By Erwin Seba

HOUSTON (Reuters) - Valero Energy Corp's and Motiva Enterprises refineries in St. Charles Parish Louisiana, west of New Orleans, will be flooded if the Morganza Spillway is not opened, the St. Charles Parish emergency preparedness director said on Wednesday.

The Mississippi River Commission is expected to decide by this weekend on opening the Morganza, which would send floodwaters from the Mississippi down the Atchafalaya River, according to the Louisiana governor's office.

"If they don't open the Morganza, those refineries would absolutely be flooded," said Scott Whelchel director of homeland security and emergency preparedness for St. Charles Parish.

Scores of U.S. heartland rivers from the Dakotas to Ohio have flooded following a snowy winter and heavy spring rains, and threaten nine Louisiana refineries as the rising waters move south down the Mississippi. (Map: http://r.reuters.com/gyt49r )

Without the release of water into the Atchafalaya, Mississippi River flood waters will pour over St. Charles parish levees, which range in height from 26 feet to 28 feet, Whelchel said.

In a Tuesday news conference, Louisiana Governor Bobby Jindal said at least 2,500 people living in the Morganza spillway would be affected.

"I don't want to dismiss what this would do to those people," Whelchel said.

Valero spokesman Bill Day said on Wednesday, that the company does not expect operations at its 185,000 barrel per day (bpd) St. Charles Parish refinery will be affected by flooding.

A representative for Motiva's 234,700 bpd Norco, Louisiana, refinery was not immediately available to discuss refinery operations.

The Mississippi River Commission can only open the Morganza spillway after the flow of water at a key point reaches 1.5 million cubic feet per second (42,475 cubic meters). As of Wednesday afternoon, the flow was 1.39 million cubic feet per second.

The flow was expected to hit the required level over the weekend.

Diverting part of the Mississippi's flow to the Atchafalaya is expected to disrupt operations at Alon USA Energy's Krotz Springs, Louisiana, refinery because barges will not be able to bring 25 percent of the refinery's crude supply, sources familiar with refinery operations have told Reuters.

Flooding the Atchafalaya Basin would also disrupt oil and natural gas production in seven parishes in south central Louisiana, according to a report by Jeffries & Co.

Rising river levels could also affect operations at the Waterford 3 nuclear plant in St. Charles Parish, Whelchel said. River levels could rise above water intake piping at the plant, which is currently offline for refueling.

(Reporting by Erwin Seba; Editing by Marguerita Choy)