Rivers that feed into the main rivers emptying a continent will be affected by several factors that will cause flooding beyond what those living along their banks suppose could ever happen. Flooding today is a comparatively local affair, in the US affecting several states at most. Where one river swells, another is not so affected, and thus the drainage happens more rapidly than it would if all the rivers were swollen. During the pole shift, there will be torrential rains everywhere, on and off, as the clouds are dropping water whipped up from the oceans. Take away the ability of the wide Mississippi to drain the Missouri River, and what would a flooded Missouri become? A lake, spreading out over the nearby lands, creeping into every low lying piece of land the fingers of the spreading waters could find. When seeking safe places in land high enough to escape water from the melting poles, and far enough inland to escape tidal waves, think in terms of seeking relatively high land to be safe during the pole shift from such flooding.