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Abstract: River ice jams can cause extreme flood events with major consequences to infrastructure, riverside communities, and aquatic life. The emerging issue of climate change and the growing appreciation of related ecological linkages underscore the need for process-based predictive capability, such as theoretical advances and numerical modelling. A key element of such capability is the internal strength of the rubble comprising an ice jam, which is quantified by a few empirical constants, most notably the angle of internal friction, φ. Though this angle is considered a material property, there is considerable variation in reported values, which derive from applications of established theoretical concepts to actual case studies. The source of this discrepancy is identified by re-examining the theoretical formulations of ice jam stability and noting certain restrictive assumptions that were made in early literature. A less restrictive analysis takes into account the three-dimensional state of stress within an ice jam and leads to a more correct formulation. The resulting φ-values are consistent with experimental data obtained with specially-designed experimental setups. Relevant information can also be gleaned from more recent developments, intended to describe the dynamic evolution of ice accumulations into ice jams, via adaptations of constitutive laws that were originally developed for sea and lake ice. Analysis of the constitutive equations indicates that it is only when full lateral confinement occurs and the static (ice jam) condition is approached that φ can be considered a material constant. For this late phase of ice jam formation, the constitutive law appears to under-predict the value of φ while over-predicting the ratio of lateral-to-streamwise stresses. [Copyright &y& Elsevier]