IN THE FOOTPRINTS OF DAN MUHS: RECONSTRUCTING LATE-QUATERNARY LANDSCAPE EVOLUTION IN RIVER VALLEYS OF THE NEBRASKA SAND HILLS

The Nebraska Sand Hills region is the largest dune field in North America and was periodically active during the late Pleistocene and Holocene. Dan Muhs, who we are honoring in this session, is among a small group of geomorphologists that has studied the Sand Hills. Muhs and others laid the foundation for understanding late-Quaternary landscape evolution in the Sand Hills, and most of their research focused on the age, sediment source, and paleoclimatic significance of the dune field. By contrast, our research is focused on the late-Quaternary stratigraphic record in valleys of major rivers within the central Sand Hills, and climatic fluctuations are inferred from that record.

Stratigraphic sequences and 51 ¹⁴C ages among 12 localities in the Snake, Loup, and Dismal river valleys revealed a consistent pattern of late-Quaternary landscape evolution. Specifically, gradual accumulation of alluvium occurred on valley floors under relatively moist conditions between ca. 12.8 and 8.0 ka, and marshes were common in those settings. In some valleys, lakes formed where dunes dammed the rivers. For example, a sequence of eolian, alluvial, and lacustrine sediments at a locality in the South Fork Dismal River valley indicates that a sand dune blocked the river around 10.5 ka. After the initial development of a wetland behind the dune dam, a lake formed and persisted until at least 8.2 ka.

Increased aridity at 8.0 ka that persisted until ca. 6.5 ka triggered landscape instability. There is a paucity of alluvium dating to that period and the middle Holocene. An erosion unconformity often separates early Holocene alluvium from late Holocene eolian or alluvial deposits. Accumulation of alluvium resumed on valley floors after ca. 3.0 ka and was punctuated by brief episodes of landscape stability and weak soil formation. Also, wetlands and lakes briefly formed behind dune dams during this period. Episodes of sedimentation and stability are attributed to periods of aridity and increased effective moisture, respectively. Rivers incised late-Holocene floodplains around 1.2 ka (Medieval Warm Period), creating broad T-1 terraces. Alluviation resumed after ca. 1.0 ka, emplacing flood deposits on T-1 terraces and forming the fills of the modern floodplain. Also, after 0.9 ka, alluvial fans that are 7-8 m thick rapidly aggraded along the margins of valley floors in response to the incision of trunk streams.