Iwan #4 Reservoir Report
Nearby: Iwan O.M. No.2 Iwan O.M. Number 1
Last Updated: December 6, 2025
Iwan #4 is a privately owned dam located in Mellette, South Dakota, along the TRIB BIG WHITE OFFSTREAM river.
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Summary
Constructed in 1957, this earth dam stands at a height of 29 feet and stretches 497 feet in length. With a storage capacity of 33 acre-feet and a maximum discharge rate of 2035 cubic feet per second, Iwan #4 serves as a crucial water resource structure in the area.
Managed by the South Dakota Department of Environment and Natural Resources (DENR), Iwan #4 is regulated, permitted, inspected, and enforced by state authorities. Despite being classified as a low hazard potential structure, the dam's condition assessment is currently labeled as "Not Rated", indicating a need for further evaluation. With emergency action preparedness and risk management measures yet to be fully assessed, there is room for improvement in ensuring the safety and reliability of Iwan #4 for the surrounding community and environment.
As a part of the St. Paul District under the supervision of the Natural Resources Conservation Service, Iwan #4 plays a vital role in water resource management and climate resiliency efforts. By staying abreast of state regulations, conducting regular inspections, and enhancing emergency preparedness measures, the owners and stakeholders of Iwan #4 can uphold the integrity of this infrastructure and mitigate potential risks associated with its operation.
Year Completed |
1957 |
Dam Length |
497 |
Dam Height |
29 |
River Or Stream |
TRIB BIG WHITE OFFSTREAM |
Primary Dam Type |
Earth |
Hydraulic Height |
29 |
Nid Storage |
33 |
Hazard Potential |
Low |
Nid Height |
29 |
Seasonal Comparison
Weather Forecast
Nearby Streamflow Levels
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.
Iwan #4