Whenever Risen Reef or another Elemental enters the battlefield under your control, look at the top card of your library. If it's a land card, you may put it onto the battlefield tapped. If you don't put the card onto the battlefield, put it into your hand.
If you put the card into your hand, you don’t have to reveal it. You also don’t have to tell your opponent whether it’s a nonland card or a land card you decided not to put onto the battlefield tapped.