FLOOD07093

• 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: 8/1/2001
• Prepared For: CWCB
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

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temperature is 90 of or higher on about 70 percent of the days. The length of the freeze-free
growing season near La Junta in the Arkansas River Valley is approximately 160 days (Berry
1972:79).

Average annual precipitation in the study area is about 12 inches, nearly 60 percent of
which result from thunderstorms in May through August (Berry 1971:2, 1972:77-79). The
annual snowfall is about 20 inches (US ACE 1965:17). Because rainfall is low and summer
temperatures are high, evaporation rates are high during the growing season. Average annual
pan (Class A) evaporation at John Martin Dam is 84 inches, of which approximately 66 inches
occurs during April through October (US ACE 1983a:4/4-4/5). The five months, from November
through March, account for approximately 20 percent of the annual pan evaporation.

2.3 PHYSIOGRAPHY, GEOLOGY, AND SOILS

The study area upstream of John Martin Dam lies along the border of the Colorado
Piedmont and Raton subsections of the High Plains section of the Great Plains Physiographic
Province (Fenneman 1931:6, Figure 2, 30-47; Abbott 1985:5, Figure 2; USACE 1983a:4/2). The
region is characterized by flat to gently rolling uplands with a few shallow valleys and many
shallow, un-drained depressions. The alluvial bottomland along the river ranges from one to
three miles wide and is bounded on the north and south by gravel terraces and sand hills.

Bedrock underlying the Arkansas River consists of Cretaceous sandstone, shale, and
. limestone that have "a very slight regional dip to the north-northeast of about 0.5 degrees to 3
degrees" (Van Couvering 1982:397). In the John Martin Dam area, most of the channel is within
the Lower Cretaceous Dakota formation that is composed of fine-grained sandstone and sandy
shale. Saturated valley alluvium consisting of gravel, sand, silt, and clay of Pleistocene to
Holocene age averages about 50-feet thick and occupies a trough eroded in the sedimentary
bedrock (Fenneman 1931:22; Elkin 1972:72; Watts and Linder-Lunsford 1992:5). The gradient
in the river valley below Pueblo varies between 5 and 9 feet per mile (US ACE 1973:34;
1977a:3).

Soils within the Arkansas River floodplain belong to the Rocky Ford-Numa-Kornman and
the Las-Apishipa-Bankard associations that consist of alluvial materials varying extremely in
texture, depth, and drainage (Larsen et al. 1972; Preator et al. 1971; USACE 1983a:412). These
soils include sand and gravel, loams, clay loams, and silty-clay loams.

2.4 WATER RESOURCES

The Arkansas River comprises Colorado's largest drainage basin. Originating in the Rocky
Mountains in the central portion of the State, the river generally flows southward for about 60
miles through mountain valleys and then eastward across the plains for about 240 miles before
entering Kansas. The drainage area above John Martin Dam is 18,915 square miles of which
18,130 square miles are contributing drainage area (US ACE 1983a:4/1; 1984:2). Snowmelt in
the upper reaches of the basin generally begins in April, with the majority of runoff occurring
from May through July.

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