Dry: March rafting flows of 1,300-1,500 cfs Friday through Tuesday. With only 200 cfs guaranteed on Wednesdays and Thursdays.
South Fork American River water year forecast as of March 8, 2018: Dry
The March Water Forecast Report for 2018 was released Thursday March 8th, with the Water-Year forecasted for Dry. Specifically it’s forecasting the Full Natural Flow (FNF) of the American River below Folsom at 1,350 million acre-feet (MAF), with Dry being considered 900 MAF to less than 1,700 MAF. This is a significant drop from the previous month’s forecast of 1,750. The biggest change will be no guaranteed Thursday water in the coming months, with no significant change to summer flows[1. Just a reduction from 6 hours on Saturday and Sunday to only 5 hours of flow].
The current forecast has an 80% probability range of 1,080 – 1,605 MAF, which would keep us firmly in a Dry water year. However, for comparison, the previous period had an 80% probability range of 1,350 – 2,280 MAF. The forecast should be more accurate as we move forward, but the change from 1,750 to 1,350 speaks to some significant differences between the previous models and what actually happened in February.
The February forecast had predicted 230 MAF of unimpaired runoff during February, however we only saw 98 MAF. That’s only 42.6% of what had been forecasted. For March they’ve only made a small adjustment down, predicting 299 MAF versus the previous expectation of 340 MAF for March unimpaired runoff.
Guaranteed Minimum Flows
The March forecast will be used to determine minimum guaranteed flows from March 11th through Mid-April[1. The new forecast takes effect 3 days after it is issued. Each forecast is typically issued around the 8th]
March through Mid-April
Monday, Tuesday, Friday: 1,300 cfs 9am-12pm
Saturday & Sunday: 1,500 cfs 9am-12pm
No guaranteed raftable flows Wednesday or Thursday
Minimum Flow: 200 cfs
Forecasted based on Current Data
If the forecast of Dry holds, the flows will be as follows.
April-May
Monday, Tuesday, Friday: 1,300 cfs 9am-12pm
Saturday & Sunday: 1,500 cfs 9am-12pm
No guaranteed raftable flows Wednesday or Thursday
Minimum Flow: 200 cfs March , 250 cfs April & May
Summer Flows – May 28th through September 3rd
Monday, Tuesday, Thursday, Friday: 1,300 cfs 9am-12pm
Saturday & Sunday: 1,500 cfs (7am-12pm Sat, 8am-pm Sun)
No guaranteed raftable flows Wednesday
Minimum Flow: 250 cfs May & June. 200 cfs July through September
September 4-30, 2018
Friday, Saturday, Sunday: 1,300 cfs 9am-12pm
No guaranteed raftable flows Monday, Tuesday, Wednesday, Thursday
Minimum Flow: 200 cfs
October through early February
Saturday & Sunday: 1,500 cfs 9am-12pm
No guaranteed raftable flows Monday through Friday
Minimum Flow: 200 cfs
The next forecast should be released around April 10th, with it taking effect 3 days after issuance.
Extra Details for Geeks
600 MAF: Full Natural Flow of American River below Folsom Lake from October through February 2018[1. Full Natural Flow for the American River below Folsom Lake]. Through March 4th: 625 MAF[1. Estimated via FNF CFS as reported by Daily Folsom Lake Data and multiplied by 1.98. (Full calculation to derive 1.98 conversion: 1 Day of FNF in AF = (Daily Average FNF CFS) * 60sec/min * 60min/hr * 24hours / 43,560cf/af)].
Reservoir Storage Amounts of Interest
Here are some reservoirs in the Upper American River Project (UARP) that might be of interest.
As of March 9th – 8 AM
Loon Lake – 6,384.94 ft (spills at 6,418 ft) with 38,510 AF (spills at ~76,500 AF)
Ice House – 5,425.10 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 – 4,833.45 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 -1,845.48 ft (spills at 1,850 ft) with 12,182 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 – 995.49 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 – table – Limited to American River Basin
About the Forecasts
Forecasts are issued February through May around the 2nd Tuesday or the 8th of each month. They take effect 3 days after being released.
The final forecast is issued in May, and is used to determine the flow schedule through October.
The final (actual) water year report is released in October and is used to determine flows for November until 3 days after the February forecast is released.
While some bias might slip in, as much as possible the above information is purely based on the forecasts and not based on gut intuition or predictions.
Note: This page is updated with current forecasts as they become available. Previous forecasts are archived for reference.
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