Occasionally, when, frequently, often, sometimes, etc. refer to time. In places, rarely, where, commonly, etc. refer to space. Interval refers to either. Not Gravel layers occur frequently in the deposit. (Why do they come and go?) but Gravel layers are common ... Not In the spring discharge commonly exceeds 25 m3/s. (...often...). When the river turns north, its gradient is steeper. (Where ...)