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<br />geologic, climatologic, and hydrologic forces influencing the Arkansas River during the: past <br />century. The Albuquerque District conducted all hydraulic and ecological investigations. <br /> <br />The Arkansas River within the study reach historically was an ephemeral, braide:d river <br />with a channel-forming discharge of about 3,000 cfs. Several small, shifting channels occupied <br />a broad, sandy river bottom and were interspersed with munerous bars and islands. In the late <br />1800s, the bankfull width of the channel was approximately 1,000 feet, and bankfull depth was <br />within the range of I to 2 feet. Currently, this reach has become a perennial, narrow, <br />meandering channel. Bankfull width has decreased to approximately 100 feet. Although the <br />channel-forming discharge has decreased to about 800 to 1,000 cfs, bankfull depth has <br />increased to 4 to 6 feet. <br /> <br />Historic flow data for the Arkansas River below John Martin Dam were evaluat,:d. <br />Flow-duration curves for the pre-dam, post-dam, and post-I 981 periods were computed and <br />compared. The comparisons show a substantial reduction in peak flows following construction <br />of John Martin Dam. This reduction occurs, expectedly, for large flow events, the type the dam <br />was designed to control, but also for the most common discharges. <br /> <br />Suspended sediment data were analyzed and indicate an apparent change in the <br />suspended sediment transport characteristics. This was, in part, expected since one of the <br />purposes of John Martin Reservoir is retention of sediment. The change, however, is more <br />complex than a simple reduction of suspended sediment load. Unlike upstream reaches of the <br />Purgatoire and Arkansas Rivers, correlations of suspended sediment load and discharge were <br />weak for the Arkansas River below John Martin Dam, indicating that the river has been placed <br />in a state of non-equilibrium. Many factors have contributed to this condition, including John <br />Martin Dam, diversion structures, local channel modification, and encroachment on the <br />floodway and channel. <br /> <br />Numerical hydraulic models were developed for each of the problem areas to analyze <br />current conditions under the 3,000-cfs operational peak discharge. The combined hydraulic <br />and sediment analyses indicate several problems. First, the conveyance capacity is less than the: <br />3,000 cfs necessary for flood control releases in 4 of the 5 Problem Areas. Secondly, the <br />channel profiles show marked disturbances resulting in local scour or deposition and inhibiting <br />effective conveyance of water and sediment. Additionally, erratic hydraulic conditions <br />contribute to seepage problems during high river flows. Thirdly, sediment transport potentials <br />are erratic. <br /> <br />Overall, the channel appears to be impacted primarily by three phenomena: a reduction <br />in peak flows due to the upstream reservoir, changes in floodplain vegetation, and <br />encroachment on the channel and floodway. Reduced peak flows has caused the channel to <br />become smaller and this shrinkage is often exhibited as narrowing. Dense vegetation <br />(primarily salt cedar) has become established on the newly formed bank and inhibits its <br />widening again during high flows. Likewise, agricultural fields have encroached on the <br />floodway with a similar result. The reduced peak flows have induced an expectation that the <br />river needs less room. Lands nearer the river have been put into production and this <br /> <br />II <br />