tl;dr: Very Very Wet. Total rainfall exceeds 1982-1983, but current snowpack is much lower at this point in time and more in line with the the Spring of 1998 or 2011.
Final South Fork American River water year forecast as of May 8, 2017: Wet
The Final Water Forecast for 2017 was released Monday May 8th, with the Water-Year forecasted for Wet. Specifically it’s forecasting the American River below Folsom at 7.810 million acre-feet (MAF), with 3.500 or greater being considered Wet[1. An increased forecast of 0.375 MAF more than forecasted in April 2017]. In fact, we’ve already had 5.922 MAF of unimpaired runoff, so even if the heaven’s remained dry through the end of this water year, we’re already locked in on it being a Wet classification.
Rainfall vs Runoff vs Snowpack
There has been a lot of talk in the media as of late, of different regions setting all time rainfall records, and well there should be. But we should be careful to understand that just because this is the biggest water year on record, does not mean that we’ll see the largest May and June flows on record.
Some important differences between the 1982-1983 water year and the 2016-2017 water year:
While this year will go down as the highest water year on record, but much of that water has already moved into the ocean. This can be most clearly seen by looking at the current California snow water content, which shows that there is significantly less water still on the mountains as compared to the 1982-1983 water year on May 12th.
Not pictured but worth noting: The Northern and Southern California current numbers are already significantly below the 1998 and 2011 numbers.
For comparison, the unimpaired runoff for the American River below Folsom in 1983 in 1,000 Acre-Feet was 983 for May and 942 for June[1. B120 October 1983 1. Table 6]. Forecast for 2017 are predicting 940 for May and 695 for June[1. B120 May 2017 page 5]. While 1998 had 694 for May and 787 for June[1. American River at Folsom Full Natural Flow], and 2011 had 684 in May and 793 in June[2. American River at Folsom Full Natural Flow]. So if those predictions hold, we should see May flows higher than we did in 1983, however June will likely drop to smaller flows than we saw in June of 1998 and 2011.
Guaranteed Minimum Flows
This final forecast is technically only valid until early October when the final numbers are crunched, but it’s a reasonable assumption that the following will not change barring an act of congress[1. I’m not intending to be flippant. It really would take a literal act of Federal Congress to change things.].
It’s worth a reminder that how quickly or slowly the snowpack melts could result in significantly higher flows during the remainder of Spring and early Summer.
May
Monday – Friday: 1,500 cfs 9am-12pm
Saturday & Sunday: 1,750 cfs 7am-1pm
Minimum Flow: 300 cfs April, 500 cfs May
Summer Flows – May 27th through September 4th
Monday – Friday: 1,500 cfs 8am-12pm
Saturday & Sunday: 1,750 cfs 7am-1pm
Minimum Flow: 500 cfs May & June, 350 cfs July, 300 cfs August, 250 cfs September
September 5-30, 2017
No guaranteed raftable flows Monday, Tuesday, Wednesday
Thursday, Friday, Saturday, Sunday: 1,500 cfs 9am-12pm
Minimum Flow: 250 cfs
October Flows
No guaranteed raftable flows Tuesday, Wednesday, Thursday
Mondays & Fridays: 1,300 cfs 9am-12pm
Saturdays & Sundays: 1,500 cfs 9am-12pm
Minimum Flow: 250 cfs
November through early February
No guaranteed raftable flows Monday-Friday
Saturdays & Sundays: 1,500 cfs 9am-12pm
Minimum Flow: 250 cfs
The final water year report should be released sometime in October.
Reservoir Storage – Of Interest
With a high snow pack and reservoirs on the Upper American River Project (UARP) filling up, here are some[1. There are many more reservoirs in the system, these are just the most impactful.] quick water storage related numbers[1. For reservoir storage, current elevation is a hard number, while current storage is a rough calculation based on estimated capacity at that elevation. Natural sediment deposits over time, and from high runoff/melt conditions, can modify (sometimes greatly) actual capacities.] that might be of interest.
As of May 12, 2017 – 10 AM
Loon Lake – 6407.9 ft (spills at 6,418 ft) with 66,397 AF (spills at ~76,500 AF)
Ice House – 5,448.6 ft (spills at 5,436.5 ft Nov-April 1, then April 2-15: 5,445 ft max , April 16-Oct: 5,447 ft max[1. April-Oct these are the maxes, but they can choose to spill earlier.])
Union Valley – 4868.7 ft (spills at 4,855 ft Nov-April 1, then April 2-15: 4,865 ft max , April 16-Oct: 4,867 max[1. April-Oct are the maxes, but they can choose to spill earlier by opening the Ogee gate.])
Slab Creek – 1,852 ft (spills at 1,850 ft) with 13,651 AF (spills at ~13,350 AF)[1. Official data claims 1,870 ft for spill and 16,600 AF of storage, but historical data shows 1,850 ft and 13,350 AF to be more realistic – I’m looking into it.]
Chili Bar – 1,000.9 ft (spills at 997.5 ft) with a max capacity of only about 1,340 AF.
Related Links and References
Quick B120 Report – Just the numbers
Detailed DWR Bulletin 120 Report – Note: this report is typically released a week or two after the quick report comes out.
American River at Folsom – Full Natural Flow (FNF) record of what actually happened.
Most Recent Snow Water Content Data – Limited to American River Basin
Note: This page is updated with current forecasts as they become available. Previous forecasts are archived for reference.
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