NEWTON, Tex. — For the streets of Newton, a small town on the Texas side of the Louisiana state line, to become impassable, “the flood would have to be biblical,” Kristen Rogers was told when she peeked into the sheriff’s office looking for guidance.

“That’s what they said about Houston,” replied Ms. Rogers, who was looking for a dry way out of rural Texas on her way to Florida.

But as Houston, the urban behemoth that has so far been the focal point in the unfolding drama of Hurricane Harvey, began gingerly to assess the devastation, the storm marched on to conquer a vast new swath speckled with small towns that are home to millions of people who were shocked anew by Harvey’s tenaciously destructive power. Officials faced a population in dire need, but far more difficult to reach.

Flooding and rain, topping 47 inches in some areas, pounded 50 counties in southeast and lower central Texas with a combined population of roughly 11 million people. The area includes more than 300 towns and smaller cities that felt the storm’s punishing force, even as Harvey was downgraded to a tropical depression on Wednesday.

The Federal Emergency Management Agency began to send out heavy-lift military helicopters carrying tons of food and drinking water, delivering it to people who could not evacuate.

Gov. Greg Abbott of Texas said officials were “immediately deploying far more” members of the National Guard to southeast Texas, increasing the total Guard deployment to 24,000, including 10,000 troops from other states.

In contrast to Houston, where the weather began to clear and a few children even returned to playgrounds, many people in these remote areas are still in desperate need of rescue. “There are a lot of places that are not accessible by car or truck or boat, and we need to get to the survivors to get them critical aid,” said Deanna Fraser, a FEMA spokeswoman.