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I I I I I I I t I , I I I I I I , I I OC18C6 4.2.5. Subreach 5: Hwy 24 Bridge to Big Union Creek (RM 3.3- 5.5) Subreach 5 extends from the Highway 24 bridge downstream past Mt. Massive Lakes to the point where Big Union Creek enters the valley (Figure 1.2). The subreach is almost 12,000 ft long, and the average channel slope is 0.68%, which is typical for the project reach (Table 4.2). The subreach is highly dynamic, as demonstrated by a wide migration corridor, multiple active channels and extensive mapped tailing deposits. The planform is characterized by extensive split flow and numerous recent bend way cutoffs. Locally very coarse cobble and boulder bars are overlain by up to 2 ft fine tailing deposits (Plate 11). Natural flow splits are very common within Subreach 5, and a diversion ditch exits the channel at RI'v[ 3.3 R. According to the geologic maps. the Mt. Massive Lakes are located on unconsolidated deposits derived from mass movements such as landslides, mudflows, rock glaciers, and talus, and these deposits extend just into the Arkansas River floodplain. 4.2.6. Subreach 6: Big Union Creek to Hwy 55 Bridge (RM 2.1- 3.3) Subreach 6 extends from the Big Union Creek confluence downstream to Kobe, a distance of approximately 6,300-ft (Figure 1.2). The most striking characteristic of the Arkansas River within Subreach 6 is that the primary channel is perched above the valley floor, which is occupied by Big Union Creek and a secondary Arkansas River channel. The channel gradient is 0.79%, which is slightly steeper than Subreach 5. The Arkansas River has relocated westward into its perched position since 1956; in the 1956 aerial photographs, the main channel is flowing along the valley bottom. As natural channels do not tend to avulse to higher elevation surfaces, the westward channel relocation is likely the result of local diversion or channelization efforts. 4.2.7. Subreach 7: Hwy 55 Bridge to Bedrock Constriction (RM 0- 2.1) Subreach 7 extends from the old Highway 55 bridge downstream to the downstream end of the project reach at the Box Creek confluence (Figure 1.2). The subreach is approximately 10,900 ft long, at an average channel slope of 0.76%. The valley bottom is relatively wide, and the historic floodplain is bisected in a north-south direction by Highway 24 and the railroad. The active channel flows on the east side of the valley bottom, often impinging on outcrop of the eastern valley wall. The planform is relatively straight and largely stable (Table 4.2; Plate 12). The eastern valley foothills of the Mosquito Range are locally gullied and contribute coarse sediment to the channel within the reach. May 7, 1999 Fluvial Geamorphological Assessmen/ Upper Arkansas River Page 30