Some consider drought to occur in the Colorado River Basin when water supplies reach drought level (2nd line down on the left graph of Lake Mead's recent elevations), and others may define it in various other ways.
The National Drought Mitigation Center at the Unversity of Nebraska/Lincoln, after declaring that drought is indeed difficult to define, says
In the most general sense, drought originates from a deficiency of precipitation over an extended period of time - usually a season or more . .
We use this, and the following two quotes, as evidence for our declaration of a 20-year drought.
Jan 5, 2014, the New York Times called it
14 years of drought nearly unrivaled in 1,250 years.
As the drought continues, rolling into 2019, it becomes an 19-year drought.
This is the worst drought period in the last 100 years, said Larry Walkoviak of the US Bureau of Reclamation.
Many archaeologists and historians believe it was a 23-year drought that caused the ancestral Peubloans to abandon the ruins we find in the southwest. The Four Corners area (the area surrounding the common corners of Utah, Colorado, New Mexico, and Arizona) has the greatest concentration of ancient cultures and ruins north of Mexico. Discover.com alarmingly suggests we may be looking at a 60-year drought.
The graph segment on the left shows the deepest and longest-term decline of Lake Mead since it started filling in 1937; on July 1, 2016, it declined to its lowest level ever - down 148 feet (45m). An abnormally large snowpack in winter 2011-2012 provided the recent increase shown on the graph, winter 2012-2013 was half of 2011-2012, so the lake level resumed its decline. The same story in 2016-17 and 20017-18: the 2017 snowmelt raised Lake Powell 41' (12.5m), the 2018 snowmelt raised the lake less than 4' (1m).
The 2000-2007 graph segment of Lake Powell on the right from NASA Observatory is strikingly similar. Lake Powell reached its lowest level ever on April 5, 2005, down 145' (44m). The huge snowpack in the Rocky Mountains in winter 2011 allowed for an equally huge release of water from Lake Powell to Lake Mead in 2012, which rebuilt sandbars and restored ecosystem damage in the Grand Canyon. The drought has since intensified, sending the lakes into a continuing decline. 9/18/2020 elevations: Lake Powell is down 102.5 feet (31.2 m); Lake Mead is down 136 feet (41.5 m).
Select the NY Times article menu button to read their story in a new window, a March 15, 2018 NPR article is more recent, but tells bascially the same story. Drought Updates continues this story with overlay graphs - 5 years for Lake Mead and Lake Powell, 7 years for the snowpack.