The water level at Kaub, a narrow and shallow point west of Frankfurt, is currently 34 centimeters. According to data from the German government, the level will rise to 148 centimeters on Tuesday. At that depth it pays for most, if not all, inland vessels to use the waterway with a decent load.
The water levels measured along the river are, incidentally, standardized values. Those values do not correspond one-to-one with the actual depth of the water. At a water level of 40 centimeters or less, it is not profitable for many inland vessels to navigate that part of the waterway.