Ish #3 (Main Dam) Reservoir Report

Last Updated: December 5, 2025

Ish #3 (Main Dam) located in Larimer, Colorado, along the Big Thompson River, stands as a historic irrigation structure completed in 1905.


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Summary

This private earth dam boasts a height of 42 feet and a storage capacity of 9065 acre-feet, primarily serving agricultural purposes in the region. With a surface area of 314 acres and a maximum discharge of 3066 cubic feet per second, this dam plays a crucial role in water resource management in the area.

Despite its age, Ish #3 maintains a satisfactory condition assessment and a high hazard potential, highlighting the importance of regular inspections and maintenance. The dam features an uncontrolled spillway with a width of 60 feet and a moderate risk assessment rating of 3. The regulatory oversight from the Colorado Department of Water Resources ensures that the dam meets state permitting, inspection, and enforcement requirements, underscoring the commitment to safety and compliance in its operation.

As climate change continues to impact water resources, Ish #3 serves as a vital infrastructure for managing water supply and irrigation needs in the region. With its rich history and critical role in water management, this dam stands as a testament to the engineering ingenuity of the past while facing the challenges of the present and future in maintaining water security and resilience in the face of changing climate conditions.

Year Completed

1905

Dam Length

6700

Dam Height

42

River Or Stream

BIG THOMPSON RIVER-OS

Primary Dam Type

Earth

Surface Area

314

Hydraulic Height

37

Drainage Area

1

Nid Storage

9065

Structural Height

48

Hazard Potential

High

Nid Height

48
       
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.