Upper Big Nemaha 21-B Reservoir Report

Last Updated: December 5, 2025

Upper Big Nemaha 21-B, located in Lancaster, Nebraska, is a vital local government-owned flood risk reduction structure designed by the USDA NRCS.


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Summary

Completed in 1973, this earth dam stands at a height of 42 feet and has a storage capacity of 1890 acre-feet, serving the primary purpose of flood risk reduction for the TR-N FK BIG NEMAHA RIVER. With a drainage area of 5.5 square miles and a maximum discharge of 787 cubic feet per second, this dam plays a crucial role in managing water resources in the region.

The dam's condition assessment in 2017 rated it as satisfactory with a low hazard potential, indicating its reliability in flood mitigation efforts. The structure's inspection frequency is set at five years, with the last inspection conducted in June 2017. Managed by the NE DNR and regulated by the state, Upper Big Nemaha 21-B showcases the collaborative efforts between federal and state agencies in ensuring the safety and effectiveness of water resource management infrastructure. Its strategic location, coupled with its significant storage capacity and operational efficiency, highlights its importance in safeguarding the local community from potential flood risks and climate-related challenges.

Year Completed

1973

Dam Length

1440

Dam Height

42

River Or Stream

TR-N FK BIG NEMAHA RIVER

Primary Dam Type

Earth

Surface Area

54

Hydraulic Height

40

Drainage Area

5.5

Nid Storage

1890

Structural Height

42

Hazard Potential

Low

Foundations

Soil

Nid Height

42
       
Seasonal Comparison

Weather Forecast

Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.