The large dunefield is actually just one of four primary components of the Great Sand Dunes geological system. The mountain watershed, dunefield, sand sheet, and sabkha are each important aspects of this complex sand system.
Great Sand Dunes System
The mountain watershed of Great Sand Dunes receives heavy snow and rain each year. Creeks flow from alpine tundra and lakes, down through subalpine and montane woodlands, and finally around the main dunefield. Sand that has blown from the valley floor is captured and carried back toward the valley. When creeks disappear into the valley floor, sand is again picked up and carried into the main dunefield. This recycling action of water and wind contributes to the great height of this dunefield.
The 30 square mile (78 sq. km) active dunefield is where the tallest dunes reside. It is stabilized by opposing wind directions (southwestly and northeasterly), creeks that recycle sand back into it, and a 7% moisture content below the dry surface. The dunefield is composed of reversing dunes, transverse dunes, star dunes, and a few barchan dunes. It is estimated to contain over 5 billion cubic meters of sand.
The sand sheet is the largest component of the Great Sand Dunes geological system, made up of sandy grasslands that extend around three sides of the main dunefield. Almost 90% of the sand deposit is found here, while only about 10% is found in the main dunefield. The sand sheet is the primary source of sand for the Great Sand Dunes. Small parabolic dunes form here, then migrate into the main dunefield. Nebkha dunes form around vegetation.
The sabkha forms where sand is seasonally saturated by rising ground water. When the water evaporates away in late summer, minerals similar to baking soda cement sand grains together into a hard, white crust. Areas of sabkha can be found throughout western portions of the sand sheet, wherever the water table meets the surface. Some wetlands in the sabkha are deeper with plentiful plants and animals, while others are shallow and salty.