At the lock and dam above Hastings, the mighty Mississippi plunges down and through the wicket gates to spin 5-foot blades in underwater turbines, which power generators that produce green electricity -- and cash.

The river town is one of seven Minnesota cities with a hydroelectric plant, and it is probably the most profitable. Since plant debt was paid off in December 2009, the 4-megawatt plant has earned about $450,000 for the city and is expected to contribute $600,000 in 2011, said Finance Director Char Stark.

"It was a big dream when they opened it in 1987," said Public Works Director Tom Montgomery. "It was almost a white elephant for the first decade. Some years we were in the hole, some years we barely broke even."

A few years of drought in the 1980s didn't help hydro production, added city Public Works Superintendent John Zgoda. He said the plant, nestled between the U.S. Army Corps of Engineers' locks and dam, produces enough electricity to serve about 3,000 of Hastings' 5,500 homes. The city sells its power to Xcel Energy.

So does St. Cloud's plant, more than 100 miles up the Mississippi. That 8.5-megawatt plant is the largest city-owned hydroelectric plant in Minnesota, according to the Department of Natural Resources (DNR).

St. Cloud generates more than twice as much power as Hastings, the next largest city-owned plant. St. Cloud is still paying off its plant debt but has built up a healthy reserve fund. The 22-year-old plant is expected to contribute, for the first time, about $300,000 to city coffers this year, said Public Service Director Patrick Shea.

"The city is at the beginning of the hydro utility starting to pay off," Shea said. "Green energy has only become more valuable as we move forward."

Hydro power is generated by capturing the energy of falling water. Minnesota has plenty of lakes and rivers, but it has few of the tall falls needed to economically produce large amounts of electricity.

About 1 percent of the state's electric power is homegrown hydro. According to the DNR, Minnesota has more than 30 hydroelectric plants ranging in size from Lanesboro's tiny 230-kilowatt plant on the Root River to Minnesota Power's 70 megawatt plant fed by the St. Louis River in Carlton County. Minnesota Power, a division of Allete, is the state's biggest hydro producer, with 11 plants that produce a total of 115 megawatts in northeastern Minnesota, according to its website.

The largest water power plants in the Twin Cities are the 17-megawatt Ford Dam plant, run by Brookfield Renewable Power of Massachusetts, and Xcel Energy's 12-megawatt plant on Hennepin Island at the upper St. Anthony Falls. Brookfield plans to open a new 9-megawatt plant this year at the lower St. Anthony Falls lock and dam.

During the permit approval process for the Hastings plant, the DNR recommended that it be a run-of-the-river plant, said Ian Chisholm, a DNR natural resource program supervisor. That means if the Mississippi drops too low, the Corps of Engineers directs plant operators to stop pulling water from the river, or pull less of it, to keep the level higher. That aids aquatic life and barge traffic.

Chisholm said that the benefits of hydro power must be balanced against environmental concerns caused by dams that fragment river ecosystems and cause sediment buildup, including pollutants, behind dams.

The other four Minnesota cities with hydro power are Rochester, Granite Falls, Redwood Falls and Thief River Falls. Those four plants range from 2.3 to 0.6 megawatts in size.

Two Minnesota counties have their own hydro facilities, including a 2.4-megawatt plant on Lake Byllesby in Dakota County, fed by the Cannon River. It is owned primarily by Dakota County and was refurbished and reopened in 1988, said Michelle Beeman, county director of environment and natural resources. It was built about 100 years ago, as was the 6-megawatt Rapidan Dam plant in Mankato, owned by Blue Earth County.

The Hastings plant started making money after improvements completed in the late 1990s and after the 2001 river flooding that inundated the plant, Montgomery said. He said the federal and state government paid for 90 percent of the more than $3 million in repairs and upgrades made after the flooding.

The improvements left the plant in good shape for about 50 years, Zgoda said. He noted the plant made money last year, even though one of its two turbines was down for five months while a leaky water seal was repaired.

Jim Adams • 952-707-9996