The Nebraska Sand Hills region is a dune sea occupying much of the northern half of the state of Nebraska. The region is the largest dune field in the Northern Hemisphere and is the largest area covered by sand in North America. The overall climate of the region is semiarid, and yet the immense dunes are stabilized by large amounts of grasses and vegetation. Beneath the Sand Hills lies the High Plains aquifer, which plays a large role in stabilizing the sand dunes. The dunes have been active throughout history, with the most recent activation ending in 1300 AD at the end of the Medieval Climatic Anomaly. During fall break of the fall 2015, Dr. James S. Aber's ES546 Field Geomorphology class took a trip to the Nebraska Sand Hills and other nearby physiographic regions, viewing the local geomorphologic features and documenting them through the use of aerial photography.
The Nebraska Sand Hills is the largest sand dune covered area in North America, with some of the largest dunes stretching up to 32 kilometers in length and extending to heights of 122 meters (Schmeisser et al. 2015; Bleed and Flowerday 1990). The sand that is present in Nebraska was deposited by ancestral rivers running from the Rocky Mountains. However, the dunes formed in Nebraska when environmental conditions were suitable for activation. These conditions related to climate, wind, and precipitation. To get dunes moving the area would need a warm dry climate, strong westerly winds, and little to no rain fall. These sand dunes thrived when conditions were met because underlying layers consist of unconsolidated alluvial sand followed by poorly consolidated Ogallala strata. Thickness of these sub dune strata go as deep as 300 m (Schmeisser et al. 2015; Swinehart and Diffendal 1990). Figure 1 shows a map of the terrestrial ecogregions of Nebraska and nearby states. Compared to the map of the Sand Hills below, figure 2, almost all of the Sand Hills can be noted to be prairie grasslands. This map can also represent the areas of the region that would be most affected by drought and reactivation of the dunes (Healy et al. 2011).
Figure 1. Terrestrial ecoregions of the Sand Hills region and surrounding areas. (Modified from Wikimedia Commons, 2010).
58: Northwestern mixed grasslands, 59: Northern tall grasslands, 60: Central tall grasslands, 61: Flint Hills tall grasslands
62: Nebraska Sand Hills mixed grasslands, 63: Western short grasslands, 64: Central and southern mixed grasslands, 65: Central forests/grassland transition
Today the sand dunes are stabilized with vegetation due to the semiarid climate in the western portion of the Sand Hills and the sub humid climate in the eastern portion. In the past 10,000 years there have been periods when the sand dunes became active. The most recent of these activations, which occurred during the Medieval Climate Anomaly, happened around AD 900-1300 (Schmeisser et al. 2015). Other such reactivation happened during the mid-Holocene climate optimum as early as 5,000 years ago. Both of these events are linked to periods of prolonged drought ranging in length from several decades to centuries in length (Swinehart and Diffendal 1990). Because there is already little rainfall in the area, when the region is hit with a drought the recharge rate of the Ogallala Aquifer that underlies the Sand Hills, which allows the vegetation to thrive, is significantly decreased and causes the dunes to become reactivated. Typically revegetation occurs from the northwest due to a rising water table in the interdunes which serves as a refuge for vegetation during the drought. Once rainfall returns to the region at its regular rate water will be quickly absorbed into the permeable soil, recharging the aquifer and fueling rivers such as the Snake, Calamus, Dismal, and the North and Middle Loup rivers (Schmeisser et al. 2015).
Figure 2. Map displaying the extents of the Nebraska Sand Hills. (Modified from Wikimedia Commons, 2010)
Events of that caliber are not required, however, for stabilized Midwest dunes to become active again (Forman et al., 2005). Droughts lasting in excess of a decade wherein there is a 25% reduction of rain during the growing season are sufficient to reduce the local groundwater below its current position, as close as 1.5 to 3.0 meters below the surface in places, and cause dune instability. This is a rather considerable reduction in annual rainfall, considering the Sand Hills region only sees a typical 500mm of rainfall in a year (Mason et al., 2004).
The Nebraska Sand Hills is the largest sand dune field in the Northern Hemisphere. (The largest sand hill region within North and South America.) The Sand Hills are about 265 miles east to west, and cover about 20,000 square miles. The area has lush prairie grass and plenty of surface and groundwater. The sand dunes reflect the wind and sand supply in a unique way. Transverse dunes are in the north & east portion of the region and longitudinal dunes are in the south and east, primarily. But, there are not just dunes of sand within the region. The aquifer system that runs through the Nebraska Sand Hills is called the High Plains aquifer system. This aquifer system thickness is about 100 meters, and surpasses 150 meters within the middle (Bleed 1990).
