Hurricane Isaac is long gone from south Louisiana, but the flood threat continues. A mandatory evacuation order was issued Saturday night for residents in parts of St. Tammany Parish north of New Orleans, along the Mississippi border.

The engorged Pearl River is threatening homes in the area near a lock that local officials fear could fail.

"We have a list of 20 to 25 neighborhoods that it could affect," parish President Pat Brister told CNN on Sunday. "I don't have a number of people at this current point and time, but it would affect several neighborhoods."

The National Weather Service says the river could crest to 19.5 feet on Monday, or five feet above flood stage. If the lock fails, at least two nearby subdivisions could be quickly inundated. St. Tammany Parish is about 200 feet above sea level.

Even at its peak, Isaac was a much weaker storm than Hurricane Katrina, which killed about 1,800 people -- mostly in New Orleans -- after it made its southeast Louisiana landfall on Aug. 29, 2005. But Isaac moved much more slowly and dumped more rain on south Louisiana than Katrina. Plaquemines Parish, just south of the Big Easy, received the brunt of Isaac's wrath.

Nearly 248,000 Entergy Corporation (NYSE: ETR) customers in New Orleans were still without power as of Saturday night; many will begin the work week Tuesday in the dark. Parts of St. Bernard Parish, just south of the Lower Ninth Ward, won't see power restored until Friday because floodwaters are still hampering operations there. (The Lower Ninth, which is part of New Orleans, and St. Bernard, the neighboring parish, were the most heavily damaged from Katrina. Some homes in St. Bernard were completely immersed above their rooftops in 2005.)

Meanwhile, Isaac's remnants were dumping much-needed water on the drought-stricken Midwest Saturday and Sunday. A flash flood warning was in effect in Kentucky and Tennessee, according to the National Weather Service.