| The
Lees Ferry gaged flow record
As the point where the river is allocated between the upper
and lower basins, Lees Ferry is the "fulcrum" for
managing the Colorado River, so it is essential to determine
how much water flows past that point. The stream gage at Lees
Ferry was installed in 1921, during the Compact negotiations,
and has operated continuously since then. For years prior to
1921, gage records upstream and downstream of Lees Ferry were
used to estimate the flow at Lees Ferry.
Like most rivers, the natural flow regime of the Colorado River
has been altered by human activities. As a result, the gaged
streamflow record does not accurately represent the natural
long-term trends and year-to-year variability in overall water
supply in the basin. The U.S. Bureau of Reclamation has made
corrections to the gaged record to account for water depletions
(usage) in the upper basin, diversions into and out of the upper
basin, and evaporative losses from upper basin reservoirs. The
resulting streamflow record is called the "natural flow"
record, since it represents what the flow at Lees Ferry would
be if the human alterations to the river did not exist. This
natural flow record is important because it shows the variability
in streamflow due to climate alone, apart from changes in the
use and management of the river.
The graph above shows the natural flow record for the Colorado
River at Lees Ferry, from 1906-2004. The annual flows are shown
in blue, a running 10-year average in red, and a cumulative
average in green. Given that the total allocation of water at
Lees Ferry is 16.5 million acre-feet (MAF) per year, and actual
depletions (use plus evaporation) are now around 14 MAF annually,
it is worth noting several features of the natural flow record:
- The annual flows over the past century have varied by a
factor of five, from about 5 MAF (1977) to 25 MAF (1984)
- The period from 1906-1930 had 10-year average flows higher
than any other part of the record except the mid-1980s
- The cumulative average annual flow declined from about 17
MAF (averaged from 1906-1930) to about 15 MAF (averaged from
1906-2004)
- The 10-year running average has varied from about 12.4 MAF
to 18 MAF--in other words, the decadal-scale variability has
been high
- From 1934 to 1984, the 10-year running average was almost
always below 15 MAF
- The 2000-2004 drought was the most severe multi-year drought
in the record, with an average annual flow of 9.6 MAF over
those five years
As the 20th century progressed, the observed range of natural
variability of Colorado River flows grew ever broader,
as "unprecedented" high and low flows were experienced.
From this broader frame of reference, it is now apparent that
the early period of the record--unfortunately used as the basis
for allocating the river's flow--was an unusually wet period.
Does the now 100-year-long natural flow record capture the
full range of natural variability of Colorado River streamflow?
This seems unlikely, given that in just the past 30 years, the
river has seen the two lowest annual flows (1977, 2002) and
the two highest annual flows (1983, 1984) on record.
On to...The Paleo Record
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