How do you shut off the water supply to the water heater?

Water is flowing out of the top of my gas water heater. I am pretty sure I was able to try off the gas - there was a switch in the control box, but I can not figure out how to turn off the water supply. I was able to find on pipe on the left that had a blue knob that I could turn a quarter of a turn clockwise, but it didn't even slow down the leak.