Snake River below Swan Falls (Ramp gage) Snake River near
Murphy Carl Rundberg Pete Vidmar
Slide 2
Gage Locations Swan Falls Dam Snake Riv blw Swan Falls Snake
River near Murphy
Slide 3
Rating curve development Snake River below Swan Falls Rating
curve development Calibrated with142 measurements Measurements made
during steady state conditions to remove travel time Measurements
over wide range of time 1995 to 2013 Range in flows from 4,370 cfs
to 40,300 cfs
Slide 4
Control features Snake River near Murphy: Rock riffle ~300yds
downstream of gage. Riffle no longer controls when aquatic growth
is in channel. Aquatic vegetation grows rapidly during warm season
with low flows Snake River below Swan Falls: Rock riffle control
regardless of season
Slide 5
Snake River near Murphy: Rating curve Shifting
Slide 6
Snake River near Murphy Rating zoomed: seasonal trends Shifting
up to ~1 ft due to aquatic vegetation
Slide 7
Snake River below Swan Falls Rating
Slide 8
Snake River below Swan Falls Rating Zoomed: small random
scatter
Slide 9
Murphy discharge up to 31% from rating Ramp discharge only 6%
from rating
Slide 10
% change in flow due to shifts
Slide 11
Shifts in feet over time
Slide 12
% difference
Slide 13
Summary Snake River near Murphy Aquatic growth causes shifting
Shifts > 0.90 ft % difference from rating > 30% Change in
flow between measurements can be as much as + -15% Snake River
below Swan Falls No apparent aquatic growth Shifts scatter up to
0.3ft (likely due to gage calibration) % difference from rating
< 6% Change in flow between measurements generally+ - 5% Some of
these inaccuracies are due to different standard of calibration and
less precise measuring equipment at the gage.
Slide 14
Potential Velocity Index Site Swan Falls Dam Snake Riv blw Swan
Falls Snake River near Murphy Potential Velocity Index Site
Slide 15
Potential Velocity Index Site
Slide 16
Slide 17
Slide 18
Snake River below CJ Strike
Slide 19
Site specific conditions on the Snake River below CJ Strike
Interference from aquatic vegetation Aeration caused by: spillway
vacuum breaker operations at the power plant
Slide 20
Possible alternatives to the current gage Reverting back to a
stage-discharge rating Moving the gage location downstream
Utilizing the Winter-Kennedy turbine flow meters Some combination
of these options