What is the "presence" of the river that is mentioned in Bless Me, Ultima?
The novel Bless Me, Ultima is an incorporation of everyday reality with mythology, visions, folklore, and magical traditions, as best represented by the curandera, Ultima, she who can heal and remove curses with great power and whose soul and life are bound up with the owl that represents her.
In native belief systems, there is a life force to trees, rivers, the earth, the wind, etc. that must be respected and may be called upon by the initiated few to assist or harm humanity--for good or ill. The "presence" of a river refers to this life force that imbues (saturates) inanimate elements of the planet with spiritual qualities and the capabilities of imitating action through volition (choice of will).