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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3.3.1 Aquatic System

Used here, the "aquatic system" includes the incised river channel and its immediate banks;
fish are the primary aquatic fauna. Fish species that have been reported to occur in the Arkansas
River and its primary tributaries in Otero and Bent Counties and the adjacent upstream and
downstream counties of Pueblo and Prowers, include brook stickleback, common carp,
stoneroller, southern redbelly dace, longnose dace, flathead chub, suckermouth minnow, fathead
minnow, red shiner, sand shiner, grass carp, tench, gizzard shad, white sucker, longnose sucker,
plains killifish, mosquitofish, and Arkansas darter (Woodling 1985).

The previous PAS study showed that the Arkansas River historically was an ephemeral,
braided river downstream of John Martin Dam with a channel-forming discharge of
approximately 3,000 cfs. Several small, shifting channels occupied the broad, sandy river
bottom and were interspersed with numerous bars and islands. In the late 1800s to 1926,
Nadler's 1978 study indicates that bankfull width in the area between La Junta, past Bent's Old
Fort, to the Otero-Bent county line averaged approximately 700 feet with a range from about 623
feet up to about 918 feet. Bankfull depth data are sparse, but was within the range of 2.5 to
about 5.5 feet (see sources in Nadler 1978). These parameters generally describe the aquatic
habitat conditions to which native fish species have adapted in this reach of the river. A similar
1999 Corps study for the reach below John Martin Dam to the Colorado-Kansas state line, and
also utilizing Nadler (1978) for a reference, reported that bankfull widths were about 1,000 feet
and bankfull depths were about 1 to 2 feet. Similarly, 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.
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
and bankfull depth has increased, resulting in further reduction of requisite shallow, low-velocity
areas.

3.3.2 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
overbank 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,
western wheatgrass, 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.

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