Complex sentence: The 474 foot long, 15 foot high concrete dam was constructed in 1914, and in 1994 its apron was raised with a 5-foot high inflatable bladder (the "longest in the world"), which can be raised or lowered in 30 minutes.
Split into simpler sentences: The 474 foot long, 15 foot high concrete dam was constructed in 1914. In 1994 its apron was raised with a 5-foot high inflatable bladder (the "longest in the world"), which can be raised or lowered in 30 minutes.