WSP02347

• 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

May 7, 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

Page 12

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quantified from the comparison. Rather, the comparison is intended to compare flow
conditions in the Arkansas River before and after the Mt. Elbert conduit was completed.

Flow duration data represent the percent of time that a given flow is equaled or exceeded
at a given data collection point. Between Leadville and Malta, the magnitudes of all but
the most frequently occurring flows (>95% duration) increase significantly between
Leadville and Malta during the 1976-1983 time frame (Figure 3,8), These curves
represent a time period during which both native and augmented flows were conveyed
down Lake Fork Creek, In contrast, the 1990-1993 time frame was characterized by
diversion of both augmented and native flows past the project reach via the Mt. Elbert
conduit (Figure 3.9). The increase in discharge between Leadville and Malta is markedly
less during the 1990-1993 time period for all but the lowest flow conditions.

Figure 3.10 shows the ratios of unit discharges (discharge per square mile of drainage
area) for incremental flow duration values at each gage, The 1976-1983 time frame was
characterized by unit discharges at Malta that were around double that of Leadville for
flows that occurred between 40% and 80% of the time, Larger magnitude events (less
than 10% duration) were of comparable unit discharges, and low flow unit discharges
(greater than 90% duration) at Malta were roughly one half of those at Leadville. In
contrast, the 1990-1993 time frame was characterized by unit discharges at Malta that
were approximately 1.5 times those of Leadville for relatively common flows (60% to
80% duration), and less than those of Leadville for flow durations of less than 25%.

In summary, annual hydro graphs and flow duration curves for the overlapping periods of
record (1976-1983 and 1990-1993) upstream and downstream of Lake Fork Creek depict
a significant influence of flow augmentations during the 1976-1983 time frame when
both native and augmented flows were conveyed down Lake Fork Creek. Annual
hydro graphs (Figures 3,2-3.4) depict block release patterns that are characteristic of
Turquoise Lake operations (Figures 3.5-3.7). Unit discharges that are equaled or
exceeded between 40% and 80% of the time downstream of Lake Fork were
approximately double those measured upstream of Lake Fork during the 1976-1983 time
frame (Figure 3.10), In contrast, the during the 1990-1993 time period, the effects of
Turquoise Lake operations are reduced peak unit discharges (low duration flows), which
are also depicted in the spring runoffhydrographs of Figure 3.5-3,7, and a moderate
increase in low flow unit discharges (high duration values),

It should be noted that the impact of reservoir releases on project reach hydrology is not
only due to the change in flow magnitude, but also stems from the patterns of release, or
the shape of the hydro graph generated. Rapid fluctuations in flow rate have the potential
to greatly impact river geomorphology, due to the effects of such fluctuations on bank
stability and sediment transport conditions. In general, the BOR attempts to keep the
fluctuations in diurnal releases to less than 10 to 15% (Studzinski, 1996), however, flow
records indicate that that objective has not always been met. Studzinski (1996) reports
that between 1970 and 1994, a reduction in daily streamflow in excess of 25% occurred
270 times.