FLOOD10000

• County: Bent
• Community: Las Animas
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Las Animas Local Protection Project Design Memo Number 1 Hydrology
• Date: 9/1/1967
• Prepared For: Las Animas
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

present flood-warning
Albuquerque District.
will not be required.

system of the Arkansas River subbasin in the
In view of the foregoinR, additional facilities

VIII - SEDIMENTATION

8-01. Sediment sources,- The drainage area of the Arkansas
River above the Las Anlmas Local Protection Project can be divided
into two general physiographic classifications for defining sediment
source areas. These consist of the Rocky Mountain or western portion
and the Great Plains or eastern portion.

8-02. The Rocky Mountain section indicates exposure of rocks of
both igneous and sedimentary origin. The igneous and metamorphic rocks
in that section cover extensive areas and are relatively resistant to
erosion. Sedimentary materials from geologic formations dating from
Carboniferous through Recent periods are found throughout the mountain-
ous area. The soil cover over most of the area is shallow but the
vegetal cover is generally good. Large areas sustain dense growths
of grasses and coniferous t,rees. Sediment production from the Rocky
Mountain section is relatively low.

8-03. The Great Plains portion of the drainage area north of the
Arkansas River is covered principally by sedimentary formations of
Cretaceous Age. The soil is clay loam, with a vegetal cover of native
grasses. Sediment production from sheet erosion is moderate, but bank
erosion is high on the Arkansas River and tributaries. This increases
the sediment load of the streams.

8-04. The Great Plains portion of the drainage area south of the
Arkansas River and west of the Purgatoire River watershed, is predomi-
nantly Niobrara formation and Benton shale with scattered areas of
limestone and shale outcrops. The soils are highly variable in depth
and texture and most of the area is easily eroded. The soils consist
of clay-loam derived from shale-bearing formations. These easily
erodible materials cause a high rate of sediment production from this
portion of the drainage area.

8-05. Sediment quantities.- The measured suspended sediment
load of the Arkansas River at the Las Animas gaging station for the
22-year period 1940 through 1962 is 1,646 acre-feet per year. The
particle-size analyses of all suspended sediment samples secured at
this station indicate an average of 15 percent sand, 66 percent silt,
and 19 percent clay. Bed samples secured from the Arkansas River in
the vicinity of Las Animas indicate an average of 12 percent gravel,
78 percent sand, and 10 percent silt. .

8-06. Aqqradation of Arkansas River. - The average annual flow
of the Arkansas River at Las Animas is about one-third of the average

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