Since elevation is obviously a factor in Arizona, knowing more information would be helpful...elevation, which part of Arizona...the more specific you are, the easier it is to eliminate what it can't be or decide what it could be.

My first thought was Populus, looks a lot like nigra.

Any trees around at all, moved/ disturbed soil recently?

I am not familiar with your region& climate. Is it in a sheltered spot?

Bye, Lin

If not Lin's Populus nigra, then some other type of cottonwood. From what I've read, cottonwoods are common along Arizona rivers, so a windblown seed is likely.

Cottonwood.