EFFECTS OF RARE, EPISODIC HIGH-FLOW EVENT ON A DRYLAND BRAIDED STREAM (PLATTE SYSTEM, WESTERN NEBRASKA) AND THEIR IMPLICATIONS FOR THE SEDIMENTARY RECORD

High-stage flows on the South Platte and Platte rivers in Nebraska in September-October of 2013 resulted from record rains in Colorado. Only five such events have occurred in 112 years of stream gaging on the South Platte River, and others likely occurred in 1844 and 1894. Major impacts from the 2013 event could be detected ~215 km downstream from the Colorado-Nebraska line or ~535 km from Denver. These impacts include the deposition of pebble and imbricated cobble gravels on bar tops, the migration of unit bars over compound bars and stabilized islands, the erosion of current crescents and larger “crescentic pools” in front of tree root wads, the production of flood-channel scours as deep as 1.8 m, and the widespread deposition of low, teardrop-shaped bars (sediment shadows)—some >30 m in length—of very coarse sand and granule to pebble gravels in the lees of trees, shrubs, and grass tussocks. These depositional features contain strata that dip downstream, obliquely downstream, and, rarely, upstream. Walls of woody debris, some > 2m in height, formed against trees and, rarely, at the margins of flood channels. The two preceding high-stage events (1965 and 1973) were within the time frame of the first published depositional models for the Platte system, yet no existing model incorporates the important impacts of episodic high-stage flows. Our observations demonstrate how rare, episodic floods can significantly shape braided streams in semiarid to subhumid regions and influence their sedimentary signatures. Moreover, because the 2013 flood was superimposed on a decades-long independent trend of channel narrowing and abandonment, there is also a basis for the interpretation of braided-stream abandonment in the sedimentary record. Depositional models for such cases should include ample vegetation-induced sedimentary structures, progressive fills of deep scours outside of main channels, thin bar-top gravels atop rooted organic silts, and associated features.