At yesterday’s monthly Colorado Basin River Forecast Center briefing, Greg Smith noted, by way of analogy, the winter of 1976-77. Smith explained that he wasn’t forecasting – the fact that the evolution of this year’s forecast is similar to 1976-77 doesn’t mean that the rest of this year will be like that year, or that this year’s runoff will be like 1976-77. But picking analog years is a great communication tool, to give us a sense of what actually happened, historically, in conditions similar to those we might see today.
So let’s look at 1976-77.
- Naturalized inflow from the Upper Colorado River Basin at Lee Ferry was 5.4 million acre feet, the lowest in the USBR’s Natural Flow Database (which goes back to 1906)
- Lake Powell dropped 3.4 million acre feet, the fourth largest one-year drop since Glen Canyon Dam was built (1990, 2002, and 2013 had bigger drops)
- I graduated from high school
None of these are encouraging analogs.
And what happened just 6 years later in 1983-1984? Both Powell and Mead were overflowing and the concern was about downstream flooding. Let’s hope history does repeat itself.
I wonder though Tom if the reservoirs were as low at the beginning of 76 as they are today. Probably not. A year as wet as 83 would certainly help but it’s not gonna be anywhere near over flowing.
Patrick, you are correct that Powell and Mead were higher at the end of 1977 than they are today (although Powell was only 8 feet higher). But natural flow in the Colorado River averaged 24 million acre-feet a year in 1983 and 1984; that may not be enough today to cause the reservoirs to spill, but it would completely change the current situation. My point is simply that one shouldn’t be discouraged that this year looks like 1977 given that average annual flow in the river over the following 10 years (1978-1987) was 18.35 MAF.
Pingback: You can check out any time you like, but you can never leave - jfleck at inkstain
Pingback: Blog: What happened in the Colorado River Basin in the winter of 1976-77? | H2minusO Blog