Arkansas River Channel Capacity and Riparian Habitat Planning Study

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The Arkansas River within the study reach historically was an ephemeral, braided river with a varied channel today of about 3,000 cfs. Several small, shifting channels occupied a broad, sandy river bottom and were interspersed with numerous bars and islands. In the late 1800s, the bankfull width of the channel was approximately 1,000 feet, and bankfull depth was within the range of 1 to 2 feet. Currently, this reach-has become a perennial, narrow, meandering channel. Bankfull width has decreased to approximately 100 feet. Historic flow data for the Arkansas River above John Martin Dam were evaluated. A Rating Curve for the stream gage at La Junta was developed to determine the-2-year flow. The 2 -year flow at La Junta is approximately 7,000 cfs and this flow transports the greatest amount of sediment over the historical record of the Arkansas River. Suspended sediment data were analyzed and upstream reaches of the Purgatoire and Arkansas Rivers show high rates of suspended sediment. Many changes have occurred as a result to this condition such as decreased channel capacities due to sediment deposition, scouring of embankments, high water table and extensive overgrowth of vegetation and encroachment on the floodway. Numerical hydraulic models were developed for each of the problem areas to analyze current conditions with flows of 100, 200, 300, 500, and 1,000 cfs. The combined hydraulic and sediment analyses indicate several problems. First, the channel capacity in some areas is less than 1,000 cfs. Second, the 2 -year flow of 7,000 cfs; would completely flood the overbank areas resulting in local scour or deposition and inhibiting Effective conveyance of water and sediment. Additionally, erratic hydraulic conditions contribute to seepage problems during high river flows. Thirdly, sediment transport potentials are erratic. Overall, the channel appears to be impacted primarily by three phenomena; a reduction in peak flows due to the Pueblo Dam and Reservoir, changes in floodplain vegetation, and encroachment on the channel and floodway. Reduced peak flows have caused the channel to become smaller and this shrinkage is often exhibited as narrowing. Dense vegetation (primarily salt cedar) has become established on the newly formed bank and inhibits the river's widening again during high flows. Likewise, agricultural fields have encroached on the floodway with a similar result. The reduced peak flows have induced: an expectation that the river needs less room. Lands nearer the river have been put into production and this encroachment inhibits rewidening of the channel. Leveeing further restricts the flow path and causes incision and bank instability. As this cycle continues, the effective conveyance capacity becomes less and less. The current floodway is tightly bounded by farmland and attendant berms. Several agricultural fields were identified as currently at risk from surface water inundation at river discharges near 1,500 cfs. 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 along the channel banks and bars. Although relatively dense cottonwood stands occurred between La Junta and Las Animas (the "Big Timbers" area), the majority resembled an open - canopied parkland ranging up to 2 miles wide, Grasslands dominated by salt grass and alkali sacaton occupied areas too saline to support cottonwood and willow. iv