WSPC02513

• File Number: 8408
• Description: River Basin General Correspondence
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 5/1/1991
• Title: Platte River Evaluation: USBR - Long Term Water Flows in the Platte River System - A Preliminary Analysis
• Water Supply Pro - Doc Type: Report/Study

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individual year annual flows and a 5 year moving average. The
data are reconstructed virgin flows for the period 1884-1983.
The reconstruction was done by accounting for man-caused
diversions into and out of the rivor system4

FIGURE 4.
A report put out by the National Climatic Data Center (NCDC)
provided summary climate data for Colorado for the period 1888-
1982. Total annual precipitation was generally high from 1904-
1928, generally low 1929-1939, generally high in the early-mid
1940s, and thereafter more in the range of pre-1900 rainfall.

FIGURE 5.
The NCDC also summarized Colorado data on the Palmer Drought
Severity Index (PDSI) which integrates precipitation and
temperature data into a monthly moving average describing
moisture conditions. The period 1904-1930 had very high indices,
indicating very moist conditions, 1931-1937 had very low indices,
and 1953-1956 were also quite low. The other periods (1888-1903,
1938-1952, 1957-1982) tended to have more average PDSI values,
with some exceptions.

FIGURE 6.
Four rainfall stations along the eastern edge of the Rockies
between Colorado Springs and Fort Collins showed no clear
indication of the annual patterns described above.

FIGURE 7.
Annual rainfall data for Nebraska (1850-1984) also showed no
clear indication of the annual patterns described above.

CONCLUSIONS

An apparent common thread between these data seems to be a
pattern of "average" flows for at least a few decades prior to
1900, high flows until 1930, then very low flows during the
19305, followed by a return to a more "average" range of flows
sometime in the 1940s, which has continued at least into the
early 1980s. The data I have discussed came from a variety of
sources and were presented in a variety of formats, so that some
degree of inconsistency between the results would be expected.

Also, as I understand the situation, all of the data that
show the pattern described above are largely influenced by
snowfall in the Rockies. This would explain the lack of the
pattern at the eastern Colorado and Nebraska rainfall stations.
Since annual flows in the Platte River system are driven largely
by snowmelt in the Rockies, these types of data should be
relevant to understanding the long term pattern of annual flows
in the Platte River, inclUding the magnitudes of annual flow that
would be moving through the river system today in the absence of
man's water management activities.