Pia Mill Reservoir Reservoir Report
Nearby: Omao Reservoir Puu O Hewa Reservoir
Last Updated: December 19, 2025
Pia Mill Reservoir, located in Koloa, Hawaii, is a privately-owned earth dam constructed in 1910 for irrigation purposes along the WaihoHonu Stream offstream.
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Summary
With a height of 16.5 feet and a storage capacity of 38.6 acre-feet, this reservoir covers a surface area of 6.1 acres and serves as a vital water resource for agriculture in the region. Despite its historical significance, the dam is classified as having a high hazard potential with a poor condition assessment, emphasizing the need for ongoing maintenance and risk management.
Managed by the Department of Land and Natural Resources (DLNR), Pia Mill Reservoir undergoes regular inspections with a frequency of 5 years to ensure its structural integrity and compliance with state regulations. Despite its moderate risk assessment, the reservoir's emergency action plan (EAP) is currently under revision as of March 2021. With a spillway width of 22 feet and a maximum discharge of 440 cubic feet per second, the reservoir plays a crucial role in water storage and distribution for local agricultural activities, highlighting the intersection of water resource management and climate resilience in Hawaii's diverse ecosystem.
As climate change continues to impact water availability and extreme weather events, the sustainability of Pia Mill Reservoir remains a key priority for stakeholders. With Tulsi Gabbard representing the area in Congress, ongoing efforts to address the dam's maintenance needs and emergency preparedness are crucial to safeguarding this essential water infrastructure for future generations. By integrating risk management measures and updating the EAP to meet guidelines, Pia Mill Reservoir can continue to support irrigation needs while adapting to the evolving challenges of a changing climate in Hawaii's water resource management landscape.
Year Completed |
1910 |
Dam Length |
450 |
Dam Height |
16.5 |
River Or Stream |
WAIHOHONU STREAM OFFSTREAM |
Primary Dam Type |
Earth |
Surface Area |
6.1 |
Hydraulic Height |
10 |
Drainage Area |
0.35 |
Nid Storage |
38.6 |
Structural Height |
14 |
Hazard Potential |
High |
Foundations |
Unlisted/Unknown |
Nid Height |
17 |
Seasonal Comparison
Weather Forecast
Nearby Streamflow Levels
Sf Wailua River Nr Lihue
|
173cfs |
Eb Of Nf Wailua River Nr Lihue
|
39cfs |
Left Branch Opaekaa Str Nr Kapaa
|
2cfs |
Waimea River Nr Waimea
|
295cfs |
Waialae Str At Alt 3
|
34cfs |
Wainiha River Nr Hanalei
|
178cfs |
Dam Data Reference
Condition Assessment
SatisfactoryNo 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
HighDams 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.
Area Campgrounds
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Pia Mill Reservoir