Swollen rivers continued rising in Southeast Texas on Saturday after a night of evacuations and rescues from floodwaters that swamped roads, stranded cars and inundated homes in the region.

Emergency responders in airboats searched the flooded streets and subdivisions around Houston, rescuing 178 people and 122 pets from stranded cars and rooftops by Saturday afternoon, according to Judge Lina Hidalgo, Harris County’s top executive.

Officials underscored the urgency of evacuation orders for residents in low-lying areas, warning that the worst was still to come.

“This threat is ongoing and it’s going to get worse,” Ms. Hidalgo said. “It is not your typical river flood.”

A sparsely populated section of northeastern Harris County along the east fork of the San Jacinto River had crested on Saturday at 77.8 feet above sea level, or more than 27 feet above normal, Ms. Hidalgo said. On Thursday, she had issued evacuation orders for about 5,000 people living in that part of the county.

Along the west fork, where a voluntary evacuation was underway, the river had not yet crested, Ms. Hidalgo said at a news conference on Saturday.

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