Finally, An Intelligent Question
What is meant by the 30' depth is, in general, that is the depth at which fish are being seen on the LCR and/or caught. Naturally, striper being a pelagic species, will use the whole water column at various times. It doesn't matter if the bottom is 40' deep or 60'. Baitfish and therefore stripers, have been holding/feeding at right around 30'. That is the depth they have been preferring during the reporting period. As the water warms, baitfish move shallower and stripers follow.
So the 30' is just a starting point. Weather conditions, current and other things will occur to change that depth during the day.