Rainbow Reservoir Report

Last Updated: December 5, 2025

Rainbow, also known as the Cth "D" Dam, is a captivating structure located in Rhinelander, Wisconsin, along the Wisconsin River.


°F

°F

mph

Wind

%

Humidity

Summary

Constructed in 1935, this gravity dam stands at an impressive height of 38.5 feet and spans 1800 feet in length. With a storage capacity of 46,006 acre-feet, Rainbow serves primarily for hydroelectric purposes and flood risk reduction, making it a vital asset in the region's water resource management.

Managed by the Federal Energy Regulatory Commission, Rainbow poses a high hazard potential due to its location and structure. Despite its age, the dam continues to play a crucial role in regulating water flow and providing renewable energy to the surrounding areas. With its controlled spillway and radial outlet gates, Rainbow is equipped to handle maximum discharge rates of up to 32,500 cubic feet per second, ensuring the safety and efficiency of its operations.

For water resource and climate enthusiasts, Rainbow represents a remarkable feat of engineering and a key player in the sustainable utilization of water resources in Wisconsin. Its historical significance, combined with its modern functionality, makes Rainbow a focal point for understanding the intersection of water management, hydropower generation, and environmental stewardship in the region.

Year Completed

1935

Dam Length

1800

Dam Height

38.5

River Or Stream

Wisconsin River

Primary Dam Type

Gravity

Drainage Area

658

Nid Storage

46006

Outlet Gates

Tainter (radial) - 5

Hazard Potential

High

Foundations

Soil

Nid Height

39
       
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.