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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Aquatic System

Used here, the "aquatic system" includes the incised river channel and its immediate
banks; fish are the primary aquatic fauna. Fish species in the Arkansas River in Prowers
County include stoneroller, longnose dace, flathead chub, suckermouth minnow, fathead
minnow, red shiner, sand shiner, white sucker, Plains killifish, and, the introduced common
carp (Woodling 1985). The Arkansas Darter, listed by the State of Colorado asa threatened
species, occurs in Big Sandy Creek.

As described above and in the geomorphological assessment (Appendix C), the
Arkansas River historically was an ephemeral, braided river with a channel-fonning discharge
of approximately 3,000 cfs. Several small, shifting channels occupied the broad, sandy river
bottom and were interspersed with numerous bars artd islands. In the late 18005, bankfull
width was approximately 1,000 feet. Bankfull depth data are sparse, but was within the range
of 1 to 2 feet (see sources in Nadler 1978). These parameters generally describe the aquatic
habitat conditions to which native fish species have adapted. The shallow, low-velocity
conditions provided abundant feeding, spawning, and refuge areas.

Currently, the river within the study reach has become a perennial, narrow, meandering
channel (Appendix C). Bankfull widths throughout the reach have decreased to approximately
100 feet, resulting in a nearly ten-fold decrease in channel area. Although the channel-forming
discharge has decreased to about 800 cfs, bankfull depth has increased to 4 to 6 feet, resulting
in further reduction of requisite shallow, low-velocity areas.

Riparian System

"

The riparian zone of a river or stream includes that portion of the terrestrial landscape
from the water edge landward where vegetation may be influenced by river-associated water
tables or flooding and by the ability of soils to hold water (Mitsch and Gosselink 1986, Naiman
et al. 1993). Historically, riparian vegetation along the Arkansas River in eastern Colorado
consisted of a wide band of sparsely distributed plains cottonwood, with scattered stands of
sandbar willow and, less extensively, peach-leaf willow along the channel banks and bars
(Lindauer 1970). Although relatively dense cottonwood groves occurred between Las Animas
and Lamar, the majority of the area consisted of an open-canopied parkland. Common native
grasses and forbs within the riparian corridor included salt grass, alkali sacaton, sand dropseed,
vine mesquite, sunflowers, and wild licorice. These species comprised the groundcover
throughout cottonwood stands and were the dominant vegetation in areas too saline to support
cottonwood and willow.

The 30-mile reach downstream of John Martin Dam historically was known as the "Big
Timbers," a scattered grove of gigantic cottonwoods (7 to 8 feet in diameter) which grew on
islands in the river and along the banks, and lacked a shrub understory (\1 estaI1939). Zebulon
Pike first noted this stand on his journey up the Arkansas River in 1806. The area was used
extensively by Native Americans, particularly in winter, and by travelers on the Santa Fe Trail;
"Big Timbers" was the only cottonwood stand of any appreciable size in the 350-mile segment