DUNE SIGNALS: SHIFTING WIND REGIMES ON THE GREAT PLAINS

The Great Plains are currently covered by numerous stabilized dune fields that were active during the late Holocene. The orientation of internal structures within these dunes preserve a record of wind directions, while meteorological stations from the last few decades record detailed information about the strength and direction of modern winds. Sand roses were produced for numerous stations in or near several late Holocene dune fields in the Great Plains using data from modern meteorological stations. Only those winds at speeds greater than that necessary to move medium sand (above 6 m/s) were used to compute the roses. In addition, the resultant sand drift directions and dune crest orientations were calculated for each station in order to determine the types of dunes that would form under the modern wind regimes.

Sand roses in the northernmost part of the study area (the Minot Dune Fields, North Dakota) indicate a unimodal wind regime, with northwesterly winds. If sand were active presently, the resultant drift direction would be to the southeast (117 degrees azimuth), with transverse dunes oriented northeast-southwest (34 degrees). Sand roses for the southernmost part of the study area (the Great Bend Sand Prairie, Kansas) indicate a bimodal wind regime, with summer southerly winds dominating, and a weaker component of winter northwesterly winds. The resultant drift direction would be to the northeast (22 degrees), while dune orientation would be roughly west-east (84 degrees), indicating the formation of oblique dunes. Finally, data from the Nebraska Sand Hills indicate a wind regime intermediary to those above. A bimodal wind regime is present with dominant northwesterly winds and lesser southerly winds. This wind regime could produce both oblique and transverse dunes with resultant drift direction roughly southeast (ranging from 100 to 164 degrees) and dunes oriented roughly northeast-southwest (ranging from 52 to 77 degrees). A comparison of the modern data with dune orientations preserved indicates a difference in dune orientation by 45 to 60 degrees and therefore a large difference between the wind regime that formed these dunes and that presently acting upon the region.