Approximately 500 visitors walked through Lake Zumbro Dam during the first two hours of tours Friday.
“And we still have tomorrow,” said Mark Kotschevar, Rochester Public Utilities' general manager.
The tours, from noon to 5 p.m. Friday and 9 a.m. to 1 p.m. Saturday, are in observance of the dam’s 100th anniversary.
Here are a few things to know about the facility located on the north end of Lake Zumbro:
1. The dam can power 1,500 homes.
The 55-foot high, 900-foot long dam has generated an average of 12,500 megawatt hours a year since 2000, which is enough power for approximately 1,500 homes.
Production hit a new record last year with 17,566 megawatt hours.
2. Energy is generated by harnessing the natural water flow.
The dam is a run-of-the-river hydroelectric system, meaning it generates electricity by harvesting energy from flowing water. Minnesota Department of Natural Resources regulations limit the flow to what’s naturally flowing into Lake Zumbro and through or over the dam, rather than allowing water retention for future energy generation. The limit helps reduce potential lake-level fluctuations.
The water flow turns a pair of turbines to generate power.
3. Enough water to fill 100 bathtubs each second can power the turbines.
When both turbines are operating at peak performance, approximately 590 cubic feet of water — more than 4,400 gallons — passes through them each second.
The typical bathtub holds 42 gallons.
4. The dam is a piece of history.
Constructed by Minnesota native Hugh L. Cooper, the dam is one of many international projects undertaken by the renowned civil engineer, including dams in Russia and Egypt. However, the Lake Zumbro Dam is his only homestate project.
In 1991, the dam was listed with the National Register of Historic Places for its feats of engineering and connection to Cooper, who was called “one of the nation's foremost hydroelectric engineers of the early 20th century” in the register paperwork.
5. The hydroplant was designed to meet growth.
RPU’s website notes the hydroelectric dam and powerhouse were built to produce enough electricity for a city of 60,000 residents at the time, which was expected to exceed anticipated needs.
Rochester had grown nearly 15 percent between 1900 and 1910, and the following decade saw the city’s population grow from 7,844 to 13,722.
6. Dam power was intended to replace steam.
Rochester’s utilities board planned to replace the city’s steam facility with the hydroelectric plant as the area's main source of electrical power, according to the application for National Register of Historic Places status.
However, local demand continued to increase at a pace that quickly required the energy created by both plants.
7. Dredging won’t impact flow.
Rochester Public Utilities contributed nearly $1.17 million to the effort to dredge Lake Zumbro, but the work isn’t expected to affect energy production at the site, said Tony Benson, RPU’s communications coordinator.
“As a member of the lake, we wanted to contribute and be a good neighbor,” he said.
8. The dam remains part of RPU's renewable energy plan.
Rochester Public Utilities estimates the dam has generated 870,000 megawatt hours of energy in its lifetime, which replaced potential need for 4,533 railroad cars of coal, each carrying 100 tons.
Kotschevar pointed out Friday that the dam is the city’s original renewable energy source.
Replacing the energy the dam produces each year would require 51 acres of 7.5 megawatt solar panels, according to an RPU presentation.