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Table 6.1 Measured channel sinuosity: 1939, 1957, and 1996 I I I I I , I I 1 In Subreach 2, sinuosity has remained relatively consistent since 1939 (Table 6.1; Figure 6.1), although the reach has been characterized by numerous cutoffs and rapid bendway migration (Chapter 8). The sinuosity trend reflects a relative balance of channel shortening through cutoff and lengthening through migration along the reach. The percent split flow has decreased, which suggests that the historic predominance of braided conditions has lessened since 1939 (Table 6.2). The aerial photographs support these trends, as they display extensive braiding in 1939, followed by the progressive development ofa single thread through 1998. Subreach River Mile Sinuosity Sinuosity Sinuosity Description (1939)" (1957) (19961 I 10.9-13.1 1.26 1.33 1.25 Increase followed by reduction; avulsion nrocess 2 9,2-10.9 1.18 1.19 1.21 SliQht increase: reduced braidinQ 3 7.0-9.2 1.31 1.23 1.29 Reduction followed by increase: natural bendwav chanQes 4 5.5-7.0 1.11 1.23 1.34 Increase: 'sol it flow caoture 5 3.3-5.5 1.25 1.19 1.18 Reduction: aQQradational 6 2.1-3.3 1.35 1.28 1.12 Reduction: channel relocation 7 0-2.1 1.09 1.06 1.14 SliQhl increase; solil flow sinuosity Table 6.2 Historic split flow conditions: 1939, 1957, and 1996 Subreach Station % Split Flow % Split % Split 1939-1957 1957-1996 1939- t996 (River (1939) Flow Flow % % % Mile) (1957) (1996) Chanpe Chanpe Change I 10.9-13.1 75 75 75 0% 0% 0% 2 9.2-10.9 37 22 10 -15% -12% -27% 3 7.0-9.2 29 23 32 -6% 90/0 3% 4 5.5-7.0 64 70 89 6% 19% 25% 5 3.3-5.5 35 19 18 -16% -1% -17% 6 2.1-3.3 71 69 95 -2% 26% 24% 7 0,2.1 15 25 18 10% -7% 3% Sinuosity has increased significantly in Subreach 4 since 1939 (Table 6.1; Figure 6.1). This change reflects the relocation of the main channel from the east channel, which runs directly against the railroad grade to the west channel, which has substantial planform diversity. A second split flow channel segment developed between 1939 and 1957. That lower split flow channel section, where 1M 4 is located, has become more sinuous through time. The sinuosity within Subreach 5 has progressively dropped since 1939 (Table 6.1; Figure 6.1). Sediment transport analyses and field observations indicate that this reach is aggradational; the sinuosity reduction supports this interpretation. Split flow lengths have decreased slightly since 1939 (Table 6.2); this reflects the abandonment of secondary channels. May 7. /999 Fluvial Geomorphological Assessment Upper Arkansas River Page 38