I also agree with PJ and Howitzer, and I think these threads are useful only insofar as they help people discover the meaning of their dreams themselves.
So here's my two cents, take with many grains of salt.
my mind kind of goes two ways on this one
1.
This dream seems to come arise from some sort of personal transition. Whatever it is in life, in the dream its the transition from tree to shore. You have some subconscious fear (founded or not) that the transition will be dangerous to your relationship, and to your girlfriend in particular. You already are conscious of the danger (the stream) but part of you feels that the danger may be greater than you know (the deepening of the water), and that the situation is different than what you think it is (everything changing in the end)
2.
The whole scene plays out mostly to set up the most important moment, where she is in the water and something stops you from going in.
I would focus on that "something" - was it closer to fear? the feeling that she had to get there on her own? something else? In this line of thought I like Howitzer's idea, and would only add that going from a tree to shore could symbolize quite beautifully a relationship moving to a more committed level.
anyhow, I'm about two weeks and many dreams late
good luck, and if the meaning still eludes you, keep an eye out to see if the message is repeated!