Mill Reservoir Report

Last Updated: December 5, 2025

Mill is a privately owned earth dam located in North Codorus Township, York, Pennsylvania, along the Codorus Creek.


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Summary

Built in 1863 for grade stabilization purposes, this structure stands at a height of 18 feet and spans a length of 250 feet, with a storage capacity of 589 acre-feet. The dam is under the regulatory oversight of the Pennsylvania Department of Environmental Protection, with state permitting, inspection, and enforcement in place to ensure its safety and compliance with regulations.

With a high hazard potential but a satisfactory condition assessment, Mill has been regularly inspected, with the most recent inspection conducted in December 2020. The dam has a normal storage capacity of 34 acre-feet and serves a drainage area of 73.4 square miles. Despite being over a century old, the structure has undergone modifications in 2005 to ensure its continued functionality. The dam's location in a picturesque setting along the Codorus Creek adds to its historical significance and importance in the local water resource management system.

Years Modified

2005 - Other

Year Completed

1863

Dam Length

250

Dam Height

18

River Or Stream

CODORUS CREEK

Primary Dam Type

Earth

Surface Area

23

Drainage Area

73.4

Nid Storage

589

Hazard Potential

High

Foundations

Unlisted/Unknown

Nid Height

18
       
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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.