Nelsonville Reservoir Report

Last Updated: December 6, 2025

Nelsonville, located in Portage, Wisconsin, is home to a hydroelectric dam on the Tomorrow River.


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Summary

This historic dam, completed in 1855, serves multiple purposes including hydroelectric power generation and recreation. With a gravity dam type and a height of 14 feet, the Nelsonville dam has a low hazard potential and is classified as having a very high risk due to its age and structural condition, which has not been rated.

The dam has a storage capacity of 180 acre-feet and a drainage area of 69 square miles, with a maximum discharge of 1685 cubic feet per second. Despite its age, the Nelsonville dam is still in operation and is regulated by the Wisconsin Department of Natural Resources (WIDNR), which oversees its permitting, inspection, and enforcement. The dam's condition is currently not rated, and it has a 10-year inspection frequency to ensure its safety and functionality.

Water resource and climate enthusiasts will be intrigued by the historical significance and operational capabilities of the Nelsonville dam. Its location on the Tomorrow River, combined with its hydroelectric power generation and recreational uses, make it a key structure in the region. However, the dam's age and risk assessment highlight the importance of ongoing monitoring and maintenance to ensure its safety and efficiency in the face of changing environmental conditions.

Years Modified

1901 - Other

Year Completed

1855

Dam Length

344

Dam Height

14

River Or Stream

TOMORROW

Primary Dam Type

Gravity

Surface Area

32

Hydraulic Height

9

Drainage Area

69

Nid Storage

180

Structural Height

14

Hazard Potential

Low

Nid Height

14
       
Seasonal Comparison

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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.