Artificially raising water levels in Lakes Huron and Michigan to compensate for drop-offs caused by human tinkering is technologically feasible but would take decades to achieve and could cost more than $200 million, according to a report prepared for a U.S.-Canadian advisory panel.

The study, obtained by The Associated Press, analyzes the pros and cons of placing structures in the upper St. Clair River to reduce outflow from Huron and Michigan, which geologically are considered one lake because they are joined at the top and have the same surface level. The river at the base of Lake Huron sends water downstream through a series of channels to Lake Erie and eventually Lake Ontario and the St. Lawrence River, which empties into the Atlantic.

Water levels have been abnormally low on Lakes Huron and Michigan at times since the late 1990s, causing millions in losses for shippers, marina owners and other businesses, while driving down shoreline property values on Huron's Georgian Bay where the shortfall has been especially severe.

Some activists are pushing officials to put turbines, underwater dams or other devices into the St. Clair River to stem the flow from Lake Huron. A study commissioned by a group called Georgian Bay Forever contends Huron is losing up to 12 billion gallons a day beyond its normal outflow.

The International Joint Commission, which advises the U.S. and Canadian governments on Great Lakes issues, asked a board of engineers and scientists to examine the matter. The board has acknowledged that dredging, channel widening, gravel mining and other activities widened the river bottom and reduced the Huron-Michigan level by nearly 20 inches for at least a century, beginning in the mid-1800s.

Still, the board concluded in 2009 that the Georgian Bay group was exaggerating more recent water losses through the river and said structures in the St. Clair weren't needed. But the commission asked the board to investigate the likely effects of leaving Huron-Michigan alone or using regulatory structures to raise them as much as 20 inches.

The study board's new report says some types of structures could return the levels of Huron and Michigan close to where they would have been without the dredging and other human interference. A set of submerged "sills" resembling 30-foot-high speed bumps would cost from $71 million to $225 million, while installing an adjustable, inflatable "flap gate" across the river's east channel to control flows would cost from $134 million to $171 million.

The sills could boost water levels by nearly 10 inches and the gate nearly 4 inches. It would take decades to gain the necessary permits and install the devices, said Eugene Stakhiv, the study board's U.S. co-chairman.

Doing so would produce winners and losers among regions, sectors of the economy and local ecosystems, the report says.

Commercial navigation on the two lakes, for example, would benefit as ships could carry heavier loads through shallow channels. But reduced flows to the lower lakes and their connecting rivers would mean less hydropower at places such as the Niagara and St. Lawrence rivers.

Higher water on Lake Huron would replenish the wetlands of Georgian Bay. But the structures would leave less water for Lake Erie and other downstream waterways for a least a decade, potentially harming their wetlands and fish habitat until the system reaches stability. They also could degrade spawning grounds of endangered fish such as lake sturgeon, for which the St. Clair River is a crucial nursery, the study says.

"It's a cost-benefit ratio" that the international commission and government officials will have to consider as they debate what to do, said Scudder Mackey, manager of the study board's group that dealt with ecosystems.

Although the report takes no position on whether to install the structures, "the consensus was almost uniform from all the fisheries biologists and people who work the wetland complexes that this really wasn't a very good idea," Mackey told the AP.

Mary Muter of the Sierra Club of Ontario, a longtime supporter of raising Lake Huron's levels, said there were proven technologies that could stem the lake's outflow without harming the environment or costing taxpayers large sums. One example would be a series of privately operated, submerged turbines that would generate hydropower, she said.

"I believe there is a way to meet everyone's needs," Muter said.

Stakhiv said a couple hundred turbines would be needed to boost water levels as much as the sills would do. "It's an obstacle course for fish" and many would be killed, he said.

Muter said there are underwater turbines operating in Canadian waters that don't harm fish or aquatic habitat.