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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O'Neill, et. al. (1997) analyzed the time series of peak discharges from the main stem of
the Upper Arkansas River and five tributary basins and concluded that flow augmentation
on Lake Fork and Lake Creek has dramatically increased flood events and resulted in
very large magnitude floods (1965, 1970, 1972, and 1978) that were not experienced by
adjacent non-augmented streams.

USGS mean daily discharge data from the Arkansas River near Leadville and the
Arkansas River near Malta depict flow conditions upstream and downstream of the Lake
Fork confluence. respectively. These gage data overlap between 1976 and 1983, and
allow a comparison of flows entering the system above California Gulch to those
downstream of Lake Fork tributary input (Figures 3.2-3.4). Although the annual
hydrographs are generally correlative, there are numerous time periods between 1976 and
1983 when the Arkansas River at Malta experienced inordinately higher discharges of
longer durations and unique hydrologic events relative to the reach upstream of
California Gulch (e.g., see 1976, 1978, 1979, 1980, and 1983). Numerous event
hydro graphs, such as that of 1978 (Figure 3.2) depicted display steep limbs that are
typical ofreservoir release patterns down Lake Fork (Figures 3.5-3.7).

Mean daily flow data from the 1976 to 1983 time period indicate that augmentations via
Lake Fork affected the shape and magnitude of the Arkansas River hydrograph at Malta.
Although the patterns depicted in Figures 3.2-3.4 are not reflective of existing hydrologic
conditions, they are important from a geomorphic perspective in that this time period, and
potentially earlier time frames as well, were characterized by relatively high flows and
probable geomorphic adjustment.

Since 1981, to the extent possible, nearly all imported and native inflow to Turquoise
Reservoir have been conveyed through the Mt. Elbert Conduit to the Mt. Elbert
Powerplant (BOR, 1999). The Bureau of Reclamation compared the estimated native
inflow into Turquoise Lake with measured reservoir outflows for spring streamflows for
the years J 992 to 1998 (Figures 3.5-3.7). The results generated indicate that during this
time period, the volume of water released from the reservoir was between 25% and 84%
less than native inflows to the reservoir. The data provided indicate that the effects of the
flow diversions through the Mt. Elbert conduit on the native inflow hydrograph range
from an almost complete diversion of runoff (1992), to a postponement of the peak runoff
event (1993), to a combined increase and postponement of peak discharge (1998).

Figure 3.8 shows flow duration curves for measured discharges for the overlapping 1976-
1983 time period at the Malta gage (07083700) and Leadville gage (07081200). The
overlapping 1990-1993 time period for the Empire gage near Malta (07083710) and
Leadville is shown in Figure 3.9. The Leadville and Malta gages were used as they
record flows upstream and downstream of the Lake Fork confluence, respectively, and
consequently provide a means of assessing augmentation inputs. It should be noted,
however, that the Malta gage also records inputs from non-augmented tributaries such as
Half Moon Creek and California Gulch, such that Lake Fork flows cannot be directly

May 7, 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

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