NEW ORLEANS -- Two huge sediment diversions slated for construction along the Mississippi River over the next four years represent by far the most ambitious effort yet to feed Louisiana's marshes with the material needed to build back the state's diminishing coastline.

But this year's record high-water event on the river is raising questions about what the introduction of so much diverted fresh water into new areas may mean for the future of bottlenose dolphins along the state's coast.

This summer, the northern Gulf of Mexico is nearing the end of an unplanned experiment that critics say might offer a preview of the effects of the diversions -- a record-breaking year of freshwater from upriver flooding and deep snowpack that has been funneled into Louisiana's brackish lakes and coastal estuaries.

The Bonnet Carre Spillway was open for much of the past year, sending tens of thousands of cubic feet of water per second into Lake Pontchartrain and from there into the Gulf, leaving a dead zone, algal blooms and devastated fisheries.

Scientists are trying to decipher the relationship between this year's freshwater deluge and the roughly 300 decomposing dolphins that washed up onshore along beaches in Louisiana, Mississippi, Alabama and the Florida Panhandle -- many of them covered in brown lesions caused by freshwater exposure.

If and when the twin diversions -- called the Mid-Barataria and the Mid-Breton -- go into operation, they will alter the salinity of the bays they dump into. Each will be capable of pouring up to 75,000 cubic feet of freshwater per second into prime dolphin habitats in Barataria Bay and Breton Sound.

"It is definitely a foreshadowing of what might happen" to dolphins, Lori Schwacke, chief scientist for marine medicine at the National Marine Mammal Foundation, said of this year's dolphin deaths. Schwacke has studied dolphins in Barataria Bay for decades.

Dolphins and their habitats are protected under the Marine Mammal Protection Act. But last year, the National Marine Fisheries Service was directed by Congress to sign a waiver of that act to allow the diversions to proceed -- a step supported by state officials who want to speed the projects along.

The waiver allows for the two diversions, and a third project in the Calcasieu Ship Channel, to result in "incidental takings," a legal term that excuses the killing of marine mammals when their habitat is disrupted unintentionally in pursuit of a broader goal.

The diversions, which will cost $2.2 billion to build, are designed to add sediment to build new land in Barataria Bay and Breton Sound and to help existing wetlands combat a global warming-fueled rise in the sea level. The state contends they will cost less over the 50-year time frame of the state's coastal master plan than repeated dredging to collect sediment.

State officials say the diversions will be operated at their maximum level only when the Mississippi is at its highest, and that their opening would be coordinated to reduce effects of the freshwater on fisheries. But they recognize that the diversions are likely to change commercial fishing practices -- such as where oysters can be grown, where certain fish can be caught, and the life cycles of various shrimp species.

But state officials must also deal with the diversions' potential effects on marine mammals. This year's uptick in dolphin deaths led the fisheries service to declare an "unusual mortality event." Since 1991, 14 such events have been declared for dolphins in the Gulf, most of them caused by toxins or undetermined causes. So far, low salinity is listed as a factor in only one of those events.

This year's investigation is still open. But it is likely that unusually high levels of freshwater had something to do with the deaths, said Schwacke, who hopes that officials are trying to figure out how low the salinity level can drop in dolphin habitats without harming the animals, and how long they can deal with such conditions.

As part of the waiver process, the fisheries service was required to consult with the Marine Mammal Commission, which was limited in its comments by the congressional action. But in an interview, Victoria Cornish, an energy policy analyst for the commission, said the freshwater event could provide information on dolphins' make-or-break point when it comes to salinity.

"What we are seeing is that it doesn't look good," she said.

Cornish said she is also concerned that, unlike past waivers, the waiver for the river diversions lacks an end date and does not limit its coverage to specific officials or organizations.

Louisiana's dolphin stocks are still recovering from the BP Deepwater Horizon oil spill of 2010, which killed over 1,100 dolphins and left others in poor health and unable to breed. The diversions could add decades to the timetable for Louisiana dolphins to recover -- if they ever do, said Schwacke.

The federal fisheries service raised concerns about the diversions before the waiver was approved. In a 2013 letter to the state's Coastal Protection and Restoration Authority, the service questioned the effects of the diversions' large freshwater flows into salty wetlands on fish, shrimp and federally managed species, like marine mammals, and on their habitats.

And in a letter to the Army Corps of Engineers in 2015, the service said the influx of freshwater would eliminate dolphin habitats.

Cornish said state and federal officials still need to prove that the diversions' positive effects will outweigh their potential damage, and questioned whether research under way as part of environmental assessments required for the projects is enough.

The fisheries service and the CPRA are studying the current status of dolphin stocks and how they might be affected by the additional freshwater that would flow from the diversions. An environmental impact statement for the Mid-Barataria diversion is scheduled for completion by July.

National Oceanic and Atmospheric Administration spokeswoman Kim Amendola said the fisheries service would work to minimize impacts on marine mammals from the diversions "to the extent practicable and consistent with the purposes of the projects."

Amendola said officials will have a better idea of how to proceed once the environmental impact statement and a separate restoration plan for dolphins required under the oil spill's "natural resource damage assessment" are completed.

Brian Lezina, a division chief for planning and research at the CPRA, said it is problematic to try to project this year's freshwater event into the future, in part because the flow from the diversions will be much smaller than the prolonged spillway opening, and it will mostly affect shallow, vegetated areas.

But the timing does not matter as much as how long fresh water is coming in, according to Schwacke. Dolphins tend to remain in the same area all year, and they stay even if conditions aren't ideal, she said.

If they move or die, that would create a new set of problems, she said.

"It would be a different ecosystem if dolphins weren't here," said Schwacke.