A late Holocene meander-braid transition of the Lower Missouri River Valley.

摘要

Historically, the Lower Missouri River is known for being a temperate river that constantly and quickly reworks its floodplain with a braided network of channels; however, the river only recently became braided. Ox-bow lakes and landscape features resembling highly sinuous meandering loops mark the floodplain of the lower Missouri River Valley as scars of what was once a fully meandering Missouri River. The unanswered question then is, when did the lower Missouri River system change from meandering to braided? To answer this question we compared a series of newly created allostratigraphic maps of the lower Missouri River valley floor to establish a record of river pattern from types of channel loops and their cross-cutting relationships. In addition, OSL samples were strategically collected in order to date the last channel loops created by the meandering system and the first braids that formed after the transition completed. This study is focused on the lower Missouri River between Yankton, SD and Omaha, NE. The data suggest that the last change from meandering to braided in this section of the Missouri River occurred abruptly approximately 1600 +/- 200 yr B.P. Down-dip, from Kansas City, MO to the confluence with the Mississippi River, previous work done by Holbrook et al. (2005, 2006) in the lower reach of the river dates the meander-braid transition from 3400--1200 yr B.P. Both shifts from meandering to braided appear to reflect changes in water and sediment supply because of climate change. We speculate the cause of the early transition onset down-dip to be due to changes in tributary inputs just south of the up-dip area caused by severe drought conditions.