The pool was formed in the same geological event that created the Balcones Fault. The water temperature is a chilly 68 degrees year-round and the main spring (under the diving board) gushes an average of 27 million gallons of water per day.
The springs have been used by humans for millenia. Native Americans considered the springs sacred and believed the waters had healing powers. In 1837, William "Uncle Billy" Barton bought the land the springs are on, along with several other tracts in the area. The park containing Barton Springs is named after Andrew Jackson Zilker, aka "Colonel Andy", who bought the land in 1901. He donated the land to the city of Austin in stages between 1917 and 1934. The land was first donated to the Public Free Schools of Austin, with the agreement that the city would buy the land from the schools at somewhat inflated prices.
In 1929, the city added the lower dam that extended the pool to its present 1,000 feet in length. The upper dam was added in 1932.
Extremely silly trivia: Robert Redford learned to swim in Barton Springs pool at the age of five, while visiting relatives in Austin.
For more info, visit: http://www.ci.austin.tx.us/parks/bartonsprings.htm