(Overgrown River isn't a spell.)
Overgrown River enters the battlefield tapped unless you pay .
: Target player exiles a card from his or her graveyard.
: Add or to your mana pool.
Put Overgrown River on top of your library: Add to your mana pool.
.
: Target player exiles a card from his or her graveyard.: Add 
or 
to your mana pool. to your mana pool. 
