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<br /> <br />1770 <br />The Arkansas River within the study reach historically was an ephemeral, braided river <br />with a varied channel today of about 3,000 cfs. Several small, shifting channels occupied a <br />broad, sandy river bottom and were interBpersed with numerOUB bars and islands. In the late <br />1800s, the bankfull width of the channel was approximately 1,000 feet, and bankfuIl depth WBS <br />within the range of 1 to 2 feet. Currently, thiB reBchhas become a perennial, narrow, meandering <br />channel. Bankfull width has decreased to approximately 100 feet. <br /> <br />Historic flow data for the Arkansas River ab$ve John Martin Darn were evaluated. A <br />Rating Curve for the stream gage at La Junta was developed to detennine the.2-year flow. The <br />2-year flow at La Junta is approximately 7,000 cfs and this flow transportB the greatest amount of <br />sediment over the historical record of the Arkansas Ri vcr. <br /> <br />Suspended sediment data were analyzed and upstream reaches of the Purgatoire and <br />Arkansas Rivers show high rates of suspended sediment. Many changes have occurred as a <br />result to thiB condition Buch as decreased channel capacities due to sediment deposition, scouring <br />of embankments, high water table and extensive overgrowth of vegetation and encroachment on <br />the floodway. <br /> <br />Numerical hydraulic models were developed for each of the problem areas to analyze <br />current conditions with flows of 100, 200, 300, 500, and 1,000 cfs. The combined hydraulic and <br />sediment analyses indicate several problema. First, the channel capacity in some areas is less <br />than 1,000 cfs. Second, the 2-year flow of 7,000 cfs would completely flood the overbank areas <br />resulting in local scour or deposition and inhibiting effective conveyance of water and sediment. <br />Additionally, erratic hydraulic conditions contribute to seepage problems during high river <br />flows. Thirdly, sediment transport potentials are erratic. <br /> <br />Overall, the channel appears to be impacted primarily by three phenomena: a reduction <br />in peak flows due to the Pueblo Dam and Reservoir, changes in floodplain vegetation, and <br />encroachment on the channel and f1oodway. Reduced peak flows have caused the channel to <br />become smaller and this shrinkage is often exhibited as narrowing. Dense vegetation (primarily <br />salt cedar) has become established on the newly formed bank and inhibits the river's widening <br />again during high flows. Likewise, agricultural fields have encroached on the f100dway with a . <br />similar result. The reduced peak flows have induced an expectation that the river needs less <br />room. Lands nearer the river have been put into production and this encroachment inhibits <br />rewidening of the channel. Leveeing further restricts the flow path and causes incision and bank <br />instability. As this cycle continues, the effective conveyance capacity becomes less and less. <br />The current f100dway is tightly bounded by fannlanl;! and attendant benns. Several agricultural <br />fields were identified as currently at risk from surfaCe water inundation at river discharges near <br />1,500 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 willow <br />along the channel banks and bars. Although relatively dense cottonwood stands occurred <br />between La Junta and Las Animas (the "Big TimberB" area), the majority resembled an open- <br />canopied parkland ranging up to 2 miles wide. GraBslands dominated by salt grass and alkali <br />sacaton occupied areas too saline to support cottonwood and willow. <br /> <br />Iv <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />