HOLOCENE LANDSCAPE EVOLUTION IN THE REPUBLICAN RIVER VALLEY, SOUTH-CENTRAL NEBRASKA

Stratigraphic investigations during the late 20^th century in the Republican River valley proposed a sequence of two distinct Holocene terraces. This differentiation, however, was based on limited age control and is problematic. Here, we present results from a systematic study of different alluvial landforms at 22 localities in the Republican River valley. A suite of over 50 bulk soil ¹⁴C ages provide age control for periods of landscape stability and soil formation.

Holocene alluvial landforms in the river valley include a broad floodplain complex (T-0a, T-0b, T-0c), a single Holocene terrace (T-1), and alluvial fans that mostly grade to the T-1 (AF-1) and T-0c (AF-0c) surfaces. Remnants of a late Pleistocene terrace (T-2), mantled by Holocene (Bignell) loess, are also preserved. Holocene alluvial fans (AF-2) also grade to the T-2 surface.

¹⁴C ages suggest that the T-1 fill and AF-1 fans aggraded between ca. 9000-1000 yr B.P. Hence, nearly all of the Holocene alluvium in the river valley is represented by these landforms. Sedimentation, however, was interrupted by several periods of landscape stability. ¹⁴C ages from the upper A horizons of buried soils in the T-1 and AF-1 fills, indicating approximate burial ages, cluster at ca. 6500, 4500, 3500, and 1000 yr B.P.

However,¹⁴C ages also indicate that the T-0c fill and AF-0c fans were aggrading between ca. 2000-900 yr B.P. Given that similar ages occur in the upper parts of the T-1 fill, we suggest that the T-1 surface was abandoned between ca. 4500-3500 yr B.P. but subsequent aggradation of both the T-1 and T-0c fills occurred due to large magnitude flooding events during the late Holocene.

δ¹³C analysis indicates a shift from ~40% C₄ biomass at ca. 6000 to ~85% at ca. 4500 yr B.P. This increase in C₄ biomass is interpreted as reflecting a shift to warmer and likely drier conditions during the Altithermal. We propose a scenario where (1) a reduction in C₃ vegetation after 6000 yr B.P destabilized the uplands, resulting in an increase in sediment supply and aggradation of the T-1 and AF-1 fills and (2) the establishment of C₄ vegetation by ca. 4500 yr B.P. stabilized the uplands, resulting in a reduction in sediment supply and subsequent incision and abandonment of the T-1 surface. The proposed timing and nature of landscape change is consistent with regional records from the Central Plains.