Groundwater/surface-water interaction near the confluence of the Elkhorn and Lower Platte Rivers

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Recent droughts in Nebraska (2000–06; 2012–13) have amplified concerns about the long-term sustainability of groundwater and surface-water resources as well as concerns about the effect of groundwater irrigation on streamflow and water supplies needed to meet wildlife, recreational, and municipal needs.  The Lower Platte River Basin-wide Management Plan is currently being developed jointly by the Nebraska Department of Natural Resources (NDNR) and seven Natural Resource Districts (NRD) to address some of these concerns by managing groundwater and surface-water resources conjunctively. To increase scientific understanding and potentially provide quantitative support for any future streamflow augmentation projects, the Papio-Missouri River NRD and Lower Platte North NRD are working cooperatively with the USGS Nebraska Water Science Center to examine groundwater/surface-water interaction along the lower Platte and Elkhorn Rivers near their confluence.

The State of Nebraska requires a sustainable balance between long-term water supplies and uses of groundwater and surface water and requires Natural Resources Districts (NRDs) to include the impact of groundwater use on surface-water systems as part of their respective integrated management plans (IMP).  Recent droughts in Nebraska (2000–06; 2012–13) have amplified concerns about the long-term sustainability of groundwater and surface-water resources in Nebraska, as well as concerns about the effect of groundwater irrigation on streamflow and water supplies needed to meet wildlife, recreational, and municipal needs.  The Lower Platte River Basin-wide Management Plan is currently being developed jointly by the Nebraska Department of Natural Resources (NDNR) and seven NRDs to address some of these concerns by managing groundwater and surface-water resources conjunctively. 

Collecting well elevation data during synoptic survey in Lower Platte/Elkhorn River study area, Nebr.

To increase scientific understanding and potentially provide quantitative support for any future streamflow augmentation projects, the Papio-Missouri River Natural Resources District (PMRNRD) and Lower Platte North Natural Resources District (LPNNRD)  are working cooperatively with the USGS Nebraska Water Science Center (NEWSC) to examine groundwater/surface-water interaction along the lower Platte and Elkhorn Rivers near their confluence. The USGS NEWSC will be characterizing and describing the interaction of groundwater and surface water along selected reaches of the Lower Platte and Elkhorn Rivers by

1. Instrumenting three groundwater-level recorder wells along a transect above the confluence of the Elkhorn and Platte Rivers. 

  • A coupled groundwater/surface-water gaging station was installed at the Platte River near the Leshara gaging station to mirror the instrumentation set-up at the Elkhorn River at Waterloo coupled groundwater/surface-water gaging station.  
  • All continuous water-level and water-temperature data collected are stored in the publicly accessible USGS National Water Information System (NWIS) and served as real-time data series on the internet from NWISweb. 

2. Analyzing and interpreting data from a transect of groundwater level recorders and coupled groundwater/surface-water gages.

  • Data collected from coupled gages can indicate whether a stream is gaining or losing near the gaging location and will improve the understanding of the effects of streamflow and seasonal climatic conditions on groundwater/surface-water interaction within the lower Platte and Elkhorn Rivers. 

Available Data

Platte River near Lashara, Nebr.: Streambank well at gaging station 06796500 || Water elevation in stream and groundwater

Elkhorn River at Waterloo, Nebr.: Streambank well at gaging station 06800500 || Water elevation in stream and groundwater

3. Planning and executing two synoptic groundwater level surveys during flow conditions that are of interest to the NRDs (most likely during a high- and a low-flow condition). 

  • Identifying and securing access to approximately 30 monitoring, domestic, or irrigation wells within the project area. 
  • All wells, if not already in the NWIS database, will be inventoried.  Accurate elevations will be established at all wells using a survey-grade RTK GPS.  

4. Using the collected groundwater-levels and surveyed surface-water elevations to interpolate a groundwater-level surface, construct contours, and interpret flow paths. 

5. Using heat as a tracer to estimate hydraulic conductivity of the streambed sediment and estimate fluxes across the streambed. 

  • This part of the study is most likely to occur during summer months when the difference in temperature between the groundwater and surface-water is greatest. 
  • Spatial scope includes two sites, each near one of the two USGS streamgages identified in (1).     

BACKGROUND

Map of the study area for groundwater/surface-water interaction near the confluence of the Elkhorn and Lower Platte Rivers
Study area.

To sustain flows in the lower Platte River that are needed for municipal water supplies, water managers have proposed projects aimed at temporary storage of surface water in upstream parts of the basin to augment streamflows in the lower Platte River during low-flow periods.  One area of interest to the LPNNRD and the PMRNRD is a reach of the lower Platte River that extends from downstream of Fremont to its confluence with the Elkhorn River. Along this 23-mile reach, the Platte River is topographically higher in elevation than the Elkhorn River and a broad, flat alluvial valley separates the two streams.  Within the study area, the Platte River has a gradient of approximately 4.4 feet per mile and the Elkhorn River flows for approximately 24 miles at a gradient 2.5 feet per mile.  Because of the differences in stream gradient, the Platte River’s bed is roughly 30 feet higher than the Elkhorn riverbed along profile A–A’  (shown on the study area map).  The lower Platte River is an important stream reach that provides

  • 100 percent of Lincoln’s drinking-water supplies,
  • 40-60 percent of drinking-water supplies to Omaha, and
  • critical aquatic and riparian habitat for threatened and endangered species. 

Flows in the lower Platte are heavily dependent on climatic conditions in the local area and upstream.  The influence of several hydrologic factors, such as increased spring flows in the Platte River caused by above-normal snowmelt, runoff conditions in the Elkhorn River Basin, or ice jams in the lower Platte River may affect the groundwater flow paths in the alluvial aquifer and produce changes in base flow of the Elkhorn River that are driven by conditions in the Platte River.  Understanding how these hydroclimatic factors affect these relations is critical to projecting effects of developing any streamflow augmentation project intended to sustain streamflows in the lower Platte and Elkhorn Rivers.