Title: The Colorado River Hot Drought

Abstract: The nation's two largest reservoirs, Lakes Mead and Powell, were brim full in the year 2000. 
Four short years later, they had lost enough water to supply California its Colorado River share for 
over 5 years. Now, seventeen years later, these reservoirs have still not recovered. A primary cause of 
the reservoir declines is the ongoing drought which started in 2000. The period from 2000 to 2014, the 
worst 15-year drought in the gaged record, had annual flow declines of over 19% (Figure 2). A similar 
15- year drought in the 1950s had annual flow declines of 18%. The 2000-14 period, however, had only 
75% of the precipitation reduction that occurred in the 1950s, begging the question of what caused the 
more recent, most severe drought on record? The answer is simple: higher temperatures. From 2000 to 
2014, temperatures in the Upper Basin of the Colorado River, where most of the river runoff is 
produced, were 1.6 Fahrenheit higher than the 20th century average.   About one-third of the 2000-14 
flow decline was likely due to higher temperatures in the basin. High temperatures cause earlier snow 
melt, which in turn leads to a longer growing season and thus more water demand from plants. Higher 
temperatures also increase plant water use, and increase evaporation from water bodies and soils. And 
finally, as it warms the atmosphere also demands more water from any available source. All of these 
effects conspire to reduce the amount of water available to flow down the river; the net impact of all 
these warming-related factors is nearly 4% less flow per degree Fahrenheit warming. With temperatures 
projected to rise as much as 9F by 2100, this bodes ill for future flows. A second problem is that the 
river is overallocated and overused. In the Lower Basin, this problem has a name: 'The Structural 
Deficit'. 'Structural' refers to a long-time water allocation and use practice that is challenging to 
fix.