Documentation of Existing Conditions in the Central Platte Valley

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SECTIONTWO Environmental Setting sandbars and islands, has been in a west to east, or upstream to downstream, progression. Woodland encroachment has been greatest in the reaches where the greatest flow reduction has occurred, such as the 130 -mile reach of the North Platte and Platte rivers above Lexington where springtime stream flows have been reduced by 85 percent since the early 1900s. Woodland expansion and encroachment has also occurred downstream of the Johnson Power Plant Return i (J -2 Return) near Overton within the study area. In 1938, approximately 24,700 acres of open channel and barren sandbars existed between Overton and Chapman. In 1969, only about 11,000 acres remained, a loss of about 55 percent during the intervening 31 (Sidle et 1989). years al. Most remaining habitat lies between Kearney and Chapman. Trends in channel narrowing and vegetation encroachment occurring over the last 25 years are not as obvious as those occurring in the earlier part of the century (FERC 1998). Lyons and Randle (1988) concluded that the channel widths near Overton had adjusted to the sediment transport regime during 1957 -1983. They concluded that channel width adjustments had also occurred at Odessa and Grand Island and further channel adjustments in the Odessa to Grand Island reach were possible. Currier (1995) and Johnson (1997) provide differing results on recent changes in channel width. Johnson (1994) reported that that the channel fraction of floodplain in the central Platte River upstream of Grand Island has remained stable or increased. Johnson's investigations suggest that this can be attributed to frequent years in which mean June flows ranged between 2,650 and 3,000 cfs and relatively high winter flows coupled with cold air temperatures. The winter flows and temperatures favored ice formation, which can effectively scour young plants. Other factors might include low summer flows leading to seedling desiccation, and occasional high summer flows in the range of 17,650 to 26,500 cfs, which likely caused sedimentation and erosion. Johnson (1997) reported that open channel area has been maintained or has increased in most sections of the Platte River in recent years, despite relatively low flows and infrequent peaks, because of the increasing erodibility of the floodplain. The greater erodibility is due to maturing of the vegetation (lower stem density) and higher floodplain surfaces with steep bank angles. In contrast to most segments, channel area has continued to decrease near Grand Island, which may be due to sediment liberated by upstream vegetation removal within the floodplain. Johnson identified vegetation management project as a "strong candidate" for localized vegetation expansion and open channel decline, but indicated that it could have other causes. Currier (1995) documented an ongoing trend towards continued loss of open channel area and wide channel habitats in the 55 -mile reach between Kearney and Chapman. The loss of remaining open channel in this reach ranged from 4 to 41 percent (average of 26 percent) during 1988 -1994. River segments that had already narrowed significantly and sites where channel habitat is actively managed declined approximately 17 to 18 percent. Sites that were unobstructed in 1988 aerial videotapes were sampled and compared with 1994 videotapes. At several sites, extensive mid - channel bar formation occurred between 1988 and 1994. These changes occurred during a time that included a relatively low flow period (1990 -1992) which allowed woody vegetation to become established. A relatively high flow event (i.e., 12,000 to 16,000 cfs) during June and July 1995 was effective in removing only 4 to 5 percent of the =swim Wbndi—mi Clyde 2-6 WFOM7286=11.d. &V1999(9:52 AM)/URSGWCFS/2 !hdW_dl Services' F1