Significant variations in the amount of rain dropped by the powerful storm system as it moved across Louisiana last week, differing by several inches of total rainfall at times even in the same parish, sent some rivers surging faster than others, said Jeff Graschel, a hydrologist with the National Weather Service’s Lower Mississippi River Forecast Center in Slidell on Sunday.

“Some (river) basins got hit by much more rainfall than others,” Graschel said. “The areas over around Covington” — where large swaths of St. Tammany Parish were inundated — “had much more rainfall than in the Amite and Comite basin.”

In Livingston Parish, Mark Harrell, the director for the Office of Emergency Preparedness, said he noticed spots just north and south of his office in Livingston that recorded significantly different levels of rain, with that disparity measuring as much as 4 inches.

“It was a strange storm,” Harrell said Sunday, adding that rainfall averaged roughly 12 inches in the parish.

“That’s a lot of water. I don’t remember ever getting that much rain at one time,” he said.

While massive flooding along the Tangipahoa River began slowly subsiding Sunday, officials in Ascension and Livingston parishes were preparing for flooding as the Amite River — which runs between the two parishes — continued rising Sunday toward major flood stage.

Differences in the landscape are playing a large role in determining when and how quickly a river crests, Graschel said.

Relatively flat and low-lying areas, like those around French Settlement on the Amite River in Livingston Parish, which is expected to flood, take longer to reach high water than stretches of river surrounded by steeper hills or with higher banks, Graschel said.

Winds also played a role in the flooding, he said,

Strong winds out of the south for several days before the heavy rains even started strengthened tides and raised water levels on Lake Ponchartrain, which eventually receives much of the water from the surging rivers in this part of south Louisiana, Graschel said.

“That has an effect on how quickly areas can drain and recede, especially as you get closer to the lake,” said Graschel.

Follow Bryn Stole on Twitter, @BrynStole.