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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, braided 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 numerous bars and islands. In the late
<br />l800s, the bankfull width of the channel was appro~imately 1,000 feet, and bankfull depth was
<br />within the range of 1 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 evaluated.
<br />Flow-duration curves for the pre-dam, post-dam, anf! post-198l periods were computed and
<br />compared. The comparisons show a substantial redhction in peak flows following construction
<br />of John Martin Dam. This reduction occurs, expec*dly, for large flow events, the type the dam
<br />was designed to control, but also for the most cOmnlon discharges,
<br />
<br />Suspended sediment data were analyzed and indicate an apparent change in the
<br />suspended sediment transport characteristics. lIDs was, in part, expected since one of the
<br />purposes of John Martin Reservoir is retention of sef!iment. 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 bas 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, Fjrst, 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 jJrimarily by three phenomena: a reduction
<br />in peak flows due to the upstream reservoir, change~ 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 exhibiteq 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, agric!lltural 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
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