Assessment of Groundwater and Surface-Water Interactions of the Niobrara River with Underlying Aquifers under Various Stress Scenarios

Executive Summary:
The 1991 Niobrara Scenic River Designation Act (Public Law 102-50) designated a 76-mile reach of the
Niobrara River to be managed as the Niobrara National Scenic River and approximately 26 miles of the
Niobrara River to be added to the existing Missouri National Recreational River and managed as a
recreational river. The designated portions of the Niobrara River are subject to the legislative mandates
contained in the National Park Service (NPS) Organic Act and the Wild and Scenic Rivers Act.

The NPS is engaged in multiple efforts to identify instream flows that will maintain and/or improve the
water-related resources and flow-dependent values of the Niobrara River within the Niobrara National
Scenic River reach. These efforts require an in-depth understanding of the contribution of groundwater
to flows of the Niobrara River. The NPS is concerned that development of the High Plains aquifer in the
vicinity of the Niobrara River could cause the eventual capture of water that would otherwise discharge
to the river and cause a reduction of instream flows. NPS has requested assistance from USGS in
addressing these concerns.
This study is planned as a two-phase study. Phase 1 will provide a hydrogeologic framework, conceptual
model, and groundwater budget for the study area. Phase 2 will consist of the construction and
calibration of a numerical groundwater flow model suitable for simulating the interaction of
groundwater and surface water under specified stress scenarios. The proposed model domain includes
the Niobrara River Basin east of the 103nd meridian in South Dakota and Nebraska.

Objective(s):
The objectives for the proposed study are to: (1) characterize the hydrogeology of the Niobrara River
Basin within the study area with respect to the interaction of groundwater and surface water and (2)
develop a numerical model that can simulate stress scenarios affecting the interaction of groundwater
and surface water.