If you’ve been reading my blog for any length of time, you know I like sources. And moreso, I like hard facts…
A while back I posted a story about Teton Dam and made several comments about American Falls Dam. These comments were based on other websites, the sites did not agree so I had to accept standard deviation. Wanting to have all the facts correct I sent in a request to the Bureau of Reclamation.
This morning I recieved the actual numbers in a very detailed response, Thanks goes to The Burley Office of the Bureau of Reclamation.
Thank you for your interest in getting correct information on American Falls Dam.
Reclamation’s first dam at American Falls was completed in 1927 with a capacity of 1,700,000 acre-feet at a normal maximum water surface of 4354.5 feet. Idaho Power Company had a smaller dam, just downstream, to divert water through their turbines before that time.
By 1973 the concrete of the dam was deteriorating from a chemical reaction between alkaline aggregate and the cement used in the original construction. At that time the capacity of the reservoir was restricted to 1,200,000 acre-feet. At 12:00 a.m., June 5, 1976, the reservoir held 1,145,060 acre-feet.
The following table shows the recorded flows and contents for the first 10 days of June in 1976. Hopefully our e-mail systems will allow the table to remain intact. Inflow and discharge are daily averages and content is the end of day, midnight, observation.Date Inflow Discharge Content . cfs cfs acre-feet 1-Jun 17,695 23,700 1,205,800 2-Jun 15,878 22,700 1,192,270 3-Jun 12,775 21,800 1,174,370 4-Jun 12,460 19,700 1,160,010 5-Jun 11,562 19,100 1,145,060 6-Jun 15,810 20,000 1,136,750 7-Jun 37,195 20,200 1,170,460 8-Jun 40,861 21,300 1,209,260 9-Jun 22,288 21,900 1,210,030 10-Jun 16,965 21,800 1,200,440
As you can see, the reservoir was full to restricted capacity, plus a little, on June 1 and began to draft. Inflow and discharge were decreasing. On June 6 discharge was increased slightly and that slight increased persisted until the reservoir returned to full.
The maximum content on June 9 resulted in a water surface elevation just .2 feet above the restricted level.
Reconstruction, completed in 1978, restored the capacity to the original water surface elevation of 4354.5 feet. In 1977, while the reservoir was drained, a new survey of the reservoir area was made.
Subsequently, the capacity of the reservoir was adjusted to 1,672,590 acre-feet. This is a small adjustment considering that more than 50 years passed between surveys.
So, contrary to what alot of other sites were saying, no they did not “drain” the lake. Intake had been decreasing till the event so discharge had been decreasing too, to 19,100 cfs before the event. Post event they increased the discharge, but since intake had been droping, they didn’t increase it much. Actual capacity was 1.2 million acre feet, the inital post-event discharge was maintained while and the lake dropped to 1.13 million acre feet before water arrived. Accroding to the data the water started arriving on the 7th with an AVERAGE DAILY INTAKE of 37,195cfs. I can only assume that for part of of the day, the intake was around normal (pre-event) and a rather large inital surge of water was well over 37,000. I did some checking and found the following data from the same 10 day period taken at a gauge on the river near Blackfoot, ID.
It shows very similar number to what was provided me in the email response. Though the gauge is upstream so the peaks and averages are going to be earlier than at the dam. This still gives us an average. I was hoping to find an instantaneous reading that would tell us what the inital surge (if any) was. I found the following data from the same gauge.
This graph represents the peak flows for recorded years. Notice that 1976 is the highest year but has a couple of close years too. Not sure the story behind those years. But the corresponding data point for 76 indicates “June 7th 1976 Gague Height 15.44 feet – 53,500cfs (dam failure).”
It would seem we’ve found what I was looking for. Inital surge would have been near 54,000cfs averaging to 37,000cfs intake for the dam for that day.
The dam did fluctuate over capacity slightly as the discharge was changed to reflect the intake. Looks like by the 10th intake, discharge and contents had stabilized to about normal.
Thanks again to the Bureau of Reclamation, Burley office for the American Falls dam history and data. It really clears up descrepancies on other sites and news reports.