Formation of a hanging dam, a large, thick sub‐surface accumulation of frazil ice that blocked river flow, altered physical environmental conditions dramatically and common carp Cyprinus carpio, and brown trout Salmo trutta, evacuated the pool in which the dam developed. A mean of 80·1% of the pool volume was filled with frazil ice. Mean and bottom water velocities in the pool increased from 6 to 27 cm s−1 and from 4 to 21 cm s−1, respectively, when the hanging dam formed, and water depth decreased from a mean of 2·25 to 0·45 m. Activity levels of fish leaving the pool varied. Two carp had higher swimming activities when in the over wintering pool, compared to the period when they were out of it, and a third showed opposite results.
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Brown et al. (2000) studied this question.