FLOOD00193

• County: Fremont
• Community: Canon City
• Stream Name: Arkansas River
• Title: Technical Addendum for Floodplain Information Report
• Date: 7/1/1981
• Prepared For: CWCB
• Prepared By: Camp Dresser & McKee
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDROLOGIC ANALYSIS

The hydrologic analysis was performed by the Corps of Engineers in their
1978 Floodplain Information Report (Reference 4), and reviewed by Camp
Dresser & McKee Inc. (COM). The fol lowing discussion is a description of
the methodology used by the Corps of Engineers and COM's review.

Basin Description

The Arkansas River's headwaters originate in the eastern slopes of the
Continental Divide at elevations in excess of 14,000 feet above mean sea
leval. The river flows in a generally eastward direction and joins the
Mississippi River near Rosedale, Mississippi.

The portion of the Arkansas River basin under consideration in this study
has its lower limits at 0.2 mile above the Minnequa Dam (see Basin Map
Figure 1). The normal annual precipitation for Canon City, Colorado is
14 inches, of which 60 percent falls from May to September. In the
Arkansas River basin precipitation varies from 11.8 inches per year at
Pueblo to more than 40 inches on the peaks west of Leadvil Ie, most of it in
the form of snow. The temperature varies along the basin from a mean
minimum and maximum of 22.0oF. and 49.50F. at Leadvil Ie to a mean minimum
and maximum temperature of 37.30F. and 68.0oF. at Pueblo. Above Canon City
the basin topography is typical mountain terrain with characteristic alpine
vegetation. Below Canon City the basin topography is much I ike that of the
lower foothil Is and plains area.

Method of Analysis - Corp of Enqineers

The discharge frequency curves for the Arkansas River (see Figure 3) were
developed by the COE from Log Pearson Type-I I I distribution analyses of the
Pueblo and Canon City gages which included peak discharge data through
1973, because of the construction of Pueblo Reservoir. The peak discharges
for the Arkansas River at Florence contained in Reference 4 Were estimated
using an area proportion method between the gage stations.

At attempt was made to util ize the Arkansas River gage at Portland Instead
of the Pueblo gage, but the prior gage had an insufficient length of
record. According to the United States Geologic Survey Portland gage was
in operation from 1939-1952 (drainage area = 4,024 sq.mi.). No further
records are available until 1965 when the new Portland gage (Drainage area
= 4,280 sq.mi.) went into operation. This later gage is currently
recording stream flow data on the Arkansas River.

COM Review and Results

Since the Corps of Eng i neers frequency curves were 'developed us i ng expected
probability, as outlined in Reference 4, the discharges were checked by
applying the hydrologic procedures as outl ined in Reference 1, to the
Arkansas River gage at Canon City. The gage data used is presented in
Table 1. The 90% confidence I imits based upon the COE results were
calculated and cheCked against COM's station analysis. It was found that
COM's results fell within these limits, as shown on Figure 2. It was
therfore recommended the Corps of Engineers' discharges be adopted for the