Water from the large glacial rivers Jökulsá á Dal and Jökulsá í Fljótsdal flows to the Fljótsdalur Station. Moreover, the rivers Kelduá, Grjótá and Saudá join up in Hraunaveita diversion, and their flow is diverted to Jökulsá í Fljótsdal.
Hálslón reservoir
Jökulsá á Dal is dammed at Fremri Kárahnjúkar, and this, moreover, is the location of the largest dam of the Kárahnjúkar HEP by far, i.e. the Kárahnjúkastífla dam. The dam is 700 m long and 198 m high, with a concrete facing on the water side. Kárahnjúkastífla dam is among the largest in the world of this type and the largest concrete-faced rockfill dam (CFRD) in Europe.
To the east of the Kárahnjúkastífla dam is the Desjarárstífla dam. A small valley on the west side contains the Saudárdalsstífla dam. These saddle dams are rock and gravel dams with central cores of glacial drift. Together, the three dams form the Hálslón reservoir, which is 57 km². The water level at full reservoir is 625 m above sea level.
It is estimated that the Hálslón reservoir will generally fill in late summer most years. When the reservoir is full, surplus water will be diverted through a spillway chute at Kárahnjúkastífla, where it will fall in a 90-metre-high waterfall into Hafrahvammur canyon.
Jökulsá diversion and Hraunaveita diversion
Jökulsá á Fljótsdal is dammed somewhat below Eyjabakkafoss waterfall and forms an intake reservoir named Ufsarlón. Kelduárlón reservoir is created in the Kelduá river, approximately 7 km², where the lake Folavatn is currently situated.
Hraunaveita diversion joins water from the rivers from Kelduá, Grjótá and Saudá and flows into Jökulsá á Fljótsdal by means of tunnels and canals.