Documentation of Existing Conditions in the Central Platte Valley

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SECTIONTWO Environmental Setting proportion (7.3 percent) of roads than all other segments. This area includes a portion of I -80 and U.S. Highway 281, and a major housing development south of the Platte. 2.3 PLATTE RIVER HABITAT CHANGE /DECLINE The Platte River was historically a wide, unvegetated channel, which has experienced a drastic reduction in channel width and increase in riparian woodland since the time of the first settlers. In the 1800s the Platte River averaged about 1.2 miles in width, varying between 3.1 and 0.75 mile ( Eschner et al. 1983). In the 73 years between 1865 and 1938, the Platte River in the Big Bend Reach narrowed to between 40 and 80 percent of its 1865 width. Between 19' )8 and 1953 it narrowed at an even greater rate. This reduction in channel width coincided with the onset of water development, which altered flow patterns, reduced flows, and reduced sediment loads. The Platte River formerly experienced large peak discharges during the snowmelt period when a multiple channel system inundated large portions of the floodplain (FERC 1998). The'large, easily moved sand supply overloaded those channels, causing them to constantly shift and destroy sandbars and islands across a wide floodplain. Several factors prevented woody plants, such as cottonwood, from establishing on the floodplain, including high flows in late spring that eroded and smothered seedlings and saplings and that inundated much of the suitable substrate available for seed germination. Low summer flows allowed the drought- sensitive seedlings to desiccate (Sidle et al. 1989; Williams 1978; Johnson 1994). Peak and mean flows are now approximately half the levels they were before creation of Lake McConaughy and Lake Glendo ( Eschner et al. 1983); and sediment supplies have decreased by more than half. With reduced flows and reduced sediment supply, channel stability has increased so that sandbars and islands are mobilized less often and become vegetated. Much of the Platte River in the study area is now a network of relatively stable braided stream channels that repeatedly split and rejoin as they flow around permanently vegetated islands. These channels are more incised and have coarser bed materials than the historic river. The Platte River channel has narrowed primarily through the process of island formation with subsequent island attachment to the floodplain ( Eschner et al. 1983; FERC 1998). Islands develop from sandbars within the channel. These bars are mobile during high flows but are inactive and exposed during lower flows. The exposed bars are invaded by vegetation, which tends to stabilize them. Subsequent vegetation growth. and sediment deposition enlarges the bar. At some point, it becomes sufficiently large to be considered an island. This growth isolates a small channel between the island and the adjacent floodplain, which is progressively filled in by sediment, allowing the former sandbar /island to be attached to the floodplain. The remnant stream channels between the islands and the floodplain are flooded primarily during high flow events that overtop the islands. These flows tend to bring finer- grained sediment into the channels that eventually transform them into meandering channels that tend to pond at low flows. Islands coalesce as the channels between them gradually lose their water and sediment- carrying capacities. Descriptions of vegetation establishment within the former channel are provided by Eschner et al. (1983), Nagel et al. (1980), Johnson (1994), and Currier (1996). The trend in encroachment of vegetation in the Platte River channels, and consequent vertical and horizontal accretion of M Oskar WbodwrardClyde Servim 66FOD97286W/rl.doc 6r211999(9:52 AM)fURSGWCFS12 2-5