Symbolism, Imagery, Allegory
The stream on Aslan's mountain has some curious properties. For one thing, it is unique. As Jill finds out, much to her dismay, the stream is the only running water on the mountain. But it also makes Jill "feel ten times thirstier than before" (2.23) when she sees it, although it quenches the worst thirst in just a few sips. It's almost as though the stream has an awareness of its own and can reach out to whomever is in need.
At the very end of The Silver Chair, Caspian's dead body appears under waters of that same stream. The water itself is not responsible for bringing Caspian back to life (it's the blood of Aslan that does the trick), but it does play an important role in reinvigorating Caspian and giving him back his life.
Because it does play this role, the waters might remind us of something biological, like the amniotic fluid that surrounds a growing baby in the womb and protects it until the time of its birth.
And since Lewis is using Christian imagery to connect Aslan to Christ, it's important to look at the Christian symbolism of this peculiar stream. For one thing, it can be connected to the Jordan River, where John the Baptist baptized people (and Jesus himself) to usher in a new era of belief. Lewis uses the water of the stream to echo the waters of Christian baptism, which promise new life to those who undergo the ritual of baptism.
Aside from the Christian symbolism, you might be reminded of other, more ancient connections to the rivers Styx and Lethe from Greek mythology. Those are two of five rivers of the underworld, Hades, that separate the underworld from the living and scrub the dead of their memories. In this sense, the rivers of the underworld prepare the dead for their new existences in the afterlife.
Whew. Water is a wickedly complicated symbol, especially in the hands of someone so steeped in ancient literature as Lewis. Believe us, all of these symbolic uses of water are meant to whiz through your brain when you see them on the page, though, so let them just wash over you, enhancing your appreciation of the story.