Mosquito Flat Reservoir Report

Last Updated: January 11, 2026

Mosquito Flat, located in Challis, Idaho, is a privately owned water resource site regulated by the Idaho Department of Water Resources.


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Summary

Constructed in 1950 by the State of Idaho in collaboration with USDA NRCS, this earth dam stands at a height of 58.4 feet and serves a primary purpose of "Other." It holds a storage capacity of 793 acre-feet, with a spillway width of 15 feet and a maximum discharge of 4360 cubic feet per second.

With a significant hazard potential and fair condition assessment as of August 2017, Mosquito Flat is closely monitored through inspections every four years. The dam's hydraulic height of 49.4 feet and stone core types contribute to its structural integrity. The site's risk assessment is moderate, indicating the need for ongoing risk management measures to ensure the safety of the surrounding areas, including Custer County, Idaho, and the Salmon River.

Despite its uncontrolled spillway type and slide outlet gates, Mosquito Flat remains a vital water management structure in the region, with a drainage area of 16.1 square miles and a surface area of 46 acres. Its location along Challis Creek and the Salmon River underscores its importance in managing water resources and mitigating potential flood risks in the area. Enthusiasts of water resources and climate management will find Mosquito Flat to be a fascinating example of collaborative efforts in water infrastructure development and regulation.

Year Completed

1950

Dam Length

450

Dam Height

58.4

River Or Stream

CHALLIS CREEK SALMON RIVER

Surface Area

46

Hydraulic Height

49.4

Drainage Area

16.1

Nid Storage

793

Outlet Gates

Slide (sluice gate)

Hazard Potential

Significant

Foundations

Rock, Soil

Nid Height

58
       
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.