A record runoff in March, a persistent outage
of the dam’s largest generator and worries that dangerous dissolved gas levels
for hatchery fish generated by more spill than normal created what officials
call a perfect storm for operations at Dworshak Dam in Idaho.
The North Fork Clearwater River basin anticipated
runoff was just a little higher than normal (about 113 percent above normal)
early in the year before record rains and runoff in March pushed the March to
June runoff forecast from an average of 3 million acre feet up to 3.98 MAF
almost overnight, according to Steve Hall of the U.S. Army Corps of Engineers’
Walla Walla District Office.
The record runoff into Dworshak’s reservoir
forced the Corps to increase outflows to as high as 25,000 cubic feet per
second, but the highest percentage of that flow was through spill, not
powerhouse flow, causing total dissolved gas downstream of the dam to climb to
over 125 percent, thought to be a dangerous level for juveniles at hatcheries.
Hall recapped Dworshak Dam operations at the
Technical Management Team’s Year End Review, December 12, in Portland.
TMT is made up of fisheries and
hydro/reservoir managers from state, federal and tribal agencies. Every
December the group looks back at actions taken during spring and summer in
managing Columbia and Snake river federal hydro/fish operations. Hall’s
presentation was among nine reviewed in the day-long session. Go to http://pweb.crohms.org/tmt/agendas/2017/1212_Agenda.html to see all presentations.
Hall described each month that led up to
March’s high runoff:
--October 2016 was very wet, but with normal
--November was very dry and warm.
--December precipitation was a little less
than normal, but it was the beginning of what he called the “frozen period” as
the area and much of the Snake and Columbia river basins were plunged into
colder than normal temperatures.
--January was relatively dry in the Clearwater
basin and cool.
The cold and wet “really didn’t get going
again until February and then it took off,” Hall said as he described a fairly
normal high elevation snowpack, but much more snow at lower elevations. At the
time, the Corps wasn’t sure how to calculate the effect of all that low-level
snow, he said.
Inflow to the reservoir hit 48 kcfs twice in
March as the low-level snow began to melt and discharge from the dam climbed to
25 kcfs and stayed close to that level through mid-April.
Unit-3, Dworshak’s largest of three turbines
(rated at about 220 megawatts), had been unavailable for more than a year so
the Corps was limited to running just 4.7 kcfs of the massive spring flows
through the dam’s two small generators. The remainder, at times exceeding 20
kcfs, spilled at the dam, and at one point TDG climbed to nearly 128 percent
(the state of Idaho and Environmental Protection Agency clean water TDG limit
at that time of year is 115 percent), threatening juvenile fish at the Dworshak
National Fish Hatchery, operated by the U.S. Fish and Wildlife Service.
Dave Swank of the Service said that the vacuum
degassing system installed at the hatchery “helped save us” and limited TDG in
the hatchery to a high of 105 percent, but mostly TDG hung around 102 percent,
even as the saturated gas in the North Fork Clearwater River rose to 125
“The good news is that we didn’t see any
additional mortality (in the hatchery among the juveniles) beyond background
levels,” Swank said.
The Corps had managed to drop flows from about
22 kcfs to 7 kcfs for two days March 18 and 19 to allow for an early release of
spring chinook smolts.
He did say that survival of spring chinook
hatchery juveniles that were released from the hatchery about one month early
was about 10 percent below average, but that hatchery steelhead survival was
above average. Some steelhead were released early, but many more were moved to
another area where reservoir water without saturated gas was available, which
helped mitigate dissolved gas within the hatchery, Swank said.
Biologists at the hatchery monitored juveniles
daily, looking at gills, fins and lateral lines for signs of gas bubbles and at
stomach contents to test whether the juveniles’ appetites were being affected.
Sometimes food consumption becomes an issue with high TDG levels, Swank said.
Idaho Department of Fish and Game was faced
with similar problems in Hells Canyon where the agency also released spring
chinook early. However, even with a river TDG of 130 percent, Russ Kiefer of
IDFG, said that he “was surprised at how well the fish did.”
Next year, “we’re thinking of just sticking to
the regular release schedule,” if the higher than normal flows reoccur, he
Adult fall chinook move into the Clearwater
and North Fork Clearwater rivers in August and spawn from October through early
December, according to Jay Hesse of the Nez Perce Tribe, who also spoke at
TMT’s end of year session. Over one-half of the redds in the Clearwater are
found downstream of Dworshak Dam, and 241 of the 6,000 redds in the rivers were
directly influenced by Dworshak releases, he said.
The effects of TDG are minimal on the eggs,
Hesse said, but the high dissolved gas levels occurred when fry were emerging
from the gravel, a critical time for the fish and a time when they are most
susceptible to high TDG.
He said that TDG related mortality for the
in-river fish could be as high as 4.5 percent, but likely mortality was less
than 2 percent.
“It wasn’t a train wreck,” he said. “We still
had successful production.”
Unit-3 has been out of service since September
2016 for a complete overhaul, but the Corps’ contractor immediately got behind
schedule when it spilled oil while disassembling the generator. The Corps
revised its expectation that the generator would be back in service from May 15
to July 15, 2017 after the oil leak. The Corps is now projecting the unit will
be fully operational by July 2018.
“If we had known a little earlier, we could
have limited the outflows to 15 kcfs with less TDG” instead of the 25 kcfs
during March and April, Hall said. That would have required the Corps to
release water earlier, even before the record rains in March, but that would
have “pushed TDG up to 113 percent and there is a lot of reluctance to go
beyond 110 percent. But that’s hindsight.”
Still, taking a step to avoid this year’s high
flow and high TDG issues, TMT and the Corps are shooting for a low reservoir level
(an elevation of 1,520 feet by December 31, 2017) at the front end of the
winter, Julie Ammons of the Corps reminded the end of year group. “In order to
have handled the flow in March, we would have had to have a low reservoir in
December 2016, and we wouldn’t have had to move so much water in March.”
Asked if the extreme runoff peak is normal for
March, Hall said it was uncertain, but he is seeing a trend toward the earlier
“I think we’re seeing somewhat of a shift to
an earlier runoff,” he said. “We’re building a body of knowledge that says that
is so,” and the Corps is considering adding March to the runoff period (April
to July) and dropping July.
--CBB, February 3, 2017, “With Dworshak
Generation Down, River Managers Balance Runoff, Flood Control Targets,
Dissolved Gas,” http://www.cbbulletin.com/438280.aspx
--CBB, December 22, 2016, “Year-End Assessment
Matches 2016 Water Supply, Stream Flow, Fish Conditions With Juvenile Migration,”