Hopkinton Milldam Reservoir Report

Last Updated: December 5, 2025

Hopkinton Milldam, located in Delaware County, Iowa, is a gravity dam constructed in 1925 along the Maquoketa River.


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Summary

With a height of 10 feet and a length of 175 feet, the dam serves primarily for recreational purposes, providing a surface area of 90 acres and a storage capacity of 433 acre-feet. Despite being uncontrolled, the spillway width is not specified, indicating a potential risk for overflow during periods of heavy rainfall.

Managed by the local government and regulated by the Iowa Department of Natural Resources, the dam is classified as low hazard potential with a moderate risk assessment rating of 3. The condition of the dam is currently labeled as "Not Rated," with the last inspection conducted in June 1999. Although the dam has not been modified in recent years, it remains subject to state permitting, inspection, and enforcement to ensure public safety and environmental protection.

With its picturesque location and historical significance, Hopkinton Milldam stands as a vital recreational landmark along the Maquoketa River. Its presence not only enhances the local scenery but also serves as a reminder of the importance of maintaining and monitoring water resources in the face of changing climate conditions. Enthusiasts of water resource management and climate resilience will find Hopkinton Milldam a fascinating case study in dam infrastructure and regulatory oversight.

Year Completed

1925

Dam Length

175

Dam Height

10

River Or Stream

MAQUOKETA RIVER

Primary Dam Type

Gravity

Surface Area

90

Drainage Area

506

Nid Storage

433

Hazard Potential

Low

Foundations

Soil

Nid Height

10
       
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.