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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cottonwoods form a continuous, though sparse band throughout the floodway. A few relatively
dense stands of younger (4- to 6-inch diameter) cottpnwood are present. Often, within about 60
feet of the channel, the understory consists of mixed willow, salt cedar, grasses, and (especially
in autumn of 1998) sunflower. Further from the ch$mel, salt cedar occurs either scattered
throughout grassland areas or in dense monotypic st~ds along abandoned meanders. Kochia
grows in very dense strips along the landward edge bf riparian in this reach and, in fact,
throughout the entire study area.

From the Carlton Bridge downstream through the Granada area (including Problem
Area 5), cottonwood is absent and willow is presentionly in small, isolated patches. Salt cedar
dominates the immediate riverbank and entire floodway.
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From the railroad bridge west of Amity dowjlstream through Problem Areas 6 and 7,
mature, scattered cottonwoods again are present. Several small stands of young trees were
observed, however, it is not known whether these originated from seed or are sprouts from
beaver cutting or other disturbance. Sandbar willow, and some peachleafwillow, occur in
patches near the channel, however, nearly all of the Immediate riverbank is dominated by dense
salt cedar. These stands extend throughout the entire floodway, particularly in areas of alkaline
soil in Problem Area 7.

Functions of Riparian Vegetation

The following discussion highlights the major functions of riparian vegetation and is
not intended to be an exhaustive summary. For concise reviews of riparian functions and
values see Brinson et at. 1981, Minshall et at. 1989,:Davis et at. 1996, and Minckley 1997.

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Bank Stabilization. Channel width, depth, and slope are determined to a large degree
by bank stability. Vegetation stabilizes banks by di~ectly reducing flow velocities and thus the
erosive forces at the soil-water interface (Davis et at:. 1996). Roots and rhizomes of bank
vegetation bind soil material, increase cohesiveness,' and reduce weakening and loosening
processes which are often the precursors of entrainment (Thome 1990). The numerous fine
roots of sedges and grasses provide greater binding strength than coarse roots of woody plants.
A mixture of vegetation is generally preferred since :the deeper rooting depth of trees and
shrubs provide additional protection to tall banks. ~egetated banks also are drier than
unvegetated slopes because soil water is removed b)( transpiration, effectively reducing the
likelihood of mass failure. The net effect of these contributing forces is generally positive;
however, in reaches such as the Arkansas River below John Martin Dam, dense bank
vegetation (i.e., salt cedar) can exacerbate channel incision and narrowing.

Resistance to flow. Vegetation within the floodway presents an obstruction to water
flow that tends to decrease flow velocities. Soil erosion is reduced in vegetated overbank areas
and deposition of suspended sediment is enhanced. !The magnitude of these effects depends
upon the density and type of vegetation. Grasses anp short herbaceous groundcover are
flattened against the ground surface by flows and present relatively little resistance to flow.
Shrubs provide higher resistance due to the stiff, less flexible branches and, if present, large