Thomson Dam Reservoir Report
Nearby: Christensen Dam Miller Dam
Last Updated: December 13, 2025
Thomson Dam, located in Defiance, Iowa, was completed in 1975 and is primarily owned and regulated by private entities.
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Summary
The dam, designed by USDA NRCS, serves multiple purposes such as fire protection, stock, and small fish pond. With a height of 27 feet and a length of 610 feet, Thomson Dam has a storage capacity of 109 acre-feet and covers a surface area of 6.3 acres, with a drainage area of 0.46 square miles.
The dam, situated on TR-WILLOW CREEK, is classified as an Earth dam with a low hazard potential and a moderate risk assessment. Although the condition of the dam is not rated, it undergoes state inspection, regulation, and enforcement by the Iowa Department of Natural Resources. Thomson Dam features an uncontrolled spillway and has not been modified in recent years. Despite the lack of detailed risk management measures, the dam's operational capacity and potential emergency response have not been fully assessed.
For water resource and climate enthusiasts, Thomson Dam serves as a vital structure for water management in Crawford County, Iowa. With its strategic location and multi-purpose functionality, the dam plays a crucial role in providing fire protection and sustaining local wildlife. The dam's design by USDA NRCS ensures its structural integrity, while ongoing state oversight guarantees compliance with regulatory standards. As a key feature within the Rock Island District, Thomson Dam contributes to the overall water infrastructure and ecosystem resilience in the region.
Year Completed |
1975 |
Dam Length |
610 |
Dam Height |
27 |
River Or Stream |
TR-WILLOW CREEK |
Primary Dam Type |
Earth |
Surface Area |
6.3 |
Drainage Area |
0.46 |
Nid Storage |
109 |
Hazard Potential |
Low |
Foundations |
Soil |
Nid Height |
27 |
Seasonal Comparison
Weather Forecast
Nearby Streamflow Levels
Boyer River At Logan
|
417cfs |
Maple River At Mapleton
|
323cfs |
Soldier River At Pisgah
|
112cfs |
North Raccoon River Near Sac City
|
312cfs |
Little Sioux River Near Turin
|
1040cfs |
Monona-Harrison Ditch Near Turin
|
158cfs |
Dam Data Reference
Condition Assessment
SatisfactoryNo 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
HighDams 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.
Thomson Dam