I don't think the fish ever just stay put in an event like that. They will just travel the shoreline, or the path of least resistance until they possibly come to an unpassable section of river, but I've never seen one that was caused by high water. Fish can make it through some incredible water. I think what's more important to know in a hypothetical situation like this, is that once the clarity of the river deteriorates to zero, and the water becomes so full of debris that you can't fish, THEN it becomes impossible to catch fish. Clarity is what you should be concerned about, not the actual volume of water coming down the river. And in saying that, you don't need much vis at all to still be successful. A change of tactics is all that's needed. Huge spinners become effective in these situations. You know, the ones that always look to ridiculously big to be useful? Those ones. Just look for water that gives the fish some respite from the heavy current, and drift a huge colorado blade through it.