Coolant is boiling in the reservoir

After driving my subaru forester turbo for like 50 minutes I realize that the car is returning water to the coolant container. The container gets full and one can see it boiling, after cooling the water goes back to the normal level. This car does not overheat because the temp gauge does not go above the normal place. I have changed the radiotor cap but still the problem remains. Whats the issue