Once you've avoided the boulder, go up that ramp. At the top, you'll find a pitcher, perfect! Come back to the pool to fill it --
-- and discover that it's some sort of puzzle jug. Blast! Read on for directions on solving it.
Click on the up and down arrows to discover that there are three different plates you can move up and down to cover the holes, green, yellow, and silver. When all the holes are covered at once, the pitcher is sealed.
One way to tackle this is to pick one color and just move it repeatedly, either up or down, and stop where it covers the most holes. Then do the same with the other two colors.
This isn't too hard, though it can be irritating. Here's how I did it:
Now that the pitcher works, fill it with water from the pool and pour the water in the bowl to the right of the tree three times. This opens the door with the evil face to let you into the River of Fire area.