There can be little doubt that this well-known passage in the Æneid is the original of Tennyson's image; that, in fact, it is an excusable plagiarism on the part of the latter, who, in introducing, his image, has, I think, missed the appropriateness, and therefore increased beauty, belonging to it in the original passage of Virgil.
When Æneas is journeying up the Tiber to visit Evander, the river, in order to lessen his labours—
"refluens ... substitit unda;"
but notwithstanding this, the journey was arduous as is shown in the whole of the three lines 94-96.
"Olli remigio noctemque diemque fatigant,
Et longos superant flexes, variisque teguntur
Arboribus, viridesque secant pacido æquore silvas."
That is to say, "They labour at the oar till night is wearied out, and day also is obliged to give place in its turn; they master one by one the long serpentine bends of the river, and, though covered and inclosed by the varied foliage above them, they cut their way through the opposing woods, which lie, as it were, in their path in the shadowy surface of the clear, still water."
The word placido is surely sufficient to prevent any one falling into the common-place interpretation alluded to by your correspondent as the one "usually given."
H. C. K.