Complex sentence: Although Glen Canyon was able to meet its required minimum release until 2014, the water level in Lake Mead has steadily declined due to the loss of the surplus water that once made up for the annual overdraft.
Split into simpler sentences: Although Glen Canyon was able to meet its required minimum release until 2014, the water level in Lake Mead has steadily declined. The decreasing water level is due to the loss of the surplus water that once made up for the annual overdraft.