On Friday April 27, 2012, releases from Anderson Ranch Dam hit 6,000 cfs. This is a flow quantity rarely seen on the South Fork Boise River. Based on records dating back to 1943 (seven years before the dam’s completion) here’s the top years where peak flows got nearly to, or exceeded, 6,000 cfs.
Since 1943 here are the years where peak flows were 6,000 cfs or better
Spring 2006 was the last time we saw flows higher than 6,000 cfs. A couple of years in the 1980s the peak flows were even higher. There were several years in the 1950s and of course the banner year 1965 with the high flows, but dam operations in that era were different than how Anderson now operates (i.e. trout anglers don’t want to know what the winter flows were back in those days).
Here is the peak date and flows for each year:
Date Peak Flow
May 25, 1956 9,770
April 17, 1943 8,720
May 31, 1986 7,890
June 12, 1983 7,610
May 28, 1952 7,130
May 23, 2006 6,490
May 18, 1965 6,170
April 28, 1946 6,040
May 30, 1958 6,030
April 27, 2012 6,006
May 26, 1951 5,920
One thing that jumps out from this list is with few exceptions the peak flow date has been a May or June event. But the exception is this year, 2012 where April 27 is the earliest since Anderson Ranch Dam was completed. Two other April dates in 1943 and 1946 were years when the dam was under construction. Natural inflows to Anderson Ranch can peak in April or early May, as will be likely for 2012. But the reservoir captures the water if there is space. In 2012 there was little space left to spare so water managers opened the spigots this past week when the rains came across southwest Idaho. The gage at Featherville shows the peak flows in the early morning hours of April 27 peaked at 8,500 cfs.
The peaks in 2006, 1983 and 1986 were in late May to mid June, most likely when the reservoir filled and inflows were passed through downstream. This year’s peak on April 27 (and sustained for a couple of days so far) represents an anticipatory action where flow releases peak early to continue to try to make a little room for snowmelt in May. Anderson pool is now 97 percent full, but as of April 29 the inflows appear to be less than the 6,006 cfs outflow.
