The Nebraska Sandhills
The Sandhills is a contiguous 19,600 square-mile sand dune formation covered by grasses and located in northcentral Nebraska. Approximately 1.3 million acres of wetlands, formed by groundwater discharge, are scattered throughout. To some, the Sandhills appears to be a continual expanse of rolling hills with wetlands in the valleys. However, a closer examination shows diverse habitats. Dunes vary from high, steep hills in the western region to small mounds in the east. The groundwater recharge and discharge associated with various dune types and geographic locations influence the type and quality of wetlands. Wetlands range from shallow, extremely alkaline basins, to deeper, freshwater lakes, to spring-fed streams. Plant communities range from isolated deciduous and coniferous forests to extensive short and tall grass prairies. Plants associated with arid conditions inhabit the top of dunes while lush stands of aquatic plants are found in the valleys a few hundred yards away. It is this broad diversity which provides homes and resting places for countless numbers of resident and migratory wildlife. This same ecosystem supports a strong ranching economy.
The landscape and economics of the Sandhills are connected to the sands and gravels that formed the area over the past 38 million years. Ancient meandering streams deposited hundreds of feet of sands, gravels and some clays to create the Ogallala formation. Wind-blown sand dunes eventually covered the water-saturated deposits. The dunes became stabilized by vegetation. Within the last 100 years, the control of wildfires and managed grazing has reduced the amount of exposed sand.
The dunes remain fragile and depend on the grasses to keep the sands in place. Many of the grasses, in turn, are dependent on the groundwater accumulated under the sands. The groundwater, recharged by precipitation, slowly discharges its excess into lakes, wetlands, meadows and streams. About 90 percent of the stream flow (2.4 million acre-feet) comes from groundwater discharge (Bentall, 1990). Lakes and wetlands cover about 1.3 million acres (Turner and Rundquist, 1980) and provide economic stability and diversity of flora and fauna.
Lands with the water table about two feet below the surface produce lush stands of native grasses. As the distance to the water table increases or decreases, vegetation shifts toward more arid or aquatic plants, respectively. In the early 1900's, landowners ditched across the wetter meadows and open-water marshes to increase grass production. The rapid movement of groundwater (up to 500 feet per year, Bleed, 1990) creates an underground continuum between the lakes, wetlands and streams. So, an alteration in one area may easily affect vegetation and wetlands over a larger landscape. Winter (1988) states:
"Drainage lowers the hydraulic head at the wetland site, initially increasing groundwater gradients, which increases groundwater discharge to the site. However, over a long period of time groundwater levels generally decline with drainage....Because the hydrologic system is a continuum, any modification of the continuum will impact contiguous parts....One well or one landscape modification generally has only local effects, but multiple modifications or development can have extensive impacts."
Another alteration in the Sandhills is the conversion of grassland to cropland. Two attempts, one at the turn of the century (the Kincaid Act) and again in the 1970's, brought financial and environmental problems. In the late 1970's, cultivation in the eastern portion was encouraged by tax laws, center-pivot technology, low land values, and high grain prices. Nebraska Natural Resource Commission (1992) reports that from 1972 to 1981, irrigated land tripled (70,550 to 215,000 acres). Crop production dropped as organic material leached or eroded away. Loss of investment tax credits and low profit margins caused many of the fields to become idle. By 1990, irrigation had stopped on 50,000 acres; much was placed in the Conservation Reserve Program. Conversion back to grassland, however, has been difficult and slow. Lands broken eighty years ago have not regained the natural plant diversity or production.
The lands' brief time as irrigated cropland had a significant impact on the local area. Water tables were lowered in some areas while other areas experienced flooding. Groundwater contamination by agricultural chemicals began to show up in domestic wells (Natural Resource Commission, 1992). Wind erosion (10 times the rate of grassland) damaged young corn and covered neighboring pastures.