Most analyses of glacial systems require a relationship between strain rates and stresses for ice deforming in creep. For conditions relevant to glacier flow, much evidence shows that strain rates increase approximately as deviatoric stress raised to a power n. Field and laboratory experiments suggest that n ≈ 3, but values span a wide range and controversy persists. Most field efforts to determine n seek clarity by examining situations with simple stress states. We instead use a fully three-dimensional model to interpret measurements of flow of an Antarctic outlet glacier that follows a tortuous path through a mountain range. By comparing observed flow to models with different parameter values, we conclude that the effective n must be between 2.6 and 5.1 (99% confidence). The best match occurs with n ≈ 3.5. We conclude that use of the traditional value is appropriate for polar ice deforming at stresses of 0.6–1.2 × 105 Pa.