Elvis' version of the song led Paul Simon to quip that the song would forever be an Elvis song, not a Simon and Garfunkel song
"Bridge over Troubled Water" is the title song of Simon and Garfunkel's final album together, Bridge over Troubled Water. It reached number one on the Billboard Hot 100 chart on February 28, 1970, and stayed at the top of the chart for six weeks. It was replaced at the number-one spot by The Beatles' "Let It Be".
This song's recording process exposed many of the underlying tensions that eventually led to the breakup of the group after the album's completion. Most notably, Paul Simon has repeatedly expressed regret that he allowed Art Garfunkel to sing this song as a solo, as it focused attention on Garfunkel and relegated Simon to a backing position. Garfunkel said that the moment when he performed it in Madison Square Garden in 1972 was "almost biblical". In recent performances on the "Old Friends" tour, Simon and Garfunkel have taken turns singing alternate verses of the vocal.

As the song ends, sounds of a thunderstorm are heard. The last note, on a violin, is a long, drawn out B-flat that lasts ten seconds.

[edit] Writing and recording
Simon wrote the song in the summer of 1969 while Garfunkel was filming Catch 22 in Mexico. It was written on the guitar in the key of G, though an early demo version Paul Simon detuned the song on his guitar to an F.

The song originally had two verses and different lyrics. He specifically wrote it for Art and knew it was going to be a piano song. He based the lyrics on a line "I'll be your bridge over deep water if you trust in me" by Swan Silvertones. It has elements of a Bach chorale as well.

Art reportedly thought Paul should sing it as he liked Paul's falsetto on the demo. Once in the studio Roy Halee, their producer, and Art thought the song needed three verses and needed to be 'bigger' sounding. Paul agreed and spent two hours writing a third verse which he always maintained you could tell was added on later.

Art attempted the lead vocal on two occasions but it didn't meet with his satisfaction. He then went to St. Bartholomews Church at 109 E 50th Street, in Manahattan -- 2 blocks from the recording studio -- and thought about the song. That day he nailed the lead vocal. Verse one and two of the song were recorded in New York. The vocal for the final verse was later recorded in Los Angeles.

Larry Knechtel spent four days working on the piano arrangement. Art came up with the intermediate piano chords between the verses while working with Knechtel.

Elvis' version of the song led Paul Simon to quip that the song would forever be an Elvis song, not a Simon and Garfunkel song