Flint Creek Dam Advisory Committee Meeting

Flint Creek Dam Advisory Committee
Update: December 9, 2014
Current Conditions
According to the USGS Georgetown Lake gage, the lake level on December 9 is 6428.61 feet. However, the
dam tender report for December 1 shows an observation of 6428.50 feet, and the USGS gage for the same day
showed 6428.57 feet. Last year this difference between observations was also present and the USGS
recalibrated the lake gage. It is possible that ice formation causes the USGS gage to show erroneously high
water level readings during the ice-on period. Period of record (since 1940) median November 30 water surface
elevation is 6428.42 feet and for the period 1999 thru this year, median is 6428.31 feet. Either way, this years’
water surface is slightly higher than the historic benchmarks.
The Georgetown Lake outflow is set at about 14 cfs, and the lake water surface is gaining slowly at this time.
With differences in water surface elevation observations come two estimates of lake inputs. Based on the
USGS measurements from October 31 through November 24, inputs calculated to about 18.5 cfs, which is
73% of the long term normal. Based on USGS gage measurements from November 30 through December 9,
inputs calculate to about 24.6 cfs, or 94% of normal. However, as mentioned above, the water surface
reading could be artificially high. We can use these two figures as low and high estimates in the model.
Autumn precipitation has been good at Peterson Meadows: October had 122%, November had 159% of normal.
That’s a good change from the summer, where April through July registered only 33% to 87% of normal and
averaged only 68% of the long term norm.
The November 20 Climate Prediction Center (CPC) forecast predicts elevated chances of below-normal
precipitation and above-normal air temperatures through March in western Montana. That would be typical of
an “el nino” system. These CPC forecasts will be updated in a few weeks.
Snow Pack
Today the snow pack for the lake sits at 119% of normal snow water equivalent. Both the Peterson Meadows
and Warm Springs SNOTEL sites also show above-normal water year precipitation, with Peterson Meadows
registering 133% and Warm Springs registering 128% of normal. The accumulation graph on the NRCS
website show that storms in late November pushed the precipitation and snow water to above-normal levels.
Snow pack for the area typically peaks in late April/early May. Right now we are about 30% of the way
through the typical snow water accumulation period. The NRCS begins its monthly Flow Volume analysis and
forecasting in January, and runs through June.
Model Predictions using 73% and 94% of normal flows
Either scenario shows plenty of water for 2015 (see graphs on following page). With the 73% scenario, spring
outflows could be tricky in order to make sure the reservoir fills to the brim. With the 94% scenario, the
concern would be having outflows high enough in the spring in order to keep from overtopping the dam. We
have a long way to go until runoff and irrigation season, and there will be adjustments to the snow pack and to
the model scenarios. The breakpoint when limited water supply becomes difficult for all interests is around
60% to 65% of normal seasonal outflow volume.
Dave Amman, Montana DNRC 444-6648
Outflows this past summer were held pretty closely to 30 cfs, with a few exceptions. In late May, outflow was
temporarily held at 78 cfs to accommodate snow melt and high inflows. There were a few other blips up to 40
cfs in July, and outflow was reduced in late October for a dam inspection and/or hydroelectric tasks. Here are
provisional monthly average outflows, along with historic period of record average mean outflows:
32 cfs
43 cfs
48 cfs
31 cfs
POR Mean
26 cfs
35 cfs
59 cfs
46 cfs
31 cfs
31 cfs
27 cfs
20 cfs
POR Mean
35 cfs
32 cfs
26 cfs
21 cfs
The provisional instantaneous
peak outflow was 82 cfs on
June 7. For the period of
record, the 2-year return
interval peak is 83 cfs. For
Boulder Creek at Maxville, an
undammed stream, the peak
was 356 cfs on May 29. The 2year return peak at this site is
372 cfs, so it was also a very
“average” peak on Boulder
The USGS provisional lake
level data reflect the outflows
described above. The lake
reached a peak of 6429.42 feet
on July 9, and then slowly
receded throughout the season.
The spillway crest sits at
6429.56 feet, so this situation
was very good.
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