WOODY VEGETATION EXPANSION AND CONTINUING DECLINES IN OPEN CHANNEL HABITAT ON THE PLATTE RIVER IN NEBRASKA

Aerial videography in 1988 and 1994 of the same selected segments of the Big Bend reach of the Platte River in central Nebraska was compared. During this 6-year period, channel area declined 4 to 41 % (median 26%). In river segments where the channel had already narrowed significantly and at sites where channel habitat is actively managed declines were less (17-18%). These substantial changes occurred during a time that included a relatively low flow period (1990-92). Flows during the summer gertttination and establishment period (mid-May through August) for cottonwood (Populus deltoides) and willow (Salix spp.), the primary woody species, were less than 17 m3/second 64% of the time during the period and less than 3 m3/second 25% of the time in 1990 and 1991. Such low flows allowed extensive exposure of the riverbed where germination could occur. Peak scouring and ice flows were also low, remaining below 80 m3/second throughout 1990-92 except for a few days in May 1991. Because the expanding vegetation recorded in this study had grown to an advanced stage, it most likely became permanently established. Furthermore, high flows in the 340-450 m3/second range, which occurred in June and July 1995, were effective in removing only a fraction of the newly developed growth