The Colorado River is a lifeline
to the Southwestern United States.
It provides water to nearly 40 million people,
is used to irrigate over 5.5 million acres of land,
generates billions of dollars in revenue
from outdoor recreation,
and can produce upwards of 4,200 megawatts
of electrical power per year.
The water in this system is precisely allocated to states
through numerous laws, agreements, and treaties,
and every drop is important.
A 20-year drought, and especially
the extraordinary lack of rainfall in the past year
has made measuring discharge below Glen Canyon Dam
vitally important to downstream communities
all the way to and beyond the Mexican border.
Normally the average amount of water
flowing out of the Glen Canyon Dam
is around 12,000 cubic feet per second, or CFS.
Recently the Bureau of Reclamation
conducted a historically low and steady release
of 4,000 CFS to lower water levels enough
to complete dam maintenance.
This provided the USGS with a rare opportunity
to confirm and/or improve the accuracy
of our streamgage at Lees Ferry.
An interesting bit of data emerged
during the planned low-flow event.
The steady discharge coming from the dam
indicated a 185 CFS difference
with the water being measured 15 miles downstream
at the Lees Ferry gage.
The 185 CFS might not seem like a large amount of water
when compared to the 12,000 CFS average,
but to put it in perspective,
this 185 CFS difference works out to be
83,000 gallons per minute.
So verifying the accuracy of the gauge
and determining what section of the 15 miles of channel
this additional water was coming from
was a crucial part of our mission during this study.
The water could be entering the river from springs
between the dam and the gage.
It could be coming directly from the dam area
or the perceived extra water could simply be an error
in equipment calibration at the gage.
To find the answers we embarked
on the most intensive and accurate
discharge measuring effort ever performed
on the Colorado River
between the dam and our Lees Ferry gage.
We collected discharge measurements from five cross sections
beginning just downstream of the dam,
1.5 miles down from the dam, 4.5 miles from the dam,
9 miles from the dam, and at the Lees Ferry gage.
The teams employed highly accurate,
Acoustic Doppler Current Profilers, or ADCPs,
to make multiple discharge measurements
at each cross section over a two-day period.
And to further reduce potential instrument error,
the teams made measurements
with two different types of ADCPs
at a few of the cross sections to verify measure discharges.
The 15 measurements taken at the gage itself
were also used to define the accuracy of the lower portion
of the stage versus discharge rating.
An opportunity that has not presented itself in 30 years.
Our data showed an average flow of 4,200 CFS
at these cross sections.
And we did not measure any significant gain or loss
over the entire 15 mile reach.
The intense measurements over the course
of the five-day, low-flow event
on the stretch of river from Glen Canyon Dam
to the Lees Ferry gage,
has not only confirm the accuracy of the gaging station,
but has also given scientists valuable pieces of data
to assist in accurately accounting
for the treasured water of the Colorado River.