Thomas Reservoir Report

Last Updated: December 5, 2025

Thomas, also known as Lewis Lake or J L Thomas Reservoir, is a privately owned water structure located in Carbondale, Colorado.


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Summary

Constructed in 1938, this earth dam stands at a height of 32 feet and serves primarily for irrigation purposes, with a storage capacity of 292 acre-feet. The reservoir covers a surface area of 15 acres and is situated along the Thomas Creek-OS, providing essential water resources for the surrounding area.

Managed by the Colorado Department of Water Resources, Thomas Dam has a significant hazard potential and is classified as being in fair condition as of the last inspection in September 2020. The dam has undergone modifications in 1999 to enhance its foundation, hydraulic, and structural components. With an uncontrolled spillway and a maximum discharge capacity of 800 cubic feet per second, the reservoir plays a crucial role in water management and irrigation activities in Pitkin County.

Despite its moderate risk assessment, Thomas Dam is an important piece of infrastructure in the region, ensuring water supply for agricultural needs and offering recreational opportunities. With continued monitoring and maintenance, this reservoir remains a vital asset for water resource and climate enthusiasts interested in sustainable water management practices in Colorado.

Years Modified

1999 - Foundation, 1999 - Hydraulic, 1999 - Structural

Year Completed

1938

Dam Length

100

Dam Height

32

River Or Stream

THOMAS CREEK-OS

Primary Dam Type

Earth

Surface Area

15

Hydraulic Height

31

Nid Storage

292

Structural Height

32

Outlet Gates

Slide (sluice gate) - 1, Valve - 2

Hazard Potential

Significant

Foundations

Unlisted/Unknown

Nid Height

32
       
Seasonal Comparison

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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.