Low spring chinook conversion rates from the
Lower Monumental Dam pool up through Little Goose Dam has fisheries managers
worried that high flows and involuntary spill are causing the fish to stall in
the pool on their migration upstream.
The interagency Technical Management Team
decided this week to cut spill during morning hours to encourage fish to pass
Little Goose Dam, a step it also took in 2017 as a way to raise conversion
rates – the percentage of fish that pass a downstream dam that make it through
dams upstream – which was in the 60-percent range Tuesday, May 29, but would
normally be in the 90 percent range, according to Paul Wagner of NOAA
Spill at the dam had been at 49.9 percent of
the Snake River’s flow and, by TMT consent, that dropped to spill of 30 percent
of the river at 4 am Wednesday, May 30, as a result of the new operation,
according to Doug Baus of the U.S. Army Corps of Engineers.
Some 21,868 spring chinook had passed Lower
Monumental Dam as of May 29, but just 13,114 of those fish had managed to pass
the upstream dam, Little Goose, a discrepancy of 8,752 fish, which is a 60
percent conversion rate.
However, after dropping spill at Little Goose
to 30 percent of the river early May 30, the daily passage of the fish rose to
2,689 fish. It had been in the 300 to 400 fish per day range for the previous
four days. The year to date count over the dam rose to 15,803 May 30 (the count
at Lower Monumental was 22,887 May 30), a discrepancy of 7,084 fish and a
conversion rate of a slightly higher 69 percent.
Passage of spring chinook at Lower Monumental
by May 30 last year was 17,664 and the 10-year average is 56,761. Similarly,
passage at Little Goose last year was 11,096 and the 10-year average is 50,468.
When the dam is spilling 30 percent (the new
operation that will be re-evaluated today, June 1, in TMT’s third meeting of
the week), the water is stored in the Little Goose pool, which rises during the
operation (30 percent spill is from 4 am to noon each day - 8 hours). That is
followed by the dam passing inflow until 4 pm (4 hours) and then raising spill
from 4 pm to evacuate water stored behind the dam until 4 am (12 hours) and
bring operations into compliance with minimum operating pool guidelines.
However, flow and spill during this latter step is not allowed to produce
saturated gases that exceed 125 percent total dissolved gas.
A three-judge panel of the Ninth Circuit Court
of Appeals ruled April 2 in favor of an April 2017 U.S. District Court
injunction allowing more spring spill at four lower Snake and four lower
Columbia river dams. With the decision, spill to the gas cap began April 3 at
lower Snake River dams and at lower Columbia River dams April 10. The
additional spill through June 15 is designed to aid migrating juvenile salmon
However, spill at the dams to gas cap levels
has been moot since early May when high flows due to snow runoff has forced
involuntary spill, often resulting in TDG levels higher than state mandated gas
caps. State spill caps are 120 percent in the tailrace of a dam and 115 percent
in the forebay of the downstream dam.
Information provided by Dan Turner of Corps’
River Control Center showed that the target spill to meet gas cap requirements
May 29 at Little Goose Dam was 26,000 cubic feet per second, but that actual
spill was 77 kcfs, producing TDG levels of 126 percent in the dam’s tailwater
and 126 percent in Lower Monumental’s forebay.
High flow, spill and TDG levels are evident at
all dams throughout the lower Snake River as well as at lower Columbia River
dams. The McNary Dam spill target is 145 kcfs, but actual spill on May 30 was
295 kcfs, which produced TDG of 129 percent in the tailwater and 119 percent in
the John Day Dam forebay.
The spill target at John Day May 30 was 90
kcfs, but actual spill was 218 kcfs, which produced TDG of 128 percent in the
tailwater and 123 percent in The Dalles Dam forebay.
The spill target at The Dalles was 90 kcfs,
but actual spill was 244 kcfs, producing tailwater TDG of 123 percent and
Bonneville Day forebay TDG of 120 percent.
The spill target at Bonneville Dam was 121
kcfs, but actual spill was 242 kcfs.
Turner said he expects involuntary spill to
continue in the Snake River through June 3 after which it will take an
additional week to flush all the water out.
He predicted involuntary spill will continue
in the lower Columbia River to June 12 to 18, then there will be “an awkward
--CBB, May 18, 2018, “Court-Ordered Spring
Spill Now Moot As High Columbia/Snake Flows Forcing Involuntary Spill At Dams,”
--CBB, April 13, 2018, “Court Ordered Spring
Spill For Fish Begins On Four Lower Columbia River Dams,” http://www.cbbulletin.com/440516.aspx
-- CBB, April 6, 2018, “New Court-Ordered
Spill Regime Based On Dissolved Gas Caps Begins This Week,” http://www.cbbulletin.com/440479.aspx