The 260-foot waterfall Systrafoss is located just past the ring road in the village Kirkjubæjarklaustur. Systrafoss is fed by a small lake called Systravatn. In summertime, melted snow feeds the lake creating this waterfall. Systrafoss means sister falls and is named after a monastery that was located in Kirkjubæjarklaustur in the medieval time.
The World of Waterfalls has been to this waterfall, which you can read about here.