During high flow years the number of dams juvenile chinook
salmon and steelhead pass on their downstream passage to the sea determines
their downstream migration survival, according to a recent study.
The study looked at survival of the fish passing one, two
and three dams on the Columbia River during a 2011 high flow event that led to
involuntary spill, creating an environment of supersaturated dissolved gas.
Researchers used acoustic transmitters and a system of
hydrophone arrays at Bonneville, The Dalles and John Day dams to calculate the
survival of yearling chinook and steelhead that pass the dams. They did this by
comparing passage data for each of the fish with the exact environmental
conditions they experienced at the moment of their passage.
According to the study, “Factors influencing the survival of
outmigrating juvenile salmonids through multiple dam passages: an
individual-based approach,” published online June 16 in the journal Ecology and
researchers “applied a unique index of biological characteristics and environmental
exposures experienced by each fish individually as it migrated downstream, in
order to examine which factors most influence salmon survival.”
The researchers found that smolt survival was strongly
influenced by barometric pressure, the speed at which the fish transited the
dams and water temperature, among other factors.
However, the effect on the fish was compounded by passing
multiple dams compared to fish that passed just one or even two of the dams.
“Despite spatial isolation between dams in the Lower
Columbia River hydrosystem, migrating smolt appear to experience cumulative
effects akin to a press disturbance.” And the frequency at which they encounter
the dams limits their ability to “recover from one set of deleterious
conditions before experiencing another significant disturbance,” the study
A press disturbance causes chronic stress in the migrants
and is long-lasting, whereas a pulse disturbance creates acute stress and,
given the opportunity, the smolt can recover. In general, the study found that
fish passing one or two dams creates a pulse disturbance, but after passing the
third dam, chronic exposure causes cumulative negative effects.
“The frequency and duration in which smolts encounter the
altered environmental conditions associated with dams in the Lower Columbia
River appears to result in chronic exposures, preventing the recovery of these
fish between exposures,” said researcher Tim Elder, a PhD Candidate at Portland
State University’s Department of Environmental Science and Management.
Elder’s co-authors are Christa Woodley, research ecologist
at the U.S. Army Engineer Research and Development Center, Mark Weiland, senior
managing scientist at Anchor QEA, and Angela Strecker, assistant professor of
Environmental Science and Management at PSU.
For example, Elder said, increased spillway discharge had
little effect on fish passing one or two dams but substantially increased
survival estimates for fish passing three dams.
Similarly, fish velocity had little effect on the survival
of fish passing a single dam but the survival of fish passing three dams was
strongly influenced by velocity, with slower fish having lower survival than
The survival of yearling chinook passing one dam was 87.8
percent, two dams was 83.8 percent and fish that passed three dams was 81.7
percent, the study said. For steelhead, survival was 90.7, 86.1 and 84.3
percent for one, two and three dams.
As this shows, the survival of chinook and steelhead is
similar and, therefore, actions taken to increase survival for one species
would likely increase survival for the other species, says the study.
“The US Army Corps of Engineers has modified these dams to
better accommodate the needs of migrating salmon and the dynamic river environment,”
Elder said. “However, additional
structural and operational modifications to the dams that account for years of
high flow volumes, involuntary spill and the ensuing elevated levels of total
dissolved gas may be needed.”
Other findings of the study are:
--The survival of both species passing one or two dams was
most influenced by temperature, dissolved gas, discharge at the dam and
barometric pressure, while fish that passed three dams were most influenced by
spillway discharge, discharge velocity and barometric pressure.
--Survival for chinook passing one dam decreased as total dissolved
gas increased above 113 percent, for both species passing two dams there is a
sharp drop in survival beyond 113 percent TDG and then again at greater than
120 percent. Steelhead appear to be more tolerant of higher dissolved gas
--Chinook passing one or two dams showed increased survival
at higher dam discharges (they move faster). There is also an increase in
survival for both species passing three dams during higher spill volumes, but
to a point, depending on other factors, including the level of dissolved gas.
--There was a slight increase in survival with increasing
spill for fish passing one dam and a sharp increase in survival for fish
passing three dams, but also to a point.
--Higher flow velocities showed the highest survival for
both species. For fish passing one dam, 2 kilometers per hour (1.2 miles per
hour) showed highest survival. For chinook passing two dams survival was
highest at 4.5 km/hr. (2.8 mph). For steelhead there was a sharp increase in
survival between 0.5 km/hr. (0.31 mph) and 2 km/hr. and little effect beyond 2
km/hr. For both species passing three dams survival increased substantially as
fish velocity increased, but survival declined slightly for chinook salmon at
velocities greater than 3 km/hr. (1.86 mph), while steelhead survival remained
high at all velocities. All measured velocities still maintained survival
greater than 80 percent through each dam, according to the study.
--For both species, survival increases as water temperature
increases to 12 degrees Centigrade (about 54 degrees), but starts to drop as
temperatures rise. Again, steelhead were more resilient to changes in water
“Our analysis,” the study concludes, “indicates that the
ecological effects of hydropower facilities are not confined to isolated areas
of deleterious environmental conditions (i.e., pulse disturbances), but rather,
exert a cumulative influence on migrating smolt, affecting survival throughout
our study system (i.e., press disturbance).”