TEPHRA AS A MARKER FOR DUNE MIGRATION

The two largest dune fields in Washington are Juniper Dunes northeast of Pasco and Potholes Dunes southwest of Moses Lake. Thirty-five years of annual measurements show that the transverse and parabolic dunes of the Juniper field migrated less than 1 m. Unlike Juniper Dunes, Potholes Dunes are far enough north to have been blanketed by Mount St. Helens ash in May 1980. As these parabolic dunes advanced northeast, the tephra was buried in the slip face sand. Erosion of the windward slope of the dunes reveals the fine-grained ash, which is more cohesive than the dune sand. Surveying indicated that a dune with abundant shrubs and grasses migrated 1 m after the Mount St. Helens eruption, and that a dune with little vegetation migrated 28 m in the same 35 years. Despite a thickness of only 1-2 cm, the white tephra has enough contrast with the darker sand to be prominent on Google Earth imagery dated 2015; measuring the distance between the arc of the 1980 ash and the arc of the current slip face allows determination of the migration rate of many of the parabolic dunes. The distances 15 dunes migrated range from 11 to 40 m, with a mean of 27 m (or 0.8 m/yr). An independent measurement of dune migration can be estimated where parabolic dunes have partially buried a forest of Russian olives. The trees began growing in this desert after the water table rose when nearby O’Sullivan Dam was completed in 1949. Some of the olive trees, many about 15 m tall, are almost completely buried by the dunes, suggesting a minimum advance of 22 m (0.3 m/yr).