In 2012 and 2013, the Colorado River basin experienced its worst consecutive water years on record, prompting a low Glen Canyon release in 2014 – the lowest since 1963, during the initial filling of Lake Powell – in the interest of recovering the level of the upstream reservoir, which had fallen to less than 40% capacity as a result of the drought. Consequently, Lake Mead's level fell significantly, reaching a new record low in 2014, 2015 and 2016. In 2014, its record low was on July 10, 2014. On June 23, 2015, Lake Mead reached another new record low when it briefly fell below , the first official "drought trigger" elevation, for the first time since the lake was filled. If the lake is below this elevation at the beginning of the water year, an official shortage declaration by the Bureau of Reclamation will enforce water rationing in Arizona and Nevada. The water year begins October 1 to coincide with seasonal Rocky Mountain snowfall, which produces most of the Colorado River's flow.