Thomsen Dam Reservoir Report
Last Updated: December 15, 2025
Thomsen Dam, located in Anthon, Iowa, along the TR-Little Sioux River, was completed in 1959 by the USDA NRCS and serves multiple purposes including fire protection, stock, and small fish pond.
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Summary
With a dam height of 38 feet and a length of 550 feet, the earth dam holds a normal storage capacity of 49 acre-feet and a maximum storage capacity of 91 acre-feet. The dam's low hazard potential and moderate risk assessment make it a crucial asset for water resource management in Woodbury County.
Managed by a private owner, Thomsen Dam is regulated by the Iowa DNR and undergoes state inspection and enforcement to ensure its structural integrity. The dam's primary source agency is the state of Iowa, and it falls under the jurisdiction of the Rock Island District. Despite its uncontrolled spillway type and lack of outlet gates, the dam's risk management measures and moderate risk assessment highlight its importance in mitigating potential flood risks and ensuring water resource sustainability in the region.
Water resource and climate enthusiasts will be captivated by Thomsen Dam's role in providing essential services such as fire protection and stock pond maintenance while also serving as a habitat for small fish species. Its location along the TR-Little Sioux River and its significant storage capacity make it a key infrastructure for water management in Woodbury County. With a focus on safety and risk management, the dam stands as a vital asset in safeguarding the surrounding communities and ecosystems from potential water-related hazards.
Year Completed |
1959 |
Dam Length |
550 |
Dam Height |
38 |
River Or Stream |
TR-LITTLE SIOUX RIVER |
Primary Dam Type |
Earth |
Surface Area |
5.8 |
Drainage Area |
0.93 |
Nid Storage |
91 |
Hazard Potential |
Low |
Foundations |
Soil |
Nid Height |
38 |
Seasonal Comparison
Weather Forecast
Nearby Streamflow Levels
Little Sioux River At Correctionville
|
327cfs |
West Fork Ditch At Hornick
|
80cfs |
Maple River At Mapleton
|
323cfs |
Floyd River At James
|
187cfs |
Missouri River At Sioux City
|
14400cfs |
Perry Creek Near Milnerville
|
25cfs |
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.
Thomsen Dam