Sand dune mobility at Grand Falls on the Navajo Nation, southwestern United States

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This article is part of the Fall 2015 issue of the Earth Science Matters Newsletter.

Nearly a third of the Navajo Nation, in the southwestern USA, is covered with sand dunes and sand sheets that have been variably active during droughts of recent geologic history. The sand mobility of these dunes, as with dune deposits worldwide, is a function of wind, sand supply, and vegetation cover. Thus, sand dune deposits are highly sensitive to changes in climate.

Grand Falls Dune Field Migration 1953 - 2010

Location of the migrating front of Grand Falls Dune Field showing the extent of dune cover from 1953 to 2010, superimposed on 2005 aerial photo. Historic positions of the dune field front were derived from archival imagery. The dune front and extent of dunes has changed rapidly in recent decades, with an approximately 70% increase in dune cover from 1992 to 2010  (modified from USGS Fact Sheet 2011-3085).

(Public domain.)

Increased aridity due to higher temperatures and a prolonged drought that began in the late 1990s is producing significant changes in dune mobility on the Navajo Nation. However, developing models that can directly link the state of dune activity to changing climatic conditions has been a challenge. Sand and dust movement in the region is closely linked to regional aridity, flood events, and wind circulation and energy. Related sand and dust storms can damage rangeland and cause dangerous travel conditions. These storms can also generate economic, cultural, and health consequences for the Navajo people. 

To understand and document the current and future potential for dune mobility on the Navajo Nation, U.S. Geological Survey scientists recently carried out a study that focused on a field of sand dunes located downwind of the Little Colorado River at Grand Falls, Arizona. These dunes formed in the early 1950s. The areal extent of the dunefield has increased rapidly during recent drought years. Seasonally repeated surveys were used to track the location of migrating sand dunes. The field survey information on dune migration rates was compared to in-situ meteorological data on wind speed and direction, temperature, precipitation, soil moisture, and vegetation to examine climatic parameters and seasonal variations that affect dune mobility.

Three years of data on migration rates and weather information were collected (2009-2012), providing a detailed snapshot of the seasonal and annual variability of active sand dune movement in the Grand Falls Dune Field. Using GPS field surveys and meteorological data, the study showed that these dunes, under the current arid and sparsely vegetated conditions, are not stable. The sand dunes migrated at rates directly proportional to local wind energy.

During the period of observation, the lowest annual precipitation rate was less than 50% of the local historical average of 160 mm, and the peak precipitation for all three years was only 75% of the long-term average. These low precipitation amounts, combined with increasing average, minimum, and maximum temperatures, result in less effective moisture needed for plant growth and dune stabilization. Additionally, the annual windy season occurs during the spring at the driest part of the year, before much of the plant growth is established. Because plant growth is at a minimum when wind storms are common, the seasonality of the climate of the Grand Falls Dune Field on the southern Colorado Plateau leaves dune deposits more susceptible to wind erosion and impacts from drought.

Average dune migration rates, calculated for wind years beginning in 2009, 2010, and 2011, ranged from 25 m/yr to 43 m/yr within the dune field; rates changed by as much as 48% annually, depending on annual variations in wind energy. The current increase in aridity, coupled with active dune movement in the western Navajo Nation, may hinder the recruitment and establishment of plants needed to stabilize mobile dunes in this region.

This paper, published in the journal Geomorphology, is available at http://www.sciencedirect.com/science/article/pii/S0169555X14004504.

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