Properties of a 5500-year-old flood-plain in the Loup River Basin, Nebraska

Flood-plain aggradation within the Loup River Basin of central Nebraska was episodic and alternated with incision throughout much of the Holocene. A widespread episode of flood-plain stability, however, occurred about 5700–5100 cal. year BP. The purpose of this paper is to describe the properties of this buried flood-plain at six sites in the basin, to consider why the properties of the buried flood-plain vary from site to site, and to evaluate possible reasons why the Loup River flood-plains stabilized 5500 years ago. Episodic valley-bottom aggradation was common during flood-plain formation at five of the six sites. The radiocarbon ages, particle-size data, and organic-carbon data for the buried flood-plain reveal that valley-bottom aggradation generally slowed between about 5700 and 5100 cal. year BP. Erratic down-profile changes in percentages of sand, clay, and organic matter indicate flood-plain sedimentation and soil formation were often episodic. Sand and clay rarely show a steady fining-upward trend. Organic matter fluctuates with depth; at some sites multiple, incipient A horizons were buried during waning valley-bottom aggradation. At two localities, the buried flood-plain is evident as a clay-rich stratum that must have been deposited in a paleochannel. Flood-plain stabilization between 5700 and 5100 cal. year BP probably occurred in response to the effects of external climate forcing on vegetation and hydrologic changes. Flood-plains of other rivers in the central Great Plains also stabilized at this time, further supporting a climatic explanation for slowing of valley aggradation and formation of a flood-plain at this time. Recognition of buried flood-plains is important to both soil mapping in valleys and to the discovery of cultural resources in valleys.