. But, in familiar usage, any river is navigable which affords passage to ships and vessels, irrespective of its being affected by the tide. See Public river.
-- Black's Law Dictionary