Grassy Creek Watershed Dam G- 40a Reservoir Report

Last Updated: December 5, 2025

Grassy Creek Watershed Dam G-40a is a vital structure designed by the USDA NRCS in Lewis County, Missouri, to serve the primary purpose of flood risk reduction.


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Summary

Completed in 2006, this earth dam stands at a height of 24.6 feet and stretches 480 feet in length, with a storage capacity of 81.9 acre-feet and a surface area of 3 acres. It is situated along the Grassy Creek, under the jurisdiction of the Rock Island District.

Despite its low hazard potential and moderate risk assessment, the dam has not been rated for its condition or undergone recent inspections. It features an uncontrolled spillway with a width of 20 feet and outlet gates that are also uncontrolled. With no associated structures, the dam's operation falls under the responsibility of the local government. Water resource and climate enthusiasts can appreciate the engineering marvel of Grassy Creek Watershed Dam G-40a, which plays a crucial role in managing flood risks in the region while supporting fire protection, stock, fish and wildlife ponds, and overall environmental conservation efforts.

Year Completed

2006

Dam Length

480

Dam Height

24.6

River Or Stream

GRASSY CREEK

Primary Dam Type

Earth

Surface Area

3

Hydraulic Height

24.6

Drainage Area

89.6

Nid Storage

81.9

Structural Height

29

Outlet Gates

Uncontrolled

Hazard Potential

Low

Foundations

Soil

Nid Height

29
       
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.