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I I' I I I I I I l , r l I I I I I I , C' ,~8 -'3 , ......- \.J of the drainage basin above California Gulch. An irrigation diversion structure is located on the left bank of the main channel approximately 1/, mile upstream from California Gulch. Table 4.2 Profile and planform characteristics of project subreaches Subreach ProfilelPlanforrn Characteristics' Sub reach Station Sinuosity. % Split Valley Channel Description (River Mile) Flow' Slope Slope (Nfl) (Nfl)" I 10.9-12.0 1.25 75 0.013 0.010 Upstream of project reach; extensive split flow; active channel relocation 2 9.2-10.9 1.2\ 10 0.0\24 0.0102 Steep aggradational wedge of tailings 3 7.0-9.2 1.29 32 0.0091 0.0070 Active widening 4 5.5-7.0 1.34 89 0.0075 0,0067 Extensive split flow; wide floodplain 5 3.3-5.5 1.18 18 0.0088 0.0067 Depositional reach; locally braided, wide floodolain 6 2.1-3.3 1.12 95 0.0091 0.0080 Perched reach; floodplain extends to valley bottom 7 0-2.\ \.l4 18 0.0089 0.0076 Relatively stable planforrn '!Tom aerial photographs flown on 1113/97 at a scale of I "~700' "URS hydraulic model (URS, 1998); Plotted profiles are shown in Appendix A 4.2.2. Subreach 2: California Gulch to Lake Fork (RM 9.2-10.9) Subreach 2 is approximately 8,650 ft long, and extends from the California Gulch confluence, which is the major source of contaminants, downstream to the Lake Fork confluence (Figure 1.2). Subreach 2 has been impacted by mine tailings from California Gulch as well as augmented flows down the Arkansas River. The channel is relatively steep at 1 % (Table 4.2; Appendix A). Local course material is contributed to the channel from alluvial terraces on the left (east) bank (Plate 1). Some efforts have been made to stabilize the toe of the eroding terrace adjacent to the wastewater ponds with bio- technical measures. Fine tailings deposits in bar swales reach up to 4-ft thickness (Plate 2). Active sediment deposition within Subreach 2 is evidenced by the presence oflocally braided channel segments. Approximately 10% of Subreach 2 has split flow conditions, mostly in locally braided areas that have extensive mid-channel bars. Limited bank protection has been constructed at the downstream end of Subreach 2 near the Lake Fork confluence (Plate 3). Two irrigation diversions arc located at RM 9.3 left that convey flows to a wide floodplain area east of the channel. May 7. 1999 Fluvial Geomorphological Assessment Upper Arkansas River Page 24