John Alley Reservoir Report
Nearby: Wanblee Lake Dam Sd Noname 186
Last Updated: December 13, 2025
John Alley is a privately owned dam located in Jackson, South Dakota, along the TR-Black Horse Creek.
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Summary
Built in 1946, this Earth dam stands at a height of 16 feet and has a length of 670 feet. With a storage capacity of 128 acre-feet, the dam serves the purpose of water resource management and flood control in the area.
Managed by the South Dakota Department of Environment and Natural Resources (DENR), John Alley is state-regulated and undergoes regular inspections, permitting, and enforcement to ensure its safety and functionality. Despite being classified as low hazard potential and not rated in terms of condition assessment, the dam plays a crucial role in protecting the surrounding community from potential risks associated with flooding events.
With a maximum discharge of 4840 cubic feet per second, John Alley is equipped to handle significant water flow during heavy rainfall or snowmelt periods. Although not currently assessed for emergency action plans or risk management measures, the dam remains a vital infrastructure for water supply and climate resilience in the region. Enthusiasts of water resources and climate change monitoring may find John Alley an intriguing site to study the intersection of human-made structures and environmental sustainability.
Year Completed |
1946 |
Dam Length |
670 |
Dam Height |
16 |
River Or Stream |
TR-BLACK HORSE CREEK |
Primary Dam Type |
Earth |
Hydraulic Height |
16 |
Nid Storage |
128 |
Hazard Potential |
Low |
Nid Height |
16 |
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.
John Alley