The Laxá á Keldum river has its main source in Rangárbotn, southwest of Laufafell at an altitude of about 600 m above sea level. From the source to the mouth is 61 km. The river is a spring river, but it also receives some runoff and a small amount of glacial water. There it flows along the edge of the Hekla lava fields until it reaches the Rauðnefsfjall mountain The watershed is 562 KM2. The average flow of the river at Tungufoss is about 20 m3/sec. The Laxá á Keldum river is a spring river, but it also has a runoff and glacial component. The glacial component is fed by Tindfjallajökull glacier.
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