WSP03817

• File Number: 8102
• Description: Arkansas River Basin Basic Hydrology
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 5/7/1999
• Author: Inter Fluve Inc
• Title: Fluvial Geomorphic Assessment of Upper Arkansas River Final Report
• Water Supply Pro - Doc Type: Report/Study

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Downstream of Lake Fork, the quantification of effective discharge is dependent on the
period of record utilized. If the entire period of record is utilized (1964-1967; 1975-
1984; and 1990-1993), the 4% flow duration value is 900 cfs. These values include a
period of significant flow augmcntations prior to the Mt Elbert Conduit of 1982. If the
most recent recorded flows are utilized (1990-1993), thc 4% flow duration value is 550
cfs, which has a recurrence interval of between 1 and 2 years (Chapter 7). Figure 6.5
shows the number of days that each of these flows were equaled or exceeded. During the
recent 1990-1993 time period, the 550 cfs discharge was exceeded an average of 13 days
per year. Between 1978 and 1984, however, that discharge was exceeded an average of
50 days per year. This earlier time period includes the pre-Mt Elbert conduit years of
1978-1982, as well as relatively wet years of 1983 and 1984. Consequently, the 1978-
1984 time frame rcflects a period of active geomorphic adjustment of the Arkansas River
downstrcam of Lake Fork.

The 4% flow duration value derived for the Leadville gage for the period of record
(1968-1983; 1991-1996) is approximately 400 cfs, which has a return interval of less than
2 years (Chapter 7), During that time period, that discharge was exceeded over 30 days
per year in 1979, 1980, 1995, and 1996 (Figure 6.6). During 1995 and 1996, thc effective
discharge was exceeded a total of 68 days. Although these two years encompass 9% of
the total time frame evaluated, they are responsible for 23% of the days during which the
effective discharge was equaled or exceeded. The 1979-1980 time frame is very similar;
during those two pe'riods, 1979-1980 and 1995-1996, almost halfofthe effective
discharge events recorded were experienced.

6.5. Geomorphic Evolution

The geomorphic analysis shows that the project reach channel width has increased
between 9% and 37% since 1939. Split flow conditions persisted since 1939. From 1939
to 1957, bendway migration rates averaged 2-5 ft per year, and bendway cutoffs
predictably occurred at radius of curvature/width values of2-3.5. In contrast, the 1959-
1997 time period was characterized by migration rates of 5-13 ft per year, and bendway
cutoffs occurred over a wide range of RcIW configurations. Prior to the completion of
the Mt. Elbert Conduit, peak and moderate flows were significantly higher downstream
of Lake Fork relative to post-conduit time frames. The occurrence of effective flows
through the reach depicts the 1976-1984 time frame as one of rapid geomorphic
adjustment, followed by additional channel forming flow periods in 1995-1996.

The results presented above indicate that the project reach has undergone progressive
widening through time, and this widening was in part driven by historic augmentations
and an associated increased frequency of channel forming flows. Flow increases due to
augmentations have been reduced since the Mt. Elbert Conduit was completed; flow
duration curves indicate that 1990-1993 channel forming flows are approximately Y, of
those of the late 1970'5.

In stable western gravel bed rivers, the channel forming flow is roughly equivalent to the
bankfull discharge (Andrews and Nankervis, 1995). In general, that flow occurs on the

May 7. /999

Fluvial Geomorphological Assessment
Upper Arkansas River

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