Estimating river discharge during ice breakup from near-simultaneous satellite imagery

•River discharge is a key driver of ice breakup and jamming processes.•Discharge is not measurable under broken and moving ice.•Near-simultaneous satellite images track motion of ice debris.•Ice velocity field and local bathymetry allow discharge computation.•Methodology proven for the first time and limitations discussed.

River ice breakup has many socio-economic and ecological implications that primarily result from the formation and release of major ice jams. A key driver of breakup processes is the river discharge, but the presence of fractured ice sheets or moving ice rubble render its measurement or estimation very difficult. Here, we exploit the fact that the acquisitions of individual images of a satellite stereo scene are separated by about one minute. Between three stereo images taken from the PRISM instrument onboard the Japanese ALOS satellite on May 20, 2010, we track the displacements of river ice debris using automatic image correlation along a 40 km long reach of the Mackenzie River, just above the entrance to its delta. This results in an almost complete ice velocity field over the river area studied with a spatial resolution of 25 m and an accuracy of ~ 0.07 m/s for the speeds. Channel bathymetry, slope, and hydraulic resistance along the study reach, known from previous studies, are then utilized to compute discharge at 15 cross sections. Calculated values vary from ~ 20,000 to ~ 27,000 m3/s and are comparable to 23,800, the value estimated by Water Survey of Canada. Variability is much smaller, and close to that of conventional flow measurements, within the relatively straight sub-reach immediately upstream of the delta, where the effects of boundary constraints caused by a sharp bend farther upstream are minimal. This study proves for the first time that it is feasible to estimate river discharge during ice breakup at a reasonable accuracy using near-simultaneous satellite images. While this work had to rely on data that were coincidentally contained in satellite archives, special satellite or aircraft acquisitions could greatly enhance the probability of obtaining suitable data and thus the applicability of the method.