Not to shoot down your theory in flames, Richard (I don't do that when the theory in question has to do with symbolism because as I think we will often find and has been correctly stated, symbols can have multiple meanings), but let me just point out there might be another way of looking at the water flowing from the tomb in the painting. http://www.swarthmore.edu/Humanities/ps ... rgod2.html
Poussin's first Et in Arcadia Ego painting, c. 1629-30, includes a figure identified by some commentators as the river god Alpheus in the lower right
. The main action of the canvas features rustics contemplating a tomb whose writing tells them `et in Arcadia ego,' that death too (as well as beauty) lives in Arcadia. Peripheral to the main action, in the lower right corner of the picture, his back to the viewer, Alpheus waits and watches water spill from his urn:
Alpheus' strong, naked back makes him seem allied with nature, not culture, and the flowing water seems to suggest the continuity and transcience; the flowing of time far beyond the brief span that will be occupied by the rustics' presence in this scene
, or even in their lifetimes in Arcadia. The god is at home on this spot, he isn't going anywhere; he and his urn were there before this scene occured and will be there after its little drama is played out.
Alpheus is the appropriate river god to have grace this scene because he is associated with love and thus with Arcadia. Ovid's Metamorphoses tells his story. He pursued a beautiful young woman, Arethusa, who swam in his waters. She appealed to Artemis, goddess of the hunt, to escape him and was transformed also into water. But to aid her escape Artemis split the earth underground between Greece and Sicily, allowing Arethusa to flow through this hidden passage and emerge as a spring in Sicily, the source of some of the purest and most sought-after waters on the island, a spot now sacred to Artemis and named after Arethusa. But Alpheus followed, intermingling his waters with hers, and legend has it that a wooden cup dropped in the river Alpheus in Greece can re-emerge in Sicily. (Cf. Edith Hamilton's Mythology.)
An irony is in the story that Hamilton does not mention: Artemis the goddess of chastity protected Arethusa in one way but in another way she did not; the means of escape she provided also became the means by which the Greek river god could unite with the woman even more fully and more permanently than she feared.
This story is not known before Ovid's famous account of it in the Metamorphosis. It's not clear whether it has Greek sources or not, unlike many of Ovid's other stories in the Metamorphosis, which clearly do have been passed down to him from Greek tradition. But this apparently precendent-less story is one of the ones that most overtly seems to represent the cultural connections between Greek and Roman culture as the Romans understood them: a deep and powerful intermingling contributes to the "purity" of the Italian spring. The story also suggests that some of the most powerful connections are not the obvious ones but the ones hidden underground.)
For Poussin in Et in Arcadia Ego, the presence of the river god Alpheus signifies the flowing and vivifying presence of the arcadian tradition in Greece and Rome (and also in Poussin's time). But he also stands for a kind of eternity in marked contrast to the mortals and lovers who try to read the writing on the tomb.