GEOMORPHIC PROCESSES CONTRIBUTING TO SEVERE DUST STORMS DURING SPRING 2009 IN NORTHEASTERN ARIZONA

Spring dust storms in northeastern Arizona are not uncommon but the severity and number of storms that occurred during spring 2009 was exceptional. The region was impacted by at least a dozen storms resulting in road closures, damage to stock watering facilities, severe rangeland damage, and dust deposition as far away as the Rocky Mountains.

Northeastern Arizona has been experiencing drought for about 15 years. This prolonged drought surely played a significant role in spring 2009 dust storms although many other factors likely contributed. Geology, geomorphology, topography, vegetative cover, and wind speed all played a role in wind erosion and deposition of dust.

The surface geology of northeastern Arizona is composed largely of Mesozoic sandstones, siltstones, and mudstones and Paleozoic limestones and sandstones, typical of the southern Colorado Plateau. Major source areas for dust appear to be locations where the surface rock is fine grained and poorly indurated and where soil derived from bedrock is not conducive to plant growth.

Two geologic units that exhibit these characteristics are the Triassic Moenkopi and Chinle Formations. Surface exposures of these formations do not necessarily result in wind erosion. In many areas the Moenkopi Formation contains indurated rock that protects it from wind erosion, while fine-grained sediments in the Chinle Formation often exhibit badlands weathering with clay crusts or desert pavements that are resistant to wind erosion. These formations seem to be most susceptible to wind erosion in areas where stormwater runoff has reworked large parts of the surface, such as on coalescing alluvial fans at the base of escarpments and in closed topographic basins.

Terrace deposits of major washes such as Oraibi Wash also contributed wind-blown material during the spring 2009 storms. One surprising observation is that the Little Colorado River, which historically contributed much of the sand and silt deposited in the area, may now be contributing significantly less because tamarisk has invaded much of its floodplain.