The gates are gradually lowered
At the time the bypass tunnel gates were closed, the flow of Jökulsá á Dal was average for the season: about 150 cubic metres per second. It had been much greater the week before, so that the decision had been made not to "shut off" the flow until there was an acceptable volume of water in the river, preventing the level from rising too quickly while the bedrock was absorbing water. The delay allowed conditions to reach an ideal state, and all appropriate preparations were concluded on schedule.
A crowd of people watched from the eastern bank of Hafrahvammagljúfur canyon, opposite the bypass tunnel, and there was also considerable excitement farther downstream in the valley of Jökuldalur, as people observed the changes there. The downstream observers saw the riverbed carrying only 10% of its normal volume, just a few hours after the gates had been closed above the dam at Fremri-Kárahnjúkur mountain.
Impregilo personnel pumped several truckloads of concrete into the bypass tunnel, behind the gates, in order to brace the closing mechanism and seal it. In addition, boulders were tipped into the river in front of the gates, which are never to be moved again. The bypass tunnel will eventually be securely blocked by creating a 20-metre thick concrete plug in the middle of it, after which the tunnel will be considered safely closed.
There is a detailed report on the Kárahnjúkar project website about the blocking of the bypass tunnel and about ongoing construction work towards Kárahnjúkar Power Plant.