Highguy,
“expound upon how dangerous (or maybe not with a packraft) high water can be, and for what reasons”.
That is kind of a subjective question that many might be hesitant to answer. High-water can be very dangerous. That said, high-water is often the best time, if only time, to float many smaller creeks and rivers. Variables that add to the level of danger include; the speed of the water, the grade or steepness, and possible snags, strainers, and other hazards.
High-water is often fast water. The faster the water the more limited your time to react to obstacles. High-water often lacks safe eddies or safe spots to hide in. It can be difficult to get out.
The steeper the creek the more dangerous. My rule of thumb; < 50 feet per mile no worries, 50-100 feet per mile will have some whitewater, 100-150 feet per mile serous whitewater but usually doable, 150-200 maybe doable, > 200 feet per mile probably un-runnable. The higher volume the river the more serous this is. Small creeks > 200 feet per mile have been packrafted, but at low flows. A high volume river < 50 feet per mile could contain dangerous hydraulics. I would never even consider doing a high volume river that exceeds 150 feet per mile.
Snags, strainers, and other hazards are more common at flood stages. As a river or creek exceeds its banks there are undoubtedly trees, fences, brush, etc, which the current is now running through, around, or over. Encountering such a hazard could be lethal. Add the fact that the water is fast you will have limited opportunity to avoid these hazards. The creek/river at high volume will have a powerful current, and in the event you are pinned, it will be difficult to deal with.
I recommend spending some time in known rivers and creeks at safe levels to develop skills and confidence. During high-water conditions start on some low volume flat creeks and build your way up. I recommend floating with other experienced paddlers and having throw ropes and the knowledge how to use them.
An old rule of thumb for packrafting is “if you wouldn’t swim it, don’t packraft it”. In recent years expert packrafters have entered waters that they would prefer not to swim. However, as a novice I would apply the old rule of thumb.
Forrest