This paper describes the spatial and temporal variability of strength in a typical river-ice cover during the pre-breakup period affected by radiation decay. Strength was determined using the borehole jack – a field-portable ice indentor system. Two cover types were analyzed: a granular white-ice/freeze-up jam and a columnar black-ice cover. Despite a protracted period of intense radiation, the strength of the highly-reflective ice in the freeze-up jam experienced no measurable change. The low-albedo, highly-transparent columnar cover, however, decreased in strength by approximately 50 % due primarily to the development of inter-granular void space. Changes are related to porosity and a comparison made to previous theoretical and laboratory work. Spatial variations in strength are discussed with respect to break-up advance and ice jamming. Possible control of break-up through modification of freeze-up processes is also presented.

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