The classification of the sand dunes themselves includes the types: compound (transverse), barchan, barchanoid-ridge, transverse-ridge, parabolic, and blowout. (Mckee and Moiola, 1975). The Barchan, barchanoid-ridge, and transverse-ridge dunes as high as 90 meters, are found scattered all over the Sand Hills. In the central part of the Nebraska Sand Hills, the dunes are highest in elevation, decreasing in size North and South of this point. The larger dunes are compound, for example, there are small transverse-ridge dunes built on the large transverse-ridge dunes. Many smaller size dunes (up to 5m) occur at the top of larger dunes, resulting in hilltop like formations that are remnants of crestal deposits of older dunes or blowout deposits.
An aerial photograph by the USGS.
The thickness of the sand in the Sand Hills of interdune surfaces ranges from 9 to 24 meters thick, with a max of 37 meters. The dune deposits do not occur at many southern locations around the Platte River. Interdune sediment contains plentiful organic substances, and these are spread throughout the Sand Hills. Lastly, fine-grained, evaporitic paleointerdune deposits, were preserved – and function as local permeability barriers (Bradley and Rainwater, 1956).
While visiting the Nebraska Sand Hills geomorphic region, there was one prominent method of capturing the exact land formations and unique features – using aerial photography. With aerial photography, it is possible to capture an image from 100 to about 500 feet legally. Two kite-flown camera rigs were used, one belonging to instructor Dr. James S. Aber and the other custom-built by a student. In the event of winds unsuitable for flying a kite for photography, a small helium blimp was brought along as a fallback. Conditions at Smith Lake were absolutely favorable, allowing for the two camera-equipped kites to take flight. Below is a collection of photos taken during field work on October 15, 2015 at Smith Lake from the perspective of Dr. James S. Aber's kite and camera setup. Photos were taken by students operating the remote-controlled camera of the aerial photography rig and are used with permission.
Aerial photo of Smith Lake, looking towards the south, with blowout sand dunes visible.
Aerial photo showing a peculiar tree line wandering across blowout dunes.
View is looking towards the west, away from Smith Lake.
Aerial photo of Smith Lake, showing a gentle distant landscape.
View is looking towards the southeast, with a student's aerial photography kite in the foreground.
Aber, J.S. and Aber, S.W. 2009. Kansas physiographic provinces: Bird’s-eye views. Kansas Geological Survey, Educational Series 17, 76 p.
Ahlbrandt, Thomas S. and Steven G. Fryberger 1980. Geologic and Paleoecologic Studies of the Nebraska Sand Hills : U.S. Geol. Survey Prof. Paper 1120-A, B, C, 58 p.
Bleed, A. and Flowerday, C. (eds.) 1990. An atlas of the Sand Hills. Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska. Resource Atlas 5a (2nd ed.), 265 p.
Brice, J.C., 1964 Channel patterns and terraces of the Loup Rivers in Nebraska: U.S. Geol. Survey Prof. Paper 422-D, 41 p.
Forman, S. L., Marín, L., Pierson, J., Gómez, J., Miller, G. H., Webb, R. S., 2005. Holocene vol. 15, issue 7, p. 973-981.
Healey, Nathan, et al. "Remote Sensing And In Situ-Based Estimates Of Evapotranspiration For Subirrigated Meadow, Dry Valley, And Upland Dune Ecosystems In The Semi-Arid Sand Hills Of Nebraska, USA." Irrigation & Drainage Systems 25.3 (2011): 151-178. Academic Search Premier. Web. 2 Dec. 2015.
Map of Nebraska Sand Hills. Wikimedia Commons. 2007. https://commons.wikimedia.org/wiki/File:Map_of_Nebraska_Sand_Hills.svg. Accessed on December 10, 2015.
Mason, Joseph A., Swinehart, James B., Goble, Ronald J., Loope, David B., 2004. Holocene, Vol. 14, issue 2, p. 209-217.
Part of the Nebraska Sand Hills. United States Geologic Service. http://water.usgs.gov/wid/html/ne.html. Accessed on December 4, 2015.
Schmeisser McKean, Rebecca L., Goble, Ronald J., Mason, Joseph B., Swinehart, James B., Loope, David B., 2015. Temporal and spatial variability in dune reactivation across the Nebraska Sand Hills, USA. Holocene vol. 25, issue 3, p. 523-535.
Swinehart, J.B. and Diffendal, R.F. Jr. 1990. Geology of the predune strata. In Bleed, A.B. and Flowerday, C. (eds.), An atlas of the Sand Hills, p. 29-42. Conservation and Survey Division, Inst. Agriculture and Natural Resources, University of Nebraska, Resource Atlas 5a.
Terrestrial ecoregions USA CAN MEX. Wikimedia Commons. 2010. https://commons.wikimedia.org/wiki/File:Terrestrial_ecoregions_USA_CAN_MEX.svg. Accessed on December 10, 2015.