Complex sentence: In these decades prior to 2000, Glen Canyon Dam frequently released more than the required to Lake Mead each year, allowing Lake Mead to maintain a high water level despite releasing significantly more water than for which it is contracted.
Split into simpler sentences: In these decades prior to 2000, Glen Canyon Dam frequently released more than the required to Lake Mead each year. This allowed Lake Mead to maintain a high water level despite releasing significantly more water than for which it is contracted.