Undines are the main water elementals that live in rivers or fresh water. Water represents the fluidity of the emotions, moving from past lives through to present life and on in to the future. The whole point of water in the tarot cards is showing that everything about life is transient and always moving along.
Water can also represent energy in the same way that the water wheel gave power to the early mills and a flowing river is a very good representative of the Roman Goddess Sulis - as in Aqua Sulis, the Roman name for the city of Bath.
Contained water that has no outlet becomes stagnant and stale, it shows a foetid and polluted state of heart and mind.
Go with the flow and run away on the River of Life. The River Styx however, ran through the Underworld and carried souls to a different form of transience.
I remember snatches of a song that as a five year old in class our teacher sang to us.
It went as following :-
The river sang softly to the leaves on the trees
I am waiting to take you on a journey with me
So the leaves fell down gently
On a bright Autumn day
and floated with the river
Far, far away
The river is never static and life does not exist successfully in stasis.