There really was a drier period beginning around 1900 but in actuality it was a return to a more normal weather pattern.
The latter half of the 19th Century was unusually wet in the West.
There is a global 60-70 year climate short cycle, oscillating above a longer (larger) 900-1100 year climate cycle (Minoan Warm Period = 1100-1000 BC, Roman Warm Period = 200-100 BC, Medivial Warming Period = 1100-1300 AD, Modern Warming period = 2000 AD.)
The short cycle (warmer in 2000-2010, 1940’s, 1885-1895) out west tends to be a bit drier. The mid-points of those hot spots are cooler (obviously!) include the record-setting high water flows measured in the Colorado River in 1915-1916. The cooler years of 1970 (Yes, Virginia, the world’s average temperature DECREASED substantially between 1940 and 1975 even though global average CO2 levels were increasing!), 1915, 1855 have more rain and more runoff in the far west.
But then the government decided that those 1915’s high flows would be continuous all years of every decade. So they negotiated the Hoover Dam water rights as if the river was flowing 20% higher than the real average value.
Excellent points, Mr. Cook. For anecdotal evidence of the cool cycle, just ask any American who celebrated Christmas in the Ardennes Forest in 1944, but better have that conversation soon.