Water from the Hálslón reservoir flows through a tunnel to the east under the Fljótsdalsheidi moor where it joins water flowing from another tunnel from Ufsárlón reservoir. The total length of the headrace tunnels to Fljótsdalur Station is 53 km.
The total tunnel length in the project is approximately 72 km. In addition to headrace tunnels, the Kárahnjúkar tunnel system consists of surge tunnels, access tunnels, tailrace tunnels, cable tunnels and adits.
The headrace tunnels were, for the most part, drilled with three giant tunnel borers (TBMs) from the company Robbins in the United States. Drilling work for the tunnels was completed on 9 April 2007, a non-stop operation since 2004. Water from Hálslón reservoir was diverted into the tunnels in early 2007, while water from Hraunaveita diversion and Ufsarlón reservoir will be diverted in August 2008.
The whole-boring of the headrace tunnels began in 2004. TBMs were used to drill 48 km of the project's 72-km tunnel system. The headrace tunnels generally lie at a depth of 100–200 m.
The drilling of the tunnels was successful for the most part, and the TBMs often set advancement records. However, there were also localised delays in the tunnel drilling and particularly so in the case of TBM2. The drill ran into an area of difficult rock with extensive faults under Thrælaháls. In addition, there was considerable leakage in the tunnels, particularly in the west section, which delayed drilling and finishing work.
The image below shows the paths taken by the three TBMs and their performance. Click on the image to enlarge.