Koolau Reservoir Reservoir Report

Last Updated: December 19, 2025

Koolau Reservoir, located in Wahiawa, Hawaii, is a federally-owned earth dam that was completed in 1919 by the US Army.


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Summary

With a height of 70 feet and a length of 270 feet, the reservoir has a storage capacity of 138 acre-feet and serves as a vital water resource for the surrounding area. The primary purpose of Koolau Reservoir is listed as 'Other', indicating its versatility in meeting various water needs.

Despite its low hazard potential, Koolau Reservoir is regulated by the Hawaii Department of Land and Natural Resources and undergoes regular inspections by the US Army. The reservoir's spillway, with a width of 60 feet, is uncontrolled, adding an element of natural flow to its operations. The risk assessment for Koolau Reservoir is moderate, highlighting the importance of ongoing monitoring and maintenance to ensure its continued safety and functionality.

Water resource enthusiasts will appreciate the historical significance of Koolau Reservoir as a vital piece of Hawaii's water infrastructure. Its location in the S Fork Kaukonahua river basin, coupled with its 138-acre-foot storage capacity, makes it a key player in managing water resources in the region. The reservoir's association with the US Army underscores its strategic importance and the need for careful oversight to mitigate any potential risks to the surrounding community.

Year Completed

1919

Dam Length

270

Dam Height

70

River Or Stream

S Fork Kaukonahua

Primary Dam Type

Earth

Surface Area

5

Nid Storage

138

Outlet Gates

None

Hazard Potential

Low

Foundations

Unlisted/Unknown

Nid Height

70
       
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.