I heard a story about Baba creating holy rivers from his toe. What does the Satcharitra say about this event with Das Ganu?

Yes, this miraculous event involving the devotee Das Ganu is detailed in Chapter 22 of the Shri Sai Satcharitra. When Das Ganu expressed his desire and asked for permission to go bathe in the holy waters of Prayag, which is the confluence of the Ganga and Yamuna rivers, Baba provided an alternative. Baba instructed him, "Know this big toe of Mine to be Prayag; bathe right here." When the devoted Das Ganu placed his head upon Baba's feet in obeisance, the waters of both the Ganga and Yamuna miraculously manifested right there from Baba's big toe. This story from Chapter 22 powerfully illustrates that for a devotee, the Guru's feet are the ultimate pilgrimage site, containing all sanctity.