Portland General Electric
Lower Deschutes River Gravel Study
Facts about the Lower
Our commitment to the Deschutes Basin
PGE and the Confederated Tribes of the Warm Springs Reservation of
Oregon are working to restore more natural conditions to the lower
Deschutes River and return healthy, sustainable migratory salmon and
steelhead runs to the tributaries upstream of the Pelton Round Butte
hydro project.
Protecting fish, wildlife, and native plants is critical to our success. We
work extensively with our partners------including tribal, local, state and
federal organizations------to improve habitat, both in water and on land.
Overview
The lower Deschutes River in Central Oregon is home to robust
populations of wild native Chinook salmon, steelhead and redband
trout that support significant sport and tribal subsistence fisheries.
Some of the most heavily used spawning gravel bars are found just
downstream of the Pelton Round Butte Project Reregulating Dam.
Native Americans traditionally camped there and harvested salmon.
Salmon spawning over the generations has shaped the gravel into
series of dune-like features that can be seen from aerial photographs.
Deschutes River
Large inputs of groundwater give
the Lower Deschutes River a
naturally stable year-round flow.
The rates of natural gravel supply
and gravel loss due to high flows
are much lower than other local
rivers.
The Pelton Round Butte
Hydroelectric project is a ‘‘run of
the river’’ project. Downstream
releases are regulated to be
within 10 percent of the inflow.
Ancient floods shaped the
channel of the Deschutes River.
Most of the flat terraces that are
now used as recreational
campsites were formed
approximately 4,500 years ago.
The need for spawning gravel
When the Pelton Round Butte Project dams were
constructed by PGE in the late 1950s and early 1960s,
gravel and sediment from upstream tributaries settled
in the reservoirs rather than being naturally deposited
in gravel bars in the lower Deschutes River.
Consequently, the first source of gravel below the
project is Shitike Creek, entering the Deschutes River
three miles downstream from the Reregulating Dam.
Tagged tracer rocks were surveyed to measure downstream
movements by river flows and spawning salmon.
Lower Deschutes River Gravel Study
The potential loss of quantity and quality of
spawning gravel and its impacts on salmon and
trout populations was a major concern raised
during negotiations in the mid-1990s for
relicensing of the Project. Fall Chinook spawner
numbers had declined and a suspected cause was
loss of spawning gravel below the project. Would
it be necessary to supplement the gravel to
replace what is moved downstream during high
flows?
The Licensees and state and federal agencies
conducted several studies during relicensing to
look at sediment supply, sediment transport,
Cross section transects of riverbed elevation were measured
historic flows, fish spawning, and the effects of
with boat mounted Acoustic Doppler Current Profiler (ADCP).
dams on these river functions in the Deschutes
Basin. Because of the volcanic geology of the watershed, the majority of mountain snowpack
percolates into the ground and emerges from large springs, creating unusually stable year-round
flows of high quality water. Consequently, the studies predicted that the rate of gravel loss below
the Project dams is very low and that gravel transport only occurs during rare major flood events.
We began The Lower Deschutes Gravel Study in 2007 to further evaluate the potential effects of
the Project on gravel transport. It included extensive field monitoring to quantify the amount of
sediment transport and changes in spawning gravel quantity, quality, and use by salmon and
steelhead in the Deschutes River.
Key findings
Results of the 2007-2014 Gravel Study showed that most gravel transport occurs in local, limited
areas and the overall rate of gravel loss is very low. Relative proportions of trout and salmon
spawning at sites upstream and downstream of Shitike Creek were consistent throughout the
study and the quality of spawning gravel was found to be excellent for survival of developing
trout embryos.
Gravel transport and channel stability studies
Methodology
We placed microchip tags in 253 chinook-spawning-size rocks at seven sites in 2007 and
resurveyed their positions annually through 2014. Acoustic Doppler Current Profilers were used to
map the bottom across the channel at 40 sites.
