Flow through the voids of breakup ice jams can be a major portion of the total river discharge, especially where the jam is so thick as to be grounded. Very little information is available on this question; it is derived from laboratory tests or from numerical modelling applications, and there is a discrepancy between respective seepage coefficients. A field program has been carried out to measure the flow through the voids in ice jams by taking advantage of favourable access conditions created by an ice-retention structure on the Credit River. The data obtained during two breakup events are described and an integral method of analysis is developed to identify areas of grounding and determine seepage coefficients. Values of the latter are in agreement with those deduced by numerical modelling.Key words: flow, ice jam, modelling, seepage, voids.