AERIAL PHOTOGRAPHIC ANALYSIS OF ACTIVE AEOLIAN LANDFORMS IN SOUTHWEST KANSAS DURING THE DUST BOWL DROUGHT (CA. 1936)

The extent of dune reactivation on the Great Plains during the 1930’s drought remains unresolved. An important archive to assess activity of dunes along the Arkansas River in southwestern Kansas is the first generation of aerial photography taken in 1936 and 1939. Dune activation occurs when the threshold wind speed is surpassed (usually >5 m/s), and there is significant sediment availability and supply for entrainment. Within the Great Plains sediment availability is a critical factor and is mostly controlled by the vegetation cover. A 30% decline in vegetation extent is sufficient to expose soils to wind erosion and potentially activate dunes. A GIS analysis of Dust Bowl era aerial photographs between Syracuse and Lakin, Kansas reveals a sand sheet dominated by nebkha dunes situated between the Arkansas River and loess covered uplands to the south. Superimposed within the sandsheet are active barchanoid ridge dunes and near the River there is both active and stable parabolic dunes. Initial morphometric analysis of dune forms shows barchanoid ridges have consistent crest spacing of 150 to 200 m and are accreted by winds from the north to northwest. Conversely, parabolic dunes appear mostly stable and are oriented with a paleowind direction from the southwest. Some nebkha dunes appear elongated by northwesterly winds. The geometry of barchanoid ridges and the presence of stable parabolic dunes may indicate form inheritance from one or more earlier, but unresolved periods of reactivation. Anthropogenic alteration within the Arkansas River valley, such as diversions and earthen dams are also examined in the aerial imagery. Agricultural diversions reduced surface water to expose sand and silt-rich fluvial sediments to aeolian deflation within the channel. However, the presence of a riparian border, including the exotic shrub salt cedar (Tamarix sp.), indicates near-surface moisture which can in turn sustain stabilizing vegetation on parabolic dune forms near the River. Further from the Arkansas River the depth to the ground water exceeded root extent of stabilizing vegetation and barchainoid ridges reactived with severe reductions in grassland communities.