FLOOD06271

• County: Otero
• County: Pueblo
• Community: Otero, Pueblo Counties
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Arkansas River Channel Capacity and Riparian Habitat Planning Study
• Date: 7/1/1999
• Prepared For: CWCB
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

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The correlations and graphical representations indicate an apparent change in the
suspended sediment transport characteristics of the Arkansas River as a result of John Martin
Dam, This is expected, since one of the benefits of a reservoir often is retention of sediment.
The Sedimentation in John Martin Reservoir, Arkansas River Basin, Colorado, Resurvey of
March 1972 report (USACE 1973) states that the reservoir has a calculated trap efficiency of
87,5%, The change, however, is more complex than a simple reduction of suspended sediment.
While the aggregate data and linear representations are relatively well behaved and consistent
for the Arkansas River at Las Animas (and the Purgatoire River, as well), there is considerably
more scatter for the Arkansas River below the dam, The correlation coefficient for the
Arkansas River below the dam is considerably less, 0,69, compared to the Arkansas River at
Las Animas, 0,85, and the Purgatoire River at Las Animas, 0,86 ror the total data sets.

For the monthly analyses, the Arkansas and Purgatoire Rivers at Las Animas show
reasonable curve fits, with correlation coefficients of 0,65 to 0,95 and 0,77 to 0,90,
respectively, while the Arkansas River below John Martin Dam values range from 0.00 to 0,78,
The graphs further illustrate this latter point - for the months of May through September the
fitted lines show a marked difference in slope, with May and July sloping in the opposite
direction from the others (Qss decreasing for increasing Q) and September showing a near-
horizontal line. Part of this is likely due to the limited data sets, but it would also seem to
indicate that the suspended load is being dispersed in a different manner during the summer
months,

It is worth noting that one method of calculating sediment discharge for sands, Colby's
relations, adjusts the base calculated value upward for increased suspended sediment
concentration (Vanoni 1975), This implies a correlation between suspended sediment
concentration and bed material transport capacity, A change in the wash load component of the
Arkansas River downstream ofthe dam could be having an impact on the sands carried by the
river. This, coupled with a reduction in the peak discharges in the river as a result of operation
of the dam for flood control, would be expected to cause deposition without an equalizing
change in the river hydraulics (e,g., channel geometry) or bed material load. It is assumed that
the reservoir also traps a significant quantity of bed material, but it is not readily apparent,
whether the reduced peak discharges or reduced sediment load is dominant. Furthermore, the
scour, which generally occurs immediately downstream of a dam, often causes deposition at
some point farther downstream,

These factors indicate that the river has been placed in a state of non,equilibrium as a
result of John Martin Dam, although there are many other factors that have also contributed to
this condition,

Degradation Range Surveys were analyzed from nine survey periods ranging from
December 1943,February 1944 to March 1987 (USACE 1965,1973,1981,1987), The cross,
sectional areas were compared by choosing an arbitrary elevation above all of the section pointsand calculating the areas between this line and the ground surface, These were then compared
to get changes in area, The resulting areas were also multiplied by the channel distances

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