Dr. Beltaos has given an excellent overview of the exsisting knowledge on river-ice processes, and has identified five areas of insufficient knowledge regarding river-ice hydraulics. I have made an attempt (the first ever, to my knowledge) to numerically model the entire river-ice breakup and jamming process (Guo and Song 1994). Based on my modeling effort, I wish to elaborate on three of those areas and point out three additional areas that I feel are critical to modeling the formation and release of an ice jam during ice-cover breadkup. To facilitate the discussion, I have include in Figure 1 the schematic layout of two typed of ice breakup front.

The six areas of insufficient knowledge identified during my modeling effort are:

1. Movement of the first type of breakup front
2. Movement of the second type of breakup front
3. Thickening of the ice jam by shoving
4. Thickening of the ice jam by frontal submergence and under-cover accumulation
5. Surface transport of ice
6. Unsteady water flow