after this, the river flows between the biskupstungur and hrunamannahreppur districts. here, the hvítá combines with three other rivers: the tungufljót, the brúará, and the stóra-laxá, doubling the volume of the river. it goes to run through the flatlands near grímsness and behind ingólfsfjall mountain. just north of the town of selfoss it meets the sog river where it becomes the ölfusá as it flows into the atlantic ocean.