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<br />encroachment inhibits rewidening of the channel. Leveeing further restricts the flow path and <br />causes incision and bank instability. As this cycle continues, the effective conveyance capacity <br />becomes less and less. The current floodway is tightly bounded by farmland and attendant <br />berms. Several agricultural fields were identified as currently at risk from surface water <br />inundation at river discharges near 3,000 cfs. <br /> <br />Historically, riparian vegetation along the Arkansas River in eastern Colorado consisted <br />of a wide band of sparsely distributed plains cottonwood, with scattered stands of sandbar <br />willow along the channel banks and bars. Although relatively dense cottonwood stands <br />occurred between Las Animas and Lamar (the "Big Timbers" area), the majority resembled an <br />open-canopied parkland ranging up to 2 miles wide. Grasslands dominated by salt grass and <br />alkali sacaton occupied areas too saline to support cottonwood and willow. <br /> <br />Currently within the study area, the band of riparian vegetation varies from 250 to 4,000 <br />feet wide and is largely restricted to the 3,000 cfs floodway. Salt cedar, a fast-growing species <br />tolerant of saline soils and shallow groundwater, was first noted in the Arkansas Valley near <br />Lamar in 1913 and has since spread substantially. It has become established in the understory <br />of remnant cottonwood galleries and has replaced broad expanses of riparian grassland <br />communities, such that it is the dominant plant species in the riparian zone. Although salt <br />cedar coverage has increased dramatically, the overall areal extent of riparian vegetation has <br />decreased significantly over the past 100 years due to urban and agricultural development with <br />the floodplain and the reduced effective discharge associated with irrigation and flood control <br />storage in John Martin Reservoir. Additionally, flood control operation has significantly <br />reduced large flood events which formerly scoured extensive areas, creating suitable substrates <br />for cottonwood and willow seed germination. Salt cedar dominates the immediate riverbank <br />throughout most of the study area. While it provides some wildlife shrub cover, its overall <br />value as wildlife habitat is much lower than native cottonwood-willow communities. <br />Additionally, salt cedar root systems can reach a depth of25 feet or more, contributing to <br />relatively high transpiration rates. <br /> <br />Several alternatives to improve water and sediment conveyance, reduce maintenance, <br />and restore riparian functions and values were evaluated. The study recommendations are as <br />follows: <br /> <br />. Nofurther reduction of the current 3,OOO-cfsfloodway capacity. Past reductions <br />already have exacerbated water conveyance, sediment transport, flooding, and <br />groundwater problems. Catastrophic floods can result from storms downstream of John <br />Martin Dam. Floods originating upstream, and of a magnitude similar to the May 1999 <br />event, could likely result in reservoir discharges substantially greater than 3,000 cfs if <br />sufficient storage is not available in the reservoir. <br /> <br />. Extensive channel improvement through excavation is not recommended due to high <br />implementation costs and significant adverse environmental impacts. <br /> <br />III <br />