Arkansas River Channel Capacity and Riparian Habitat Planning Study

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1771 Currently within the study area, the band of riparian vegetation varies from 250 to 4,000 feet wide and is largely restricted to the 1,000 to 1,500 cfs floodway. Salt cedar, a fast - growing species tolerant of saline soils and shallow groundwater, was first noted in the Arkansas Valley near Lamar in 1913 and has since spread substantially. it has become established in the understory of remnant cottonwood galleries and has replaced broad expanses of riparian grassland communities, such that it is the dominant plant species in the riparian zone. Although salt cedar coverage has increased dramatically, the overall areal extent of riparian vegetation has j decreased significantly over the past 100 years due to urban and agricultural development within the floodplain and the reduced effective discharge associated with irrigation and flood control storage in Pueblo Reservoir. Additionally, flood control operation has significantly reduced large flood events which formerly scoured extensive areas, creating suitable substrates for cottonwood and willow seed germination. Salt cedar dominates the immediate riverbank throughout most of the study area. While it provides some wildlife shrub cover, its overall value as wildlife habitat is much lower than native cottonwood - willow communities. Additionally, salt cedar root systems can reach a depth of 25 feet or more, contributing to relatively high transpiration rates. Several alternatives to improve water and sediment conveyance, reduce maintenance, and restore riparian functions and values were evaluated. The study recommendations are as follows: • No further reduction of the current 1,000 to 1,500 -cfs floodway capacity. Past reductions already have exacerbated water conveyance, sediment transport, flooding, and groundwater problems. Catastrophic floods can result from storms downstream of Pueblo Dam. Floods originating upstream, and of a magnitude similar to the May 1999 event, could likely result in reservoir discharges substantially greater than 1,000 to 1,500 cfs if sufficient storage is not available in the reservoir. • Extensive channel improvement through excavation is not recommended due to high implementation costs and significant adverse environmental impacts. • Smaller -scale channel modification techniques cannot be approached generically. Specific points along the river may benefit temporarily from channel modification to avoid or reduce structural damage. Existing erratic hydraulic and sediment transport characteristics demand site - specific design, determination of upstream and downstream effects, and consideration of overall channel stability. Because of the widely varying hydraulic characteristics throughout the study reach, a generic channel configuration addressing conveyance, sediment transport, and vegetation encroachment on banks could not be identified. • Inundation of agricultural fields by river flows can be addressed in various ways: - Structurally, through rehabilitation of existing berms by local authorities; - Non - structurally, through retirement of lands from production and conversion to ' pasture or riparian vegetation. This may be pursued by individual landowners, or facilitated by the Corps of Engineers' ecosystem restoration authority (Section 1135).