What is riverine island?
an island in a river.(might be correct in the general sense )
if it was required to elaborate - then, speaking in case of northern plains of india -the rivers comming from the north are always involved in depositional work . in the lower course , due to gentle slope ,the velocity of the river flow decreases and then the water drains/flows ahead leaving behind depositionl materials like sediments , aluvium etc . and which hence results in a formation of a riverine islands .