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Subreach Cross-section Characteristics Subreach Station Average Qbf Average Qbf Average 500-yr Average Qbf (River Mile) Channel WidthlDepth Floodplain Width Recurrence Topwidth (ft)" Ratio. (ft)" " Interval (vears)" I 10,9-12.0 nla n/a 900 nla 2 9.2-10.9 61 40 700 1.1 3 7.0-9.2 87 45 uls: 800 dls: 1000 1.3 4 5.5-7.0 104 49 1200 4.4 5 3.3-5.5 109 73 1200 1.2 6 2,1-3.3 nla nla 1300 nla 7 0-2.1 95 48 900 1.9 " Chapter 7; Qbf=bankfull discharge ""URS hydraulic model (URS, 1998) nla = not available 1 I I I I I , I I I 1 j 4.2.3. Sub reach 3: Lake Fork to Railroad Bridge (RM 7.0-9.2) Subreach 3 is approximately 12,000 ft long, and extends from the Lake Fork confluence (Plate 4) to the onset of split flow at the railroad bridge crossing (near Iowa Gulch; Figure 1.2). The average channel slope of the sub reach is 0.70%. The subreach displays evidence of channel response to flow augmentations and increased sediment loads, including coarse bar development (Plate 5), local braiding (Plate 6), and active bank undercutting and channel widening (Plate 7). Tailings deposits within Subreach 3 have proven difficult to identify as they are commonly overlain by organic-rich floodplain deposits that support thick stands of grasses and willows (URS, 1997). Upstream of the B. Smith bridge, remnant channels to the west have been ponded by beavers; the ponding has perched the water table above the active channel. Table 4.3 Cross-section characteristics of project subreaches 4.2.4. Subreach 4: Railroad Bridge to Hwy 24 Bridge (RM 5.5-7.0) Subreach 4 is approximately 8,000 ft long, and extends from the railroad bridge crossing (near Iowa Gulch) downstream to the Highway 24 bridge (Figure 1.2). The 0.67% slope of the sub reach is typical of the entire project reach (Table 4-2). A primary characteristic of Sub reach 4 is extensive split flow; at the upstream end of the subreach, the channel bifurcates, and one channel crosses under a railroad bridge to the east side of the grade (Figure 1.2). For about 2,000 ft, the two primary channels are separated by the railroad grade. About halfway down the subreach, the channel on the west side of the grade flows under a bridge and joins the other channel. A diversion ditch leaves the western channel (RM 6.2 R) just prior to the bridge crossing. Just downstream of the confluence, the channel splits again, forming two channels that flow semi-parallel for approximately 1,600 ft until they rejoinjust upstream of the Highway 25 Bridge. Bank erosion is active in the lower portion of the subreach, where grazing pressure is intensive (Plate 8), May 7, 1999 Fluvial Geomorphological Assessment Upper Arkansas River Page 27