Findings:
• Over 90 percent of the rocks were relocated.
• Three-quarters of the rocks had moved less than three feet from where they were placed.
Lower Deschutes River Gravel Study
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Most of the moved rocks were in areas used by Chinook salmon for spawning.
During the highest flow of 9,000 cubic feet per second, only sand and small gravel particles
moved.
At most sites, Acoustic Doppler Current Profilers found little change in depth, indicating that
scour and deposition rates are very low.
Trout and salmon spawning
Methodology
We mapped Redband trout and steelhead spawning use at six gravel bars within the Gravel Study
area including three sites between the Project and Shitike Creek and three sites downstream of
Shitike Creek. We also mapped areas with suitable sized gravel (half an inch to two inches) at the
study sites. We also measured the suitability of spawning gravels for successful trout egg survival.
Findings:
• We determined the proportion of total
spawning at the upstream sites from 20082014. There was no trend indicating that
relative use had changed during that time
period.
• A decrease in use of upstream sites might
indicate degradation of spawning habitat
between the project and tributary supply of
replacement gravels.
• Over the study period, suitable spawning gravel
area appeared to decrease somewhat at all
sites except the one immediately downstream
Total area of redband and steelhead spawning
from Shitike Creek. We don’t know if the
gravel was mapped throughout the study.
declines were due to variation in our ability to
accurately map patches, expansion of rooted vegetation over the trout spawning gravel, or
mixing of the patches of trout gravels by large Chinook salmon. We did not identify gravel
loss from the reach, and the area of suitable gravel was not reduced to the point of limiting
spawning use by trout.
• Several indexes of survival based on the permeability, oxygen concentration, and percentage
of fine materials mixed with the gravel showed that predicted survival during egg incubation
was high at all sites.
• Chinook salmon spawning activity is monitored annually by the Oregon Department of Fish
and Wildlife and the Tribes. Biologists count Chinook spawning redds in the lower Deschutes
River each fall by helicopter from the Columbia River to the project. Total numbers of
spawning Deschutes River fall Chinook have been at historic highs in recent years. The
relative proportion of redds in the three mile reach above Shitike Creek compared to
downstream has not changed in more than four decades.
Lower Deschutes River Gravel Study
Experimental gravel augmentation
Methodology
We constructed three experimental gravel bars in 2008. A total of 300 cubic yards of gravel half
an inch to six inches in diameter was placed in the Deschutes River downstream of the project
using a hopper suspended from a cable stretched across the river. The gravel was placed in areas
that would naturally have similar deposits. The contours of the bars were surveyed following
construction and after spring runoff. We tagged rocks and placed them on the surface of the
experimental bars to determine the extent of movement. We also mapped spawning use by
redband and steelhead trout and Chinook salmon.
Findings:
• Most of the tagged rocks remained close to their
original placement locations throughout the study.
• There were no floods during the study period that
caused general movement of the streambed or
major loss of gravel from the experimental bars.
• Salmon, redband trout and steelhead used all
three bars.
• Chinook salmon spawning activity, as well as scour
from large wood that came to rest on the bars,
has reshaped the gravel in limited areas.
Our Next Phase of Work
Three independent river science experts reviewed the
results of the Gravel Study. The experts concluded that
further analyses would be needed before a decision to
start large-scale gravel supplementation. The Gravel
Study was conducted during a drought period, and
there were no flows greater than 10,000 cubic feet per
second. Future monitoring of gravel transport and
channel dimensions will be done only after flows of
10,000 cfs or higher. The experts recommended placing
additional experimental gravel at the margins of islands,
A digital camera suspended from a weather balloon was
used to make high resolution images of our spawning
and gravel augmentation study sites.
and constructing known-area deposits of trout spawning gravel to determine how they change
over time.
Pelton Round Butte’s Fish Committee approved the Phase II Gravel Study in 2016. We are
initiating study plans and will start work in 2017.