Shell Rock Milldam Reservoir Report

Last Updated: December 13, 2025

Shell Rock Milldam, located in Shell Rock, Iowa, is a rockfill dam constructed in 1928 for recreational purposes along the Shellrock River.


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Summary

This local government-owned structure stands at a height of 7 feet and has a storage capacity of 229 acre-feet, with a normal storage level of 152 acre-feet. The dam spans 221 feet in length and covers a surface area of 45 acres in Butler County, Iowa.

Managed by the Iowa Department of Natural Resources, Shell Rock Milldam is regulated, permitted, inspected, and enforced by state authorities to ensure its safety and compliance with regulations. Despite being classified as having a low hazard potential, the dam is considered to have a moderate risk level, with a risk assessment score of 3. The last inspection of the dam was conducted in September 1989, and its condition is currently rated as "Not Rated."

For water resource and climate enthusiasts interested in exploring the intersection of dam infrastructure and environmental conservation, Shell Rock Milldam offers a unique insight into the management of recreational water structures in rural Iowa. With its picturesque location along the Shellrock River, this dam serves as a vital recreational resource for the local community while also highlighting the importance of maintaining and monitoring dam safety in the face of changing environmental conditions and regulatory requirements.

Year Completed

1928

Dam Length

221

Dam Height

7

River Or Stream

SHELLROCK RIVER

Primary Dam Type

Rockfill

Surface Area

45

Drainage Area

1752

Nid Storage

229

Hazard Potential

Low

Foundations

Soil

Nid Height

7
       
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.