Twin Granite Reservoir Report

Last Updated: December 21, 2025

Twin Granite, also known as Twin Lakes Dam, is a private water resource structure located in Adams County, Idaho.


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Summary

Built in 1920 for irrigation purposes, the dam stands at a height of 24.5 feet and has a hydraulic height of 19.9 feet. With a storage capacity of 600 acre-feet and a surface area of 40 acres, Twin Granite plays a crucial role in managing water resources for agricultural use in the region.

Owned and regulated by the Idaho Department of Water Resources, Twin Granite is classified as a low hazard potential structure with a fair condition assessment. The dam features a controlled spillway with a width of 5 feet and a maximum discharge capacity of 64 cubic feet per second. Despite its age, Twin Granite has undergone regular inspections, with the last assessment conducted in July 2019, ensuring its continued safety and functionality for the community.

Situated along the Twin Lakes and Goose Creek waterways, Twin Granite serves as a vital infrastructure for water management in the area. With its stone core and earth construction, the dam stands as a testament to early 20th-century engineering and continues to play a crucial role in sustaining agricultural activities in the region.

Year Completed

1920

Dam Length

620

Dam Height

24.5

River Or Stream

TWIN LAKES GOOSE CREEK

Surface Area

40

Hydraulic Height

19.9

Drainage Area

0.6

Nid Storage

600

Hazard Potential

Low

Foundations

Rock, Soil

Nid Height

25
       
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.