DUNE-DAMMED PALEOVALLEYS OF THE NEBRASKA SAND HILLS - INTRINSIC VERSUS CLIMATIC CONTROLS ON THE ACCUMULATION OF LAKE AND MARSH SEDIMENTS

Although running water is the dominant geomorphic agent on Earth, eoli an processes can gain ascendancy in regions where the climate is arid, vegetation is sparse, and abundant sand is available for transport. W ith climate change, the boundaries between fluvial-dominated and eolia n-dominated areas may shift. Although there have been few reports in t he North American literature of river systems blocked by dune sand, ou r work in the Nebraska Sand Hills provides evidence of multiple episod es of such blockage events. During prolonged arid intervals in latest Pleistocene and middle Holocene time, eolian dune sand blocked two lar ge valley systems in western Nebraska. These blockages raised the wate r table of the High Plains aquifer as much as 25 m over an area of 700 0 km(2) and created over one thousand lakes. Wetlands far removed from the discharge points of the buried paleovalley system are strongly al kaline (exceeding 250 000 mg/L total dissolved solids [TDS]). Relative ly fresh (280 mg/L TDS), flow-through lakes are present at the distal end of the system where the gradient of the water table is steep and t he cross section of the buried valley is large. Anomalously thick mars h and lake sediments accumulated in deep paleovalleys upstream of dune dams near the southern margin of the Sand Hills. Our cores and radioc arbon dates from Blue and Crescent Lakes reveal their histories to be quite distinct from adjacent Swan Lake; these differences are best exp lained by multiple blockage events. Our work explains why lakes are mo st abundant in the driest part of the Sand Hills. It also provides ano ther line of evidence for major dune activity in the Sand Hills region during Holocene time and shows that factors other than regional clima te, specifically location, height, and hydraulic conductivity of dune dams, can control the rise and fall of the ground-water table and the chemistry of lakes.