One might not imagine hundreds of lakes in a sea of sand and grass, but indeed, the Nebraska Sandhills contain more than fifteen hundred lakes scattered throughout its hills and valleys. Strange as this might seem, lakes in dry, sand-dominated regions also occur in other places across the globe, including the Badain Jaran Desert of Inner Mongolia and in some of the sand seas of the Sahara. These lakes exist because of the large aquifers beneath the fields of dunes, which provide a continuous source of water. In the Sandhills, it is the High Plains Aquifer that rises to the surface in areas of low topography and is the major source of water for many of the region’s lakes and wetlands.
Ironically, most of the Sandhills lakes likely formed during past intervals of extreme drought. Some lake basins are relatively small depressions formed by deflation of the sand by wind in places with sparse vegetation cover. When the climate got wetter, and the rains returned, these wind-carved basins filled with rainwater. In other cases, river and stream drainages were blocked by blowing sand during major droughts, forming a dam. Subsequently, the water table rose behind the dam, flooding low-lying areas and giving rise to lakes and wetlands.
These deflation depressions and dune-dammed lakes likely formed during multiple so-called megadroughts over the millennia. The most recent of these megadroughts occurred about eight hundred years ago, but these processes have occurred for thousands of years, yielding lakes of varied ages across the landscape. Some lakes that exist today are only hundreds of years old, whereas others have been present for millennia. In addition, remnants of ancient lakes that existed in the past, but were filled with sediment over time, are found along the Niobrara River and in other Sandhill drainages. These “paleolakes” are evident as large exposures of layered lake sediment adjacent to modern rivers and streams. Radiocarbon dating of the outcropping sediment indicates that some of these lakes existed more than forty thousand years ago, during wetter intervals of the last glacial period.
Because of their differing origins, the lakes of the Sandhills range in size from small, isolated basins of less than a hectare to large linear lakes that extend for miles between the dunes. These interdunal areas are sometimes filled with shallow lakes and in other cases are occupied by wetlands, depending on the steepness of the valley walls and the amount of water inflow from groundwater and rainfall. All Sandhills lakes are relatively shallow, ranging in depth from tens of centimeters to a few meters. The shallow depth of the basins combined with strong winds, which stir up and mix muds from the lake sediments into the water column, cause many of the lakes to be extremely turbid (opaque). This high turbidity limits the penetration of sunlight through the water column, limiting photosynthesis and the growth of aquatic plants.
The chemistry of Sandhills lakes is tremendously variable because of complex interactions between the mineral composition of deposits at the surface, the connectivity of basins to groundwater, the topography of the land surface, and climate (precipitation, evaporation). The lakes have pH values from mildly alkaline (~8) to highly alkaline (>10), and salinity ranges from freshwater to several times higher than seawater. In general, lakes at lower elevation are closer to the water table and have more groundwater inflow than those lakes higher in the regional flow system. Because most groundwater in the Sandhills region is relatively fresh (<2 g L-1), the salinity of low-elevation lakes is lower than that of nearby lakes at higher elevation. Yet even in low-lying basins, salts are concentrated by evaporation, producing lake water that is saltier than the groundwater.
In regions of higher elevation, near the crests of dunes, many lake basins are fed mainly by precipitation and are cut off from groundwater inputs. In these areas, the intense evaporation during the warm season concentrates salts over time, and these so-called perched lakes often have extremely high salinity and sometimes dry out. In contrast to the ocean, which is dominated by sodium chloride salts, lakes in the Sandhills may be dominated by carbonate, sulphate, or chloride brines dependent on the geology; carbonate salts are the most common. Among the more interesting of the Sandhill lakes are the alkaline lakes in Sheridan and Garden Counties, which are characterized by potassium salts and in some places were mined for potash for fertilizer.
The Sandhills lakes are an important water source for migrating waterfowl, other wildlife, and cattle. In addition, the lakes themselves harbor a rich diversity of microscopic plants and animals that vary in lakes of different depth and water chemistry. In comparison with other regions of Nebraska, the Sandhill lakes have not been affected by substantive human impact. Nonetheless, the rich grassland soils and warm temperatures favor high algal productivity during the growing season, including blooms of cyanobacteria (blue-green algae that sometimes produce toxins) during summer months. Analysis of the history of Sandhill lakes via the record preserved in their sediments indicates these algal blooms existed prior to widespread regional settlement and the expansion of ranching.
Through the years and centuries and millennia, the size, depth, chemistry, and biology of Sandhills lakes have varied in response to changes in climate, particularly changes in precipitation and temperature-driven evaporation. As organisms died and sank to the lake bottom and layers of sediment accumulated over time, a record of each lake’s history and the associated climate was preserved in the sediment layers, with the oldest layers deep in the mud and recent history at the sediment surface. Various mineral, chemical, and biological fossils contained in those layers of sediment can be used to reconstruct the climate and environment at the time of deposition and to put together a sequential history of the lake and its watershed.
Fossilized clues from lake sediments have been used in combination with other records of geological history, such as the records from the dunes themselves, to unravel the long-term history of the Sandhills region. Together these geologic records indicate that repeated droughts spanning decades and centuries—droughts that were more extreme and persistent than those of the last century or two—are a recurrent part of regional history. This long-term history preserved in Sandhills lakes and dunes shows the dynamic interaction of hydrology, vegetation, and wind over time in this sensitive grassland region, and the record suggests that drought is the rule rather than the exception for the Sandhills and its various long-term inhabitants.
Sandhill lakes are an enduring feature of the landscape that have provided resources and refuges for the region’s people and wildlife throughout time. They exist because of the interplay between the region’s geology and hydrology and are shaped by the interaction of sand, water, wind, and plants. Because of their sensitivity to changing climate and local environments, they serve as sentinels of the dynamic character of the Sandhills as they evolve and change in space and in time.