Four years passed from the beginning of construction work at Kárahnjúkar in 2003 until the Fljótsdalur Power Station reached full operational capacity in 2007. Concurrent with the construction work at Kárahnjúkar, an aluminium plant was built in Reyðarfjörður. Most of the energy generated is sold to the Reyðarfjörður plant.

On its long journey from the reservoirs in the highlands to the station’s intake, the water drops approximately 200 m. Two-thirds of the total head runs through an approximately 400 m high vertical pressure tunnel near the Fljótsdalur Station.

The water’s total drop therefore, is more than 600 m. The water drives six powerful turbines in the powerhouse and then flows through a tailrace tunnel and canal into the river Jökulsá in Fljótsdalur, east of Valþjófsstaður Mountain, at an altitude of 26 metres. The underground powerhouse is located inside Valþjófsstaður Mountain and is accessed through an 800 m tunnel. Electricity is transmitted from the station through a separate cable tunnel to the switchgear house and from there through high-voltage lines to Alcoa’s aluminium plant in Reyðarfjörður.

Fljótsdalur Station came on-line in 2007. The Station’s catchment area covers over 2200 km2 and the station’s reservoirs are formed by five dikes that are over five kilometres in length. The water is diverted to the Fljótsdalur Station’s turbines from the reservoirs in the highlands north of the Vatnajökull Glacier, through a tunnel system that is approximately 72 km long.

Kárahnjúkar Dam is the tallest concrete-faced rockfill dam in Europe and among the largest of its kind in the world. The River Jökulsá á Dal is dammed at Fremri Kárahnjúkar with the largest of the Kárahnjúkar dams. Most of the rockfill was quarried just upstream of the dam within the reservoir area and placed in compacted layers. During construction, the river was diverted through two diversion tunnels under the dam on the western bank. Two smaller saddle dams were built at Kárahnjúkar, Desjará Dam to the east and Sauðárdalur Dam to the west. Together, the three dams form the Hálslón Reservoir which covers an area of 57 km2, and reaches all the way to the Brúarjökull Glacier.

Most years, Hálslón fills up in late summer. When this happens, the spillover is diverted through a chute down to the canyon edge, where it becomes the waterfall Hverfandi (Vanisher). Almost 100 metres high, Hverfandi can, at times, become more powerful than Dettifoss, its neighbour to the north and one of Europe’s most powerful waterfalls. The water from all the reservoirs comes together in the headrace tunnel, which runs almost level at 100-200 metres depth under the Fljótsdalsheiði Moor. The headrace tunnel ends in two pressure shafts, where the water falls 400 metres vertically into the Fljótsdalur Station’s Powerhouse, about 1 km inside the mountain.

Fljótsdalur Power Station came online on 30 November 2007 and is underground at Mt. Valþjófsstaðarfjall.

2 diversion tunnels Hraunaveita (dia: 4.5 m) 

2 vertical pressure tunnels (dia: 4.0 m) 

Access tunnel to power station (dia: 7.5 m)

Installed capacity	690 MW
Generating capacity	4600 GWh/year
Turbines	6 Francis
Rated output per unit	115 MW
Rated discharge per unit	24 m³/s
Total head	599 m
Rated discharge (maximum discharge)	144 m³/s
Average discharge	115 m³/s

Area of full reservoir	63 km²
Length of reservoir	25 km
Live storage	2.210 Gl
Reservoir full supply level (FSL)	625 m.a.s.l
Reservoir minimum operating level (MOL)	560 m.a.s.l
Catchment area	1.806 km²
Average inflow	107 m³/s

Maximum dam height	198 m
Dam length	700 m
Fill materials	8,4 million m³

Maximum dam height	68 m
Dam length	1.100 m
Fill materials	2,9 million m³

Maximum dam height	29 m
Dam length	1.100 m
Fill materials	1,6 million m³

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Sultartangi Power Station

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Sigalda Power Station

Area of full reservoir	1 km²
Reservoir full supply level (FSL)	625 m.a.s.l
Catchment area	430 km²
Average inflow	31 m³/s

Maximum dam height	37 m
Dam length	600 m
Fill material	0,5 million m³

Area of full reservoir	7,5 km²
Reservoir full supply level (FSL)	669 m.a.s.l
Live storage	60 Gl

Maximum dam height	26 m
Dam length	1.700 m
Fill materials	0,7 million m³

Tunnels	Approx. 72 km
Headrace from Hálslón (dia: 7.2-7.6 m)	39,7 km
Headrace from Ufsarlón (dia: 6.5 m)	13,3 km
3 adits to headrace (dia: 7.2-7.6 m)	6,9 km
2 diversion tunnels and adit at dam	2,4 km
Grouting galleries at dam	500 m
Surge tunnel (dia: 4.5 m)	1,7 km
2 diversion tunnels Hraunaveita (dia: 4.5 m)	3,7 km
2 vertical pressure tunnels (dia: 4.0 m)	800 m
Access tunnel to power station (dia: 7.5 m)	1,0 km
Tailrace tunnel (dia: 9.0 m)	1,3 km
Cable tunnel (dia: 4.0 m)	1,0 km