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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. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes frazil ice events detected using upward looking sonars, one high (546kHz) and one low (235kHz) frequency, during the 2009/2010 freeze-up field deployment on the North Saskatchewan River, Edmonton, AB, Canada. The field acoustic data have been processed to compute time series of the depth averaged volume backscatter strength, S vd (dB). A review of three scattering models: the sphere, prolate spheroid, and disk models, together with their formulation, assumptions and limitations are presented. Estimation methods for the concentrations and sizes from S vd (dB) using scattering models are discussed. The applicability of the three scattering models to measuring suspended frazil ice was investigated using laboratory data reported by Ghobrial et al. (2012a). It was found that the laboratory measured concentrations were in a good agreement with concentrations estimated with these scattering models. It was also found that the disk model is the most applicable because it provides more realistic particle sizes. In total, eight frazil events were detected with the sonars during the field deployment. Preliminary linkages between the meteorological (air and water temperatures) and surface ice conditions measured at the site, and the duration and magnitude of the detected frazil events are presented. Both the laboratory results and the disk scattering model were used to estimate frazil ice concentration and sizes from the field data. Concentrations ranging between 0.01 and 0.05% and disk radii between 0.13 and 0.21mm have been estimated. [Copyright &y& Elsevier]