A large ice-dammed lake drained catastrophically into Lake Tuborg, Ellesmere Island, beginning on 25 July 2003. Limnological, sedimentological, and hydrological parameters were recorded before, during, and after this event. For several weeks prior to the jökulhlaup, water overtopped the ice-dammed lake and flowed into Lake Tuborg’s freshwater basin. A shallow sill separates the freshwater basin from a larger, deeper basin containing ∼25 PSU (practical salinity units) salt water. The sill blocked underflows from entering the saltwater basin before the jökulhlaup. The ice-dammed lake drained completely and catastrophically when englacial or subglacial conduits developed, and a glacier portal formed 980 m from the Lake Tuborg shore, marking the beginning of the jökulhlaup. The level of Lake Tuborg increased by 7.6 m in 84 h. This jökulhlaup is the largest known to have occurred in the High Arctic, and the largest witnessed in Canada since 1947. Strata of very cold water flowed above the chemocline for about 14 km, from the sill to the southwest end of the lake. The cold strata turbulently mixed with underlying salt water, allowing for saltwater flocculation of suspended sediment, causing rapid settling. The saltwater layer very slightly freshened and cooled. Close to the sill, near-surface sediments derived from the jökulhlaup are coarse and laminated; however, no erosion occurred toward the distal end of the lake, where a fining upward unit with a coarse base was deposited.