For the fourth time since 2008, federal officials are opening the Bonnet Carre Spillway, sending cold, fresh Mississippi River water tumbling into the normally brackish Lake Pontchartrain.

This year's opening is starting small — beginning at 10 a.m. Thursday — with only eight of the spillway's 350 gates likely to be opened. More will follow as the volume of water flowing down the river increases over the next few days. 

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The release of water into the lake reduces pressure on the river levees in the New Orleans area.

The freshwater plume flowing into the lake will slowly push out the brackish water that normally is found in the estuary, according to John Lopez, of the Lake Pontchartrain Basin Foundation. 

"After about two weeks, the whole lake will be river water," he said of the plumes, which are often visible in satellite photos. The water will bring with it some freshwater fish species, including freshwater catfish, he added.

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One unwanted species that has not set up residence in the lake during past openings is invasive carp, and Lopez is hopeful that that will stay true in 2018.

"I am sure there will be some carp coming through the structure itself," he said. But as the lake's brackish water returns, it likely will make conditions hostile to carp survival, he said. One worry, though, is that the carp may settle in the freshwater bayous and rivers that flow into Lake Pontchartrain and Lake Maurepas, he said.

The Lake Pontchartrain Basin Foundation conducts regular water quality testing in the lake, and what it has found in the past is that the lake returns to its normal state within two or three months after the spillway closes.

"Usually by July or August, the lake is pretty much back to normal," Lopez said. "I don't think we are seeing any cumulative effect" from the repeated recent openings of the spillway, he said.

The spillway is a mile-long structure in St. Charles Parish near Norco. Sitting in the outside of a bend in the river, its 7,000 wooden "needles," or heavy beams, hold back the river. When it is opened, a crane pulls the beams up one by one, allowing the river water to flow through. 

Completed in 1931, the Bonnet Carre Spillway was opened eight times between 1937 and 1997, including three times between 1973 and 1979. 

Since Hurricane Katrina, the spillway was opened in 2008, 2011 and 2016. By far the largest of the recent openings was in 2011, when all 350 bays were opened and at least part of the spillway was open for 42 days as the river reached unusually high levels.

There is a temptation to attribute the frequency of recent openings to things like climate change, said John R. White, a professor of oceanography and coastal science at LSU. However, he said, "It might be too early to tell that yet."

The amount of water flowing down the river is subject to a number of variables, he noted.

This year, snow melting in the river's upper basin and heavy rains are the main drivers of the high water.

"When the rain comes in at the same time (as snow melt), we get that big push of water," he said.

One thing that officials, scientists and shipping industry executives will be watching carefully will be shoaling, or the depositing of sediment on the bottom of the river. When the spillway is opened, it slows the river down, which means more sediment settles.

Silting is already an issue at Southwest Pass, said Sean Duffy of the Big River Coalition.

Dredges are already working there to keep the pass clear for shipping, and Duffy said Army Corps of Engineers officials have told him that more are on the way to help deal with increased sediment.

"The first one is going to arrive Saturday. A couple more are on the way," he said. "That's the real impact on shipping."

In what Duffy called the worst case, the Corps would implement draft restrictions, limiting the size of ships allowed on the river, as it did in 2016. "That was a huge economic impact on the river system," he said. 

A high river also means currents above 8 knots, which makes navigation harder and requires additional ship security measures, Duffy said. 

"We are in a heightened awareness phase, with extra security measures in place," he said. "It's just so much water."

Follow Faimon A. Roberts III on Twitter, @faimon.