Modeling groundwater upwelling as a control on river ice thickness

The Tanana River flows through interior Alaska, a region characterized by discontinuous permafrost. Studies link degrading permafrost to increased winter river discharge due to greater groundwater (GW) recharge increasing GW input to river baseflow. In winter, interior Alaskan rivers are exclusively fed by GW, which provides an external source of heat. In fact, some portions of rivers fed by GW maintain thin ice cover throughout the winter, or remain ice-free, despite very cold air temperatures. These ice conditions represent a significant danger to rural Alaskans who extensively use rivers for wintertime travel in this largely roadless area. A physically based, numeric model was developed to examine the consequences of permafrost degradation in explaining unfrozen river conditions in the winter. Results show that the ice melt was amplified by increased water column temperatures, flow velocities, air temperature, and snowfall. Abrupt changes in snowfall were illustrated to contribute to decreased ice thickness and more hazardous conditions for winter travelers. The model examines the physical mechanisms that underlie dangerous ice conditions in winter and early spring, and suggests that GW flow parameters need to be better characterized to model mid-winter ice degradation in sub-arctic environments.