SOME SEDIMENTOLOGIC EFFECTS OF SEPTEMBER 2013 HIGH-STAGE FLOWS ON THE SOUTH PLATTE AND PLATTE RIVERS IN NEBRASKA, USA

Exceptional rains in Colorado in September, 2013 produced major downstream effects in Nebraska. The main channel of the South Platte River was remodeled and widened, but some anabranches showed only minor deposition, if any significant changes at all. In places (e.g., Hershey), gravel-transporting flood channels were produced at least 2.4 m above January, 2014 stage in wooded overbank areas (parts of the braidbelt many decades ago). Crystalline pebble and cobble gravels were deposited on many bar tops and in flood channels. Coarse unit bars with 20-60 cm fronts bars migrated over compound bars in the main channel. Wooded islands and banks overtopped by the flood exhibit flood channels a few tens of meters in width and scoop- to trough-shaped scours as deep as 1.8 m. Vegetation-induced sedimentary structures—most notably low, ridgelike to teardrop-shaped bars of very coarse sand and granule to pebble gravels formed in the lees of shrubs and trees—were commonly produced on wooded islands and overbank areas, atop which flow depths must have exceeded 1.5-2 m. Some of these bars exceed 15 m in length. Walls and ramparts of woody debris remain widespread.

On the Platte River, as far as ~100 km downstream from North Platte, new deposition of sand and fine gravel on channel and side bars was common, but flood effects varied depending on local vegetation cover, engineering works, and pre-existing bar and channel conditions. At Cozad, crystalline cobbles were reworked downstream of a bridge and deposited with imbrication on bar tops and the floor of a channel. Here also, large scours—as much as 16 m in length parallel to local flow and 110 cm in depth—formed on bar surfaces behind vegetation rooted in preflood bar sediments; channel scouring occurred to a minimum depth of 2.5 m below bar tops. Some cobbles were deposited downstream from a bridge south of Darr. High-stage sedimentological effects diminished markedly downstream of Lexington.