EVOLUTION OF LAKES IN THE SEMI-ARID NEBRASKA SAND HILLS: INTERACTION OF TOPOGRAPHY, CLIMATE, HYDROLOGY, AND EOLIAN TRANSPORT

The Nebraska Sand Hills is located in a semi-arid region and contains more than 1600 permanent shallow lakes and ponds with salinity ranging from 0.2 to over 100 g/L. Lakes in the Western Sand Hills contain the highest contents of carbonate saline waters in North America. Hydrodynamic models show that variability of lake salinity in semi-arid environments is strongly dependent on differences between flow through and purely recharging regimes. Purely recharging regimes and saline lakes form when the regional head gradient is flat. These regimes are controled by geologic, climatic, and topographic factors.

Spatial analysis of geologic and hydrologic data revealed the existence of a remarkable topographic feature buried beneath the sand dunes and a thin veneer of late Pleistocene fluvial sand in southern Sheridan and northern Garden County. This feature, the Sand Hills Tableland (SHT) developed on top of the Ogallala Group sediments in Sheridan and Garden County covers about 4000 sq km. No other landscape in Nebraska of equivalent area has such a low slope. We propose that the SHT has greatly influenced the location and history of lakes in the western Sand Hills.

Eolian transport of salts is also an important mechanism contributing to variability in lake salinity. New core data from the saline Alkali Lake were obtained to supplement previous core and C¹⁴ data for paleo-reconstruction of lake evolution in the Sand Hills. Radiometric and optical age dating and diatom analysis show that the Alkali Lake core had only a maximum of 2.5 m of lake sediments during its 14,000 years of formation while cores from freshwater lakes in the area had up to 13 m of peat and lake mud. Evolution of Alkali Lake suggests periods when the lake dried out and its sediments were blown away.