The "ideal" depth for beaver? Depends on the slope of the river bank, could be up to 18 inches or more. Four, 6 or 8 inches would be my favorite water depth. As far as "guessing" the water-level, one thing you can depend on is a scanner or "weather-box", and listen to the local NOAA weather station. I assume you're trapping the Shenandoah River? As mentioned above, rising water is a pain, just one of those things where you're hands are tied, about the only thing to do is listen to the weather/stream report, make your best guess (where to set the traps) and hope for the best. Here where I live, it's not unusual for the rivers to rise/fall 6 or 8 inches overnight, just depends on the temps. Water trapping requires you to stay on top of the weather even more than dirt-trapping.