I’m inclined to say that rivers are not illusions. But I think it all boils down to our problem with defining “a thing.” If “a thing” is the sum of its parts, and those parts are continually changing, then that “thing” can never really exist. Its name is merely a useful placeholder.
I guess if you put a gun to my head I would say the river and the ego (You) technically exist, as they can be perceived. But it’s the reality of that perception that I challenge. Our memories of subjective experience also change from moment to moment (see research by Elizabeth Loftus), so I think your definition is lacking as well.