PLATTE RIVER RESTORATION

The Lexington to Chapman reach of the Platte River in Nebraska is the focus of a proposed endangered-species-recovery program to restore some of the bird habitats that have been lost through channel narrowing and vegetation growth. One goal of the program is to restore channel habitat by making some areas of the main river channel shallower and wider, with water filling the channel during the spring and fall crane migration seasons, and with braided, dry sand bars emerging from low water during the summer nesting season. Minimal vegetation encroachment within the channel is also critical for the endangered birds. There is no intent to return the Platte to historic widths, but rather to restore a few wide, braided, channel reaches.

The data and analyses from our study associate the channel narrowing with substantial reductions in river flow and the resulting expansion of vegetation, primarily caused by water development and drought, and by a decrease in sediment supply and resulting channel incision. Reductions in river flow account for most of the narrowing. Vegetation growing on sandbars creates wooded islands, dividing the river into narrow channels.

Clear water from dams and canals causes channel incision, which initially has maximum scour impact near the sources of clear water. Local bed coarsening causes the erosion to migrate downstream over time. Channel narrowing and bed coarsening are geomorphic consequences of incision. Significant incision can currently be found in the reach immediately downstream from the Johnson-2 Canal Return, while smaller amounts of incision have been noted as far downstream as Kearney. With no additional changes to flow or sediment, it is anticipated that this channel narrowing will continue downstream to Chapman.

The proposed plan for channel restoration includes: 1) the clearing and lowering of vegetated islands, and 2) annual pulse flows of short (3 day) duration from Kingsley Dam. The clearing and lowering of fine-grained river islands would immediately increase the area of wide, open channel, and add finer sand to the riverbed. The annual pulse flows would build sandbars for summer nesting and would keep the sandbars clear from the previous year's seedlings. Implementation would occur by adaptive-management to ensure habitat improvement and avoidance of adverse impacts.