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May 7,1999 Fluvial Geomorphological Assessment Upper Arkansas River Page 23 I I I I I I I I I I I I I I I I I I , characteristic of downstream reaches are absent and vegetative cover is relatively dense. The channel sinuosity is 1.3 (Table 4.2), and the average floodplain width is approximately 900 ft (Table 4.3), Table 4.1 Project subreaches Subreach Delineations Subreach Statiou (ft) River Mile Hec-Ras Cross- Length Description Sections' (ft) 1 573+59 to 62+59 10.9-12 940 and above 5,500 Upstream of project reach; old road crossing to California Gulch 2 487+ 12 to 573+59 9,2-10.9 790-940 8,647 California Gulch and Rte 300 Intersection to Lake Fork confluence 3 368+46 to 487+ 12 7,0-9.2 645-790 11,866 Lake Fork confluence downstream to railroad bridge crossing 4 289+03 to 368+46 5.5-7.0 562-645 7,943 Raiiroad bridge crossing downstream to Hwy 24 bridge 5 175+49 to 289+03 3.3-5.5 380-562 11,354 Hwy 24 bridge downstream to Big Union Creek 6 108+86 to 175+49 2.1-3.3 281-380 6,663 Big Union Creek downstream to Hwy 55 bridge at Kobe 7 0+00 to 108+86 0-2.1 0-281 10,886 Hwy 55 bridge at Kobe downstream to bedrock constriction '(URS, 1998) Upstream of the Rte 300 bridge, the Arkansas River channel pattern is anastomosing, which means that is has multiple channels that diverge and rejoin. Anastomosing channels are generally characterized by low, fine-grained banks, a highly sinuous planform, and multiple channels, In an anastomosing system, new channels develop by avulsion (rapid relocation of a long channel length, as opposed to a bendway cutoff), as old sinuous channels become hydraulically inefficient, causing the frequency of overbank flooding to increase. Concentration of overbank flows on the floodplain results in formation of new channel segments. In Subreach I, the Arkansas River flows within 1 to 3 distinct channels, all of which convey some flow during high flow events, Relic channels on the floodplain have been dammed by beavers. The channels are highly sinuous, and display active channel relocation; in 1992, a major channel relocation occurred on the Seppi property (Plate I). Currently, multiple active channels .comprise approximately 75% of the reach channel length. The multi-channeled pattern of Sub reach 1 is probably similar to that of the project reach prior to the impacts of resource development. The wide floodplain, dense vegetation, active beaver activity, and presence of multiple, sinuous channels are typical