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 ∼0.4 m from 1984 to 2010, but complicating factors may be influencing this result. Winter‐discharge has increased by ∼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. Key Points First study of river‐ocean interface hydrology in a ice‐covered Arctic river Peak water levels in large lake‐rich delta are now 8 days earlier than in 1964 Earlier water peaks are a result of more rapid melting and breakage of river ice