Within the valleys of the undulating Sandhills landscape lies a network of flowing waters. Flowing southeasterly across the Sandhills, the streams include the Calamus, Dismal, Middle Loup, North Loup, and Snake Rivers. Flowing easterly, the Niobrara and Elkhorn Rivers trace the Sandhills’ edge in the north, and the North Platte River traces its edge in the south.
Streams and rivers derive their water from a variety of sources. In grassland regions, this generally includes water directly from overground sources, such as rain or melting snow, called overland flow, and belowground sources, like springs or groundwater seepage. The belowground sources derive from a vast body of sediment, saturated with groundwater, that lies beneath the Sandhills. Those saturated sediments start as much as a thousand feet below the surface, extending upward to the water table. The higher land and water table elevations in the west drive groundwater generally to the east, where it finds an exit at lower elevations. The lowest elevations on the landscape are usually streambeds. The varying streamflow quantity (and even the color of the water) throughout the year is largely an expression of that hidden groundwater.
To examine the characteristics of streamflow in the Sandhills more closely, consider the Loup River system. The Loup River is fed by headwater creeks and straightened drainage ditches that flow from wet meadows in the western valleys in the Sandhills. The creeks and ditches then flow into meandering streams before feeding into the Loup. In dry times, the streams are smooth and clear, although sometimes stained the color of tea from organic matter that is leached out of valley soils. During wet seasons, the valleys may flood, straining or overtopping culverts at ditch crossings and downstream road crossings. Where the channels are sinuous, the turbulent stream water almost appears to boil as it swirls at the channel bends. At these times, the stream has a light-brown color in part due to fine white sand that is suspended in the water, a reminder of the power of these waters to shift and reshape the stream channel over time. Along the edges of the stream, the waterlogged fine sands can produce pockets of quicksand, an unpleasant surprise for the unsuspecting newcomer.
Scientists studying the streams in the Loup River system have made some exciting discoveries. In particular, they have discovered that the river and its tributaries are receiving groundwater from different parts of the dune sands and from the aquifer below. In the headwaters of the Middle Loup River, measurements of the age of groundwater, just before it seeps into the stream, suggest that much of the groundwater discharging to the headwater creeks has been traveling through the aquifer for years to decades before discharging into the stream. Farther east, in the main channel of the Middle Loup River near Seneca, Nebraska, at least some of the groundwater feeding the river is likely thousands of years old! This vast reservoir of “ancient” groundwater is the reason that these streams have flowed so reliably over the years. For those who “tank” in the Middle Loup, that cool water at your feet was recently groundwater that may have traveled for a thousand years or more to greet you.
Riverine Flora and Fauna
The Sandhill streams and rivers provide habitat for a multitude of plants and animals. In contrast to the surrounding landscape full of grasses and other prairie species, the riparian areas stretching on either side of the flowing waters support a variety of sedges, rushes, bulrushes, reeds, and forbs. The larger rivers can support some forested communities along their banks, although the tree canopy does not extend across regions of flowing waters at the stream’s center. Yet this lack of canopy cover does not mean that river and stream waters are always warm. Instead, the consistent groundwater inflows generally keep waters cool. Indeed, this groundwater influence tends to minimize temperature variation in Sandhills streams throughout the year. The cooler waters are necessary for a favorite fish of anglers, the trout, to live and reproduce. Brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss), and brook trout (Salvelinus fontinalis) can all be found in Sandhill streams, although the rainbow trout are often stocked. Perhaps the most interesting fish of all is the plains topminnow (Fundulus sciadicus). This olive-green fish, no more than the length of a playing card, with an upturned mouth, prefers the heavily vegetated, slower moving backwaters of streams and rivers. Stream degradation, nutrient pollution, and other stressors have decreased its population size over much of its range in the central United States; hence, the Sandhill streams remain important in maintaining this fish population.
Human Impact on the Flow of Rivers and Streams
While the Sandhills region is considered less impacted by human activities than much of the Great Plains, the region still faces local and regional pressures. Indeed, area ranchers can recall streams that were once fordable by truck, foot, or hoof and are now so downcut or entrenched that they resemble small canyons and require some ingenuity to cross. Ditching and straightening of streams, done to aid agricultural endeavors, increases streamflow and, ultimately, rates of erosion. Often, this increased erosion has occurred on the streambed, resulting in a further lowering or downcutting of the stream channel. While many ranchers have maintained a sustainable balance in managing their land, many have not. Thus, in some places, unsustainably managed grazing has not only led to increased stream bank erosion but loss of riparian vegetation and compromised water quality as well.
The overall water balance in the region has also changed. Within some local areas, extraction of groundwater for irrigation has diverted water from streams and rivers, moving it to cropland systems. At a regional level, climate change has increased precipitation in the Sandhills over the last hundred years, mostly through an increase in heavy precipitation events, which can cause catastrophic flooding, as witnessed in the March 2019 flood. While it may be difficult to pinpoint whether specific rain or flood events are caused entirely by climate change, the overall trend toward more extreme weather events is consistent with the output of climate model experiments that investigate the impacts of increased greenhouse gases in the atmosphere.