Mianus Filter Plant Dam Reservoir Report

Last Updated: December 6, 2025

The Mianus Filter Plant Dam, also known as Mianus Mill Pond, is a historic masonry dam located on the Mianus River in North Mianus, Greenwich, Connecticut.


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Summary

Built in 1875, the dam serves as a vital water supply structure for the area, with a storage capacity of 118 acre-feet and a normal storage level of 101 acre-feet. The dam has a height of 31 feet and a length of 130 feet, with a spillway width of 65 feet for uncontrolled overflow.

Managed by the Connecticut Department of Energy and Environmental Protection (DEEP), the Mianus Filter Plant Dam is state-regulated and undergoes regular inspections to ensure its structural integrity. The dam has a high hazard potential but is currently assessed as being in fair condition, with a moderate risk level. Despite its age, the dam continues to play a crucial role in providing water to the surrounding community and mitigating flood risks along the Mianus River.

With its picturesque location and rich history, the Mianus Filter Plant Dam is not only a functional infrastructure but also a significant cultural landmark in Fairfield County, Connecticut. Its association with the Mianus River and its role in water resource management make it a compelling site for water resource and climate enthusiasts to explore and appreciate the intersection of human engineering and natural processes in maintaining a sustainable water supply for the region.

Year Completed

1875

Dam Length

130

Dam Height

31

River Or Stream

MIANUS RIVER

Primary Dam Type

Masonry

Surface Area

7.2

Drainage Area

29.9

Nid Storage

118

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

31
       
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.