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<br />characteristic of downstream reaches are absent and vegetative cover is relatively dense. <br />The channel sinuosity is 1.3 (Table 4.2), and the average floodplain width is <br />approximately 900 ft (Table 4.3). <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />1 <br />l <br />I <br /> <br />Table 4.1 Project subreaches <br /> <br /> Subreach Delineations <br />Subreach Station (ft) River Mile Hec-Ras Cross- Length Description <br /> Sections. (ft) <br />1 573+59 to 62+59 10.9-12 940 and above 5,500 Upstream of project reach; old <br /> road crossing to California <br /> Gulch <br />2 487+ 12 to 573+59 9.2,10.9 790-940 8,647 California Gulch and Rte 300 <br /> Intersection to Lake Fork <br /> contluence <br />3 368+46 to 487+ 12 7.0-9.2 645- 790 11,866 Lake Fork confluence <br /> downstream to railroad bridge <br /> crossing <br />4 289+03 to 368+46 5.5-7.0 562-645 7,943 Railroad bridge crossing <br /> downstream to Hwv 24 bridge <br />5 175+49 to 289+03 3.3-5.5 380-562 11,354 Hwy 24 bridge downstream to <br /> Big Union Creek <br />6 108+86 to 175+49 2.1-3.3 281-380 6,663 Big Union Creek downstream <br /> to Hwv 55 briMe at Kobe <br />7 0+00 to 108+86 0-2.1 0-281 10,886 Hwy 55 bridge at Kobe <br /> downstream to bedrock <br /> constriction <br /> <br />*(URS, 1998) <br /> <br />Upstream of the Rte 300 bridge, the Arkansas River channel pattern is anastomosing, <br />which means that is has multiple channels that diverge and rejoin. Anastomosing <br />channels are generally characterized by low, fine-grained banks, a higWy sinuous <br />planform, and multiple channels. In an anastomosing system, new channels develop by <br />avulsion (rapid relocation of a long channel length, as opposed to a bendway cutoff), as <br />old sinuous channels become hydraulically inefficient, causing the frequency of overbank <br />flooding to increase. Concentration of overbank flows on the floodplain results in <br />formation of new channel segments. <br /> <br />In Subreach I, the Arkansas River flows within 1 to 3 distinct channels, all of which <br />convey some flow during high flow events. Relic channels on the floodplain have been <br />danuned by beavers. The channels are highly sinuous, and display active channel <br />relocation; in 1992, a major channel relocation occurred on the Seppi property (Plate 1). <br />Currently, multiple active channels comprise approximately 75% of the reach channel <br />length. The multi-channeled pattern of Subreach I is probably similar to that of the <br />project reach prior to the impacts of resource development. The wide floodplain, dense <br />vegetation, active beaver activity, and presence of multiple, sinuous channels are typical <br /> <br />May 7. 1999 <br /> <br />Fluvial Geomorphological Assessment <br />Upper Arkansas River <br /> <br />Page 23 <br />