MEMPHIS, Tenn.—Insured losses from flooding along the Mississippi River will be spotty, with some property owners covered by insurance written by the commercial market and the National Flood Insurance Program, and others without any coverage, experts say.

The river, bloated by heavy rains and snowmelt, has washed over its banks, flooding homes and businesses. Areas around Memphis, Tenn., and Tunica, Miss., were hit the hardest, suffering economic losses estimated to be hundreds of millions of dollars.

Late last week, the Army Corps of Engineers was considering whether to open the Morganza Spillway in Louisiana to protect Baton Rouge and New Orleans from catastrophic flooding, a move that would put about 13,000 structures upriver from those cities at risk of being swamped, according to the Corps.

Dockside casinos along the Mississippi were shuttered last week and oil refinery workers scrambled to place barriers aimed at holding back the water.

Some, but not all, commercial operations along the river are covered for flood damage said Duncan C. Ellis, managing director and U.S. property practice leader at Marsh USA Inc. in New York.

While large companies likely are covered, middle-market and smaller operations may not have insurance, Mr. Ellis said. “It's very spotty,” he said, and often depends on whether the companies have loans with banks that require the insurance.

Patrice Collingwood, senior vp at Marsh in San Antonio, said businesses in high-risk areas can purchase up to $500,000 in flood coverage from the NFIP. The insurance will respond to property losses, but does not provide business interruption coverage, she said.

Mr. Ellis said commercial property policies typically attach above the coverage provided by the NFIP.

Without the NFIP, insurers likely would write flood coverage that includes a large deductible, said Al Tobin, national property practice leader with Aon Risk Solutions in New York.

“If it's available, you should be buying it,” Mr. Tobin said of NFIP coverage.

Among businesses that were trying to ward off flood waters, Dallas-based Alon USA Energy Inc.'s unit in Krotz Springs, La., was constructing a levee.

The refinery is on the Atchafalya River, which is at risk of flooding from water released from the Morganza Spillway. Barge traffic and access to oil pipelines would be interrupted, causing the refinery to operate at less-than-full capacity perhaps for several weeks, Alon said in a statement.

Coverage for preparations such as those by Alon would be covered under a property policy's “sue-and-labor” provisions, said Mr. Ellis. Such coverage pays for expenses incurred to protect property from “imminent damage,” he said.

Business interruption insurance generally responds only when triggered by physical damage, Mr. Ellis said, but property policies typically respond if flooding causes “ingress and egress” problems that lead to losses because access to a facility is restricted.