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Problem Area is also aggrading, as indicated by the PZF plot of the Arkansas River at Las Animas showing aggradation for the period afterl974 (see Figure 6). The effective average channel width is approximately 150 feet. The channel improvement template for this problem area uses a 70-foot bottom width for the low-flow channel, with a slight meander that results in a channel slope of 0.00111. The 2,500 cfs channel has a 200-foot bottom width and follows the valley slope, estimated to be approximately 0.00113. . 3.2.6 Hydraulic and Sediment Summary The hydraulic and sediment analyses performed indicate several things. First, the maximum conveyance capacity within the channel is approximately 1,000 cfs. Second, the erratic hydraulics likely cause seepage problems fro#llocalized high stages in other areas. Third, the channel form and profile are widely variable and :these in tum cause a broad range of sediment transport potentials within the individual reaches. The "fits and starts" nature of the areas inhibits effective conveyance of water and sediment. The sediment transport potentials along the river area are erratic. The channel appears to be largely impacted by these phenomena: hydrologic changes due to flow regulation and urbanization upstream, changes in the vegetal makeup of the floodplain, agricultural encroachment, and the backwater affect of John Martin Reservoir. The dominant discharge appears to have undergone dramatic changes. Vegetation moves into the newly formed banks and inhibits widening again during high flows. Likewise agriculture moves closer in and functions in essentially the same manner. Reduced peaks induce the sense that the river needs less room. In much the same way this encroachment inhibits rewidening of the channel. The battles are fought, sometimes at considerable cost, and the river responds with further instability. The cycle continues and the effective capacity becomes less and less. Compounding this, the available energy slope decre\lses as the reservoir is neared. 3.3 ECOLOGICAL RESOURCES Historic and current ecological conditions were determined from scientific literature, govemment reports, consultation with private individuals and agency representatives, and site visits. Otero and Bent Counties lie within the Southwestern Tablelands ecoregion which is transitional between the Southern Rocky Mountain \lnd Western High Plains ecoregions (Bailey 1976). The native plant community outside the Ar~ansas River floodplain is comprised of short grass prairie. Common species include blue grama" side-oats grama, buffalo grass, galleta, alkali sacaton, sand dropseed, western wheatgrass, and three-awn. Throughout the Arkansas River valley, irrigated agricultural cropland that includes most of the floodplain and upland areas that are now below irrigation canals predominates and often directly abuts the river channel. There are a few small areas of irrigated pasture, dry cropllind, or rangeland. 38 I I I I I I I I I I I I I I I I I I I