In the sixth century St. Gregory composed a commentary on the book of Job. Upon beginning he realized the task was more difficult than imagined: “[When] I learned the extent and character of the task to which I was compelled, being overwhelmed and wearied with the mere burden of hearing it, I confess I sank under it.”
Gregory was interpreting Job according to the Four-Fold Senses which was common in his time (the Quadriga): the historical sense (plain sense), the allegorical sense (typological), the moral sense (tropological), and the anagogical sense (pertaining to the last or ultimate things). Here was Gregory’s challenge: on one hand, he sought to avoid missing the obvious meaning that stared him in the face in the historical or literal sense; but on the other hand, he wanted to avoid missing the spiritual senses that lie a bit “deeper.”
So, to explain the way in which Holy Scripture is both shallow (easily accessible in its historical sense) and deep (requires spiritually discernment) Gregory used the metaphor of a “river.” The Bible is deep enough that the most devout and skilled among us can never reach the bottom, and shallow enough that the simplest among us can swim.
The word of God, by the mysteries which it contains, exercises the understanding of the wise, so usually by what presents itself on the outside, it nurses the simple-minded. It presents in open day that from which the little ones may be fed; it keeps in secret that whereby men of a loftier range may be held in suspense of admiration. It is, as it were, a kind of river, if I may so liken it, which is both shallow [planus] and deep, wherein both the lamb may find a footing, and the elephant float at large. Therefore as the fitness of each passage requires, the line of interpretation is studiously varied accordingly, in that the true sense of the word of God is found out with so much the greater fidelity, in proportion as it shifts its course through the different kinds of examples as each case may require.
Bernard McGinn gives an alternative translation
Just as the divine Word exercises prudent persons by mysteries, so, for the most part, it refreshes simple persons by the surface sense. Indeed, it is surely like a river, if I may say so, which is wide and deep, in which a lamb may walk and an elephant swim. Thus, as the fitness of each individual passage requires, so the level of exposition is carefully varied.
It is a powerful and very sticky metaphor for describing the depth of Scripture. Like a river deep enough for elephants to swim but shallow enough for lambs to wade, Holy Scripture never disappoints either the mature or the immature.