2. Whenever any clear and definite usage of Shikast paiwast respecting the disjunction and junction of land by the encroachment or recess of a river may have been immemorially established, for determining the rights of the proprietors of two or more contiguous estates divided by a river (such as that the main channel of the river dividing the estates shall be the constant boundary between them, whatever changes may take place in the course of the river, by encroachment on one side and accession on the other), the usage so established shall govern the decision of all claims and disputes relative to alluvial land between the parties whose estates may be liable to such usage.