Timing, duration, and magnitude of peak annual water-levels during ice breakup in the Mackenzie Delta and the role of river discharge

North-flowing rivers of the pan-Arctic region have important effects on the Arctic Ocean, but their river-ocean interfaces, including some with vast deltas such as the Mackenzie, have complex hydrology and remain poorly understood. Analysis of 39 years (1973-2011) of water-levels and river discharge at the head of the Mackenzie Delta, 48 years (1964-2011) of water-levels in the mid-delta, and 28 years (1984-2011) of water-levels in the outer delta permitted evaluation of changes in the timing, duration, and magnitude of peak annual water-levels during river-ice breakup. The initiation date of freshet-discharge into the delta has not changed, but the duration from freshet initiation until peak water-levels in the central delta (i.e., duration of ice clearance) has shortened from 35 to 27 days since 1964. The height of annual water-level peaks in the outer delta at Reindeer Channel may have declined by approximate to 0.4 m from 1984 to 2010, but complicating factors may be influencing this result. Winter-discharge has increased by approximate to 21% from 1973 to 2011, but this amount is too small to cause a trend in total Mackenzie discharge. Breakup-discharge (i.e., occurring during ice clearance through the central delta) has not significantly changed. The lag time from freshet-discharge initiation into the delta until initial breakage of the river ice-sheet has declined by 6.6 days from 1974 to 2007 and is sufficient to account for the shortened period of river-ice clearance. Declining snow-pack depths during April suggest that river-ice may be melting earlier and more rapidly.