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The Karahnjukar dam spillway is the spillway for the Halslon reservoir, the main reservoir for the 690 MW Karahnjukar Hydroelectric Project in East Iceland. The spillway consist of a 140 m long overflowing weir with a capacity of 2250 m3/s which discharges into a side channel, followed by a 450 m long chute. The chute terminates at a canyon edge where the jet drops some 100 m into a narrow gorge downstream of the Karahnjukar dam. During the design phase of the project, the flow conditions in the spillway were modelled in the hydraulic model tests in 1:45 scale model at the Laboratory for Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich. The power plant went into operation in 2008 and since then considerable experience has been obtained from operation of the spillway. In particular, flow depths and air discharge measurements have been obtained during spilling periods. In this paper, these measurements are presented and compared with results from the hydraulic model tests. In particular, flow depths are discussed as well as aeration effectiveness.