There are four basic thermal features in Yellowstone, Geysers, Hot Springs, Mud Pots and Fumaroles. Each is unique and has specific thermal characteristics. On of those is that they tend to be very hot with most of the water being almost boiling. You will notice that the color of the water will change with depth. This is a result of different bacteria (all being extremophiles) living at different temperatures and the bacteria is what gives the water its color.

It is interesting that Arsenic is one required element for the metabolism of extremophiles.

http://www.theguardians.com/Microbiology/gm_mbm04.htm