Mississippi River Lock And Dam 21 Reservoir Report

Last Updated: December 19, 2025

Mississippi River Lock and Dam 21, also known as Pool 21, is a federal-owned structure located in Hannibal, Missouri.


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Summary

Completed in 1938, this concrete dam plays a crucial role in navigation along the Mississippi River, with a hydraulic height of 11 feet and a structural height of 48 feet. The dam's purpose is primarily for navigation, and it features a single lock that is 600 feet long and 110 feet wide, allowing boats to pass through the river.

With a hazard potential deemed significant, the US Army Corps of Engineers (USACE) actively manages flood risks associated with the dam. Regular inspections, maintenance, and collaboration with local emergency managers are key components of the risk management measures in place. The dam has limited water storage capacity, and water releases may be necessary to manage water levels and ensure the dam's structural integrity. USACE works diligently to address all types of flood risk and maintain the safety and functionality of the Mississippi River Lock and Dam 21, serving as a critical infrastructure for water resource management and climate resilience in the region.

Year Completed

1938

Dam Length

3116

River Or Stream

MISSISSIPPI RIVER

Primary Dam Type

Concrete

Surface Area

8843

Length Of Locks

600

Hydraulic Height

11

Drainage Area

135000

Nid Storage

62000

Structural Height

48

Outlet Gates

Roller - 3, Tainter (radial) - 10, Uncontrolled - 0

Hazard Potential

Significant

Foundations

Soil

Nid Height

48
       
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.