Tumblesom Reservoir Report

Last Updated: December 5, 2025

Tumblesom is a private fish and wildlife pond located in Lyons, Colorado, along the South St.


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Summary

Vrain Creek. Constructed in 1918, this rockfill dam stands at 19 feet tall and spans 186 feet in length, with a storage capacity of 149 acre-feet. The dam serves multiple purposes, including providing habitat for fish and wildlife, as well as offering recreational opportunities for visitors.

Managed by the Colorado Department of Water Resources, Tumblesom has a low hazard potential and is in satisfactory condition as of the last inspection in October 2020. With a moderate risk assessment rating, the dam is equipped with an uncontrolled spillway and has a maximum discharge capacity of 96 cubic feet per second. While no emergency action plan is currently in place, the dam is regularly inspected every six years to ensure its safety and functionality.

Located in the Omaha District, Tumblesom is overseen by private owners and falls under state jurisdiction for regulation, permitting, inspection, and enforcement. The dam plays a crucial role in supporting the local ecosystem and providing a scenic backdrop for recreational activities, making it a notable water resource and climate enthusiast destination in Boulder County, Colorado.

Year Completed

1918

Dam Length

186

Dam Height

19

River Or Stream

SOUTH ST. VRAIN CREEK-TR

Primary Dam Type

Rockfill

Surface Area

16

Hydraulic Height

19

Nid Storage

149

Structural Height

19

Hazard Potential

Low

Nid Height

19
       
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.