The strength of an ice cover significantly affects the timing and severity of breakup on northern rivers. During the pre-breakup period, thermal and radiative warming is known to significantly decrease such strength. Although reductions in strength with temperature are reasonably well known, there is a dearth of data concerning strength changes due to internal radiative melt. In the spring of 1989, near Floral, Saskatchewan, Canada, a detailed series of experiments were conducted to determine changes in flexural strength, porosity and optical properties of the ice cover. The test data strongly support existing theoretical models of strength reduction due to porosity development in columnar grained ice. Experimental methods and results with respect to flexural testing, porosity determinations and radiation energy balance are discussed.