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River ice conditions in the Nelson drainage system MacKay, Donald Kenning

Abstract

Hydrological and meteorological observations related to ice conditions on rivers in the Nelson drainage system are compared statistically to determine the measure of agreement between them. The results show a high degree of positive correlation. The areal variability of ice formation, ice disintegration, mean length of the ice-free season, and the standard deviation of first-ice and last-ice are plotted on maps. The data has been based upon ten-year mean dates and also the entire 1921 to 1950 period. The progress of ice formation and disintegration is examined statistically both latitudinally and also along the major tributaries of the Nelson River. Results indicate that first appearance of ice affecting discharge generally follows the expected north to south pattern; no systematic progression along tributaries in either an upstream or a downstream direction is apparent. On each major tributary tested, ice disintegration progresses downstream. Latitudinal progress follows a south to north pattern with the exception of the southeasterly-flowing portion of the Assiniboine River. Trends and fluctuations in ice formation and disintegration are studied by five-point filtered series. Break-up (last-ice) occurred earliest in the mid-1940’s whereas freeze-up showed no definite trend. Filtered series of ice formation dates appear to exhibit greater co-variability than those of ice disintegration dates. The variability of break-up on headwater streams could be a factor in limiting the covariability between last-ice records due to the dependency of break-up at downstream sites on upstream conditions. Trends in the length of ice-free (open) season for 1921 to 1950 are examined using cumulative percentual deviations from the mean. At most locations in the Nelson basin, the length of the ice-free season was shorter than average from 1921 to the early l930’s and longer than average in the last half of the record. Cumulative percentual deviations from the mean ice-free season are compared to those from the mean annual air temperature. Mean annual air temperatures and lengths of ice-free season do not appear to be significantly correlated in the Nelson basin. The study of factors affecting the formation and disintegration of river ice in the Nelson basin is limited primarily to a discussion of the relationships among air temperatures, ice conditions, and river discharge. The extent and variability of freezing and melting degree days before ice formation and disintegration are examined for the period 1921 to 1950. Local air temperatures are extremely variable before first-ice and last-ice dates. Ice may be reported when temperatures are above the freezing point; break-up may occur when temperatures are below the freezing point. Two possible explanations for first-ice being observed under thawing conditions are: (a) ice formed under freezing conditions upstream moving downstream: and (b) the pooling of cold air in entrenched valleys resulting in ice formation. The occurrence of last-ice under freezing conditions may be caused by freeze-thaw cycles weakening the structure of the ice cover combined with increases in discharge raising and cracking it. Mean discharge rates (1921 to 1950) prior to ice disintegration are computed and graphed for seven locations in the Nelson basin. Mean rates increase three to five times in the ten-day period preceding break-up. Increases in discharge appear to be one of the prime factors contributing to ice disintegration.

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