According to the walkthrough for the game:
Combine the spear and the bow to find out that the spear is too big. Look at the waterfall that was frozen before and see that it is flowing. Stand at bottom water basin and see that Geron is large. Go to the upper water basin and see that Geron is now smaller. Place the spear in the water at upper water basin. Go down to lower basin and take the shrunken spear - arrow.
Let's also see if someone else can help you with this or send you a saved game.