Find A Way In

You are malnourished, weary, and weak.  Your kingdom has been at war for a short time but, for a warrior like yourself, it seems like it has lasted forever.  You are in need of food and rest but you must find ten of your comrades who have been captured by the ogreish, pitiless Grogaians.  You have trudged many miles and now stand before the caverns where the Grogaian warriors have imprisoned your friends.  There are two entrances to inside the caverns, but they are both equally dangerous.  There is a boulder, easy to move, blocking a doorway and a river flowing deep inside.