Analysis of a Persistent Early Winter Open Water Zone Within the Ice‐Covered Tanana River Near Fairbanks, Alaska Using Field Studies, Remote Sensing, and Hydraulic Modeling

Abstract Frozen rivers serve as important transportation corridors for Alaskans during winter. Open water zones (OWZs, i.e., open leads) in otherwise ice‐covered rivers present a hazard because their location and causes are poorly understood. We studied one persistent OWZ on the Tanana River near Fairbanks using field studies, remote sensing, and hydraulic modeling. Interannual occurrence and duration of the OWZ was identified from 2014 to 2023 using optical satellite images and synthetic aperture radar. In eight out of 10 years, an OWZ occurred when an ice jam occurred at an upstream channel constriction during freeze‐up. In the other 2 years, a partial ice cover developed downstream of the reach and no OWZ occurred. We suspected this initial freeze‐up ice jam where the channel narrowed played an important role in OWZ occurrence. To test this, we performed simulations of various ice covers and discharges in HEC‐RAS to evaluate flow hydraulics restrictive or conducive to ice cover formation. Model results demonstrated that initial ice jam locations influence a range of potential velocities that may prevent ice formation. Long‐term records showing increasing discharge on the Tanana during freeze‐up may impact the occurrence of persistent OWZs and future river ice regimes. Our findings indicate that channel form, discharge, and ice jams may explain the occurrence of many OWZs in otherwise ice‐covered rivers. Thus, considering channel form, hydrology, and ice‐affected flow hydraulics will allow for better prediction of current and future river ice conditions, leading to improved winter travel safety across Alaska.