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6.3.2. Cross-section As described previously (Section 6.1), a primary impact of flow augmentations on project reach tributaries has been active channel widening to accommodate increased erosive energy and flow volumes. Similarly, sediment load increases and livestock grazing generally result in the formation of a wider, shallower channel cross-section. I I I I I I I I I I 1 I I I I Subreach 6 experienced a dramatic reduction in channel sinuosity between 1939 and 1998 (Table 6.2; Figure 6.1). This subreach has experienced channel migration, cutoffs, avulsion, and, potentially, some relocation. The primary cause of the reduction is a major relocation of the main channel segment to a current position on the western margin of the floodplain against the railroad grade. This channel is currently perched above the floodplain to the east, and appears to have been intentionally moved. The increase in sinuosity in Subreach 7 reflects some development of relatively sinuous split flow reaches 1939 and 1996 (Table 6.2; Figure 6.1). This trend may indicate that Subreach 7 is currently being impacted by relatively high sediment loads, such that channel capacity in the main thread is being lost, and old secondary channels are progressively capturing more flow. Aerial photographs were utilized to estimate changes in channel width through time in the project reach. The scale of the historic photos (1939 and 1957) was on the order of 1"= 2000', which made precise width measurements difficult. To compare widths, relatively straight channel segments were selected within each project reach and measured on the photos. The measurements reflect the low flow channel within a straight reach, and do not represent reach average bankfull topwidths, which tend to be significantly wider. The measurements indicate that the low flow channel width has increased within each sub reach between 1939 and 1998 (Table 6.3, Figure 6.2). The long-term average rate of width increase has remained relatively constant at approximately 0.25 ft/yr. These consistent increases in channel width through the project reach reflect the predicted response of the channel to combined influences of increased flows, increased sediment, degradation of the riparian corridor due to substrate toxicity, and livestock access. Table 6.3 Measured low flow topwidth, 1939, 1957, 1996 Subreach Station Width Width Width 1939-1957 % 1957-1996 1939-1996 (River Mile) (1939) (1957) (1996) Change % Change % Change I 10.9-13.1 26 30 30 13% 0% 13% 2 9.2-10,9 35 35 45 00/0 22% 22% 3 7.0-9.2 44 66 70 33% 6% 37% 4 5.5-7.0 55 55 65 0% 15% 15% 5 3,3-5.5 64 69 70 7% 1% 9% 6 2.1-3.3 55 55 65 0% 15% 15% 7 0-2.1 60 60 70 0% 14% 14% May 7, 1999 Fluvial Geomorphological Assessment Upper Arkansas River Page 40