Upper Big Nemaha 20-C Reservoir Report

Last Updated: December 6, 2025

Upper Big Nemaha 20-C is a vital local government-owned earth dam located in Johnson, Nebraska, serving multiple purposes including flood risk reduction.


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Summary

Built in 1980 by the USDA NRCS, this dam stands at a height of 35 feet and has a storage capacity of 542 acre-feet. With a satisfactory condition assessment and low hazard potential, Upper Big Nemaha 20-C plays a crucial role in protecting the surrounding area from potential flooding events.

Managed by the Nebraska Department of Natural Resources, this dam is subject to state regulation, inspection, and enforcement to ensure its continued safety and effectiveness. The dam's structure consists of stone core and soil foundation, with a length of 828 feet and a maximum discharge capacity of 1893 cubic feet per second. Despite its low hazard potential, regular inspections every 5 years are conducted to maintain its satisfactory condition and readiness for emergencies.

Located on the TR-Big Nemaha River, Upper Big Nemaha 20-C is an essential part of the local water resource infrastructure in Nebraska. With its strategic design and purposeful construction, this earth dam serves as a reliable barrier against flooding and contributes to the overall water management efforts in the region. Its presence underscores the importance of sustainable water resource management and climate resilience in the face of changing environmental conditions.

Year Completed

1980

Dam Length

828

Dam Height

35

River Or Stream

TR-BIG NEMAHA RIVER

Primary Dam Type

Earth

Surface Area

14

Hydraulic Height

34

Drainage Area

1.5

Nid Storage

542

Structural Height

40

Hazard Potential

Low

Foundations

Soil

Nid Height

40
       
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.