Adam Clarke on Genesis 41:18

Seven kine, fat-fleshed - See Clarke on Gen 41:2 (note). And observe farther, that the seven fat and the seven lean kine coming out of the same river plainly show, at once, the cause both of the plenty and the dearth. It is well known that there is scarcely any rain in Egypt; and that the country depends for its fertility on the overflowing of the Nile; and that the fertility is in proportion to the duration and quantity of the overflow. We may therefore safely conclude that the seven years of plenty were owing to an extraordinary overflowing of the Nile; and that the seven years of dearth were occasioned by a very partial, or total want of this essentially necessary inundation. Thus then the two sorts of cattle, signifying years of plenty and want, might be said to come out of the same river, as the inundation was either complete, partial, or wholly restrained. See Clarke on Gen 41:31 (note).