a slightly lower water supply in the North Fork Clearwater River basin this
week, the U.S. Army Corps of Engineers opted to continue a combined generation
and spill operation at Dworshak Dam, maintaining total dissolved gas levels at
just under 110 percent, but still within clean water limits.
Corps made the decision Wednesday, February 8, after conferring with fisheries
managers at the interagency Technical Management Team meeting. The operation
will continue until next Wednesday’s TMT meeting when river operators and
fisheries managers will again review weather and water supply, inflows and
outflows, as well as potential operations options at the dam.
is the third week running that TMT has overseen operations at Dworshak Dam to
ensure TDG in the river below the dam doesn’t exceed 110 percent while also
ensuring the reservoir is drawn down to meet April flood control concerns.
Total flow through the dam is 8,100 feet per second: 4.8 kcfs through two
turbines and 3.3 kcfs spill.
North Fork Clearwater runoff forecast is still higher than normal at 2.55 to
2.6 million acre feet, although that
is down from last week’s estimate of 2.6 to 2.7 MAF (average is 2.45 MAF).
According to operational graphs provided by Steve Hall of the Corps’ Walla
Walla District Office, that should temporarily ease the dam’s operations and
not require higher spill and, consequently, TDG in April, as he had previously
thing the current operation does do, however, is drop the Dworshak reservoir
level below the elevation required by April 15 – 1,520 feet -- under the flood
control rule curve, and that worries Erik Van Dyke of the Oregon Department of
Fish and Wildlife. He said Oregon would prefer to maintain the reservoir level
at its current elevation (about 1,524 feet), rather than drop it further and
risk refilling beginning at the end of April. He asked the Corps to stop spill
but continue passing water through the two small powerhouses.
control operation and maintaining TDG below 110 percent would be much simpler
if all generation were available at the dam. However, just two smaller
generators are working as the dam’s largest generator is out of service for an
overhaul. That makes it difficult to drain enough water from the reservoir to
meet April flood control targets and still meet water quality standards for TDG
the dam’s largest generator, has been out of service since September for a
complete overhaul, but the Corps’ contractor is behind schedule by two months.
The Corps expects the generator to be back in service July 15 – it was
initially to have been in service May 15 – with field tests and validation
testing into early August.
updated schedule to bring unit-3 back on line is at
outage limits the powerhouse capacity at the dam to about 45 percent of normal,
but it also limits the amount of discharge from the dam to about 4.7 kcfs per
second through the two small turbines. All other water must be spilled and that
is causing TDG to hover in the 110 percent range, a state of Idaho clean water
limit. If TDG rises above 110 percent, the Corps must apply for a state waver.
worrisome is the impact of high TDG at the Clearwater National Hatchery
downstream of the dam. The hatchery uses North Fork Clearwater River water and
dissolved gas exceeding the clean water limit could impact juvenile salmon in
gas levels stirred up by water plunging into dam tailraces can negatively
affect fish and other aquatic life.
this week, Hall provided a range of operating options in four scenarios, two at
2.55 MAF and two at 2.6 MAF
scenarios show a sudden spike in inflow Saturday, February 11, going from about
4 kcfs to almost 12 kcfs overnight due to snow and rain storms expected late
this week. That will cause a nearly imperceptible rise in reservoir elevation
for a few days, partially alleviating Van Dyke’s fears of a reservoir dropping
below a recovery level for refill.
first scenario (2.55 MAF) is favored by the Corps at this time. Although inflow
rises considerably in April, TDG remains below 110 percent.
second scenario (2.55 MAF) shows just outflow through the turbines through
March 1, predicting a needed rise in outflow in April causing TDG to rise to
over 120 percent.
third scenario (2.6 MAF) holds outflow to about 110 percent TDG until April 1,
when flows and TDG rise to about 115 percent.
fourth scenario (2.6 MAF) is similar to scenario two, dropping outflow to that
of the two small generators without spill and with TDG rising to more than 120
percent April 1.
water quality report for TDG is at
February 3, 2017, “With Dworshak Generation Down, River Managers Balance
Runoff, Flood Control Targets, Dissolved Gas,” http://www.cbbulletin.com/438280.aspx
September 23, 2016, “Dworshak Oil Spill Into North Fork Clearwater Slows
Turbine Overhaul, Cleanup Continues,” http://www.cbbulletin.com/437598.aspx
May 13, 2016, “Needed Work, Low Flows At Dworshak Dam Pose Challenge As Water
Needed For Juvenile Sockeye Migration,” http://www.cbbulletin.com/436712.aspx
April 8, 2016, “Agencies Set For Spill Tests At Dworshak To Judge Impacts To
Hatchery Fish During Generator Overhaul,” http://www.cbbulletin.com/436412.aspx