— The rains started innocently enough — a refreshing break from a summer-long drought that had parched the landscape and sent residents of this water-loving city for relief, swimming and boating in its spectacular man-made lakes.

But what has happened is more insidious: The season has become one of the wettest autumns on record, causing five deaths in the Central Texas region and tainting an entire city’s water supply.

This city of one million people took the unprecedented step this week of asking residents to boil drinking water for three minutes to kill any bacteria, the culmination of a series of floods that have deposited large amounts of sediment from the soil, as well as oil and other pollutants into its water system, overwhelming its water-treatment plants.

The warning intensified Tuesday, when the contamination triggered a state-mandated boil-water notice.

Texas’s rainfall this autumn has been historic: Since Sept. 1, Central Texas has seen anywhere from 200 to 500 percent of the normal rainfall it receives, according to the National Weather Service. Austin has received 15 inches so far, marking the 18th wettest fall.

“Most of the state was in drought,” said meteorologist Brett Williams. “And that’s basically all been wiped out in a matter of several weeks.”

The floods led to the deaths of four people who were swept away from an RV park in Junction, a town about 140 miles west of Austin. A fifth person died in a low water crossing.

Texas Gov. Greg Abbott (R) has put the State Operations Center on heightened readiness as floods this month rampaged through the nearby Hill Country, washing away a bridge and ripping into homes.

The dramatic measures reflect the challenges to a city familiar with the historic hazards of sudden flooding that have been exacerbated by unusual weather patterns, creating a high-velocity torrent that swept across a vast rural region and kept sediment suspended as it raced toward the city.

“Because of changes in the climate, big floods have become more frequent,” said Raymond Slade, an Austin-based hydrologist.

Residents who in the past left water worries to engineers and other experts are now feeling the effects on their daily lives. On Wednesday, Austin opened seven water distribution centers for them. “We’re giving out water to people who can’t boil it or can’t afford any more bottled water,” said Bryce Bencivengo, a spokesman for the city. Bencivengo said most of the water was purchased, some was donated and some was provided by the state.

Wearing a neon yellow vest and holding a rainbow umbrella, Robert Aleman stood next to a blue signs bearing the words “WATER AGUA,” directing motorists to a distribution site in Southwest Austin, where city employees from a smattering of departments loaded cases of bottled water into vehicles.

Sabrina Lau Marquez, 37, came to pick up water with her mother. She is used to boiling water for formula for her 1-year-old son, she said, but she was grateful for the bottles as a backup. Originally from New York City, Marquez said she was impressed by the civility of her fellow residents.

“If this was New York, there would have been a riot already,” she said.

The distribution centers will be open until the boil-water advisory is lifted.

Help in addressing Austin’s water crisis has come from various places. A few breweries gave out water they had boiled to brew beer, and several businesses handed out free water, too.

Since Monday, the San Antonio Water System has supplied water for institutions including the Travis County Jail, the city’s animal shelter and the Emergency Operations Center, Bencivengo said. More bulk water from San Antonio and trucks from Fort Worth were expected to arrive soon.

Central Texans know their floods. Flash Flood Alley, as this area is sometimes called, is one of the most flood-prone regions of the country thanks to a combination of geography and geology. Heavy rainfall is created when moisture from the ocean meets the cooler mountain air of the Hill Country. The rain swooshes rapidly off the granite and limestone landscape, funneling into mounting torrents as it rushes downward.

Among the most memorable floods was in 1935, when Colorado’s swelling tributaries destroyed the bridge in Llano before splitting Austin in two and rushing, unhindered, toward the sea.

The response has been to create a massive chain of dams that form the Highland Lakes. They act as drinking-water reservoirs, provide a source of hydroelectric power and are part of a system of waterways that course like veins through Central Texas, supporting life and offering recreational opportunities. Lakefront property is a coveted escape from the summer heat.

Lady Bird Lake, which anchors downtown Austin, is often filled with paddleboarders, kayakers and rowing teams at practice. Just days after thousands filled Zilker Park for the annual Austin City Limits music festival, floodwater turned the lake into a turbulent vat of what resembled chocolate milk. Someone put an orange life vest on a statue of musician Stevie Ray Vaughan, his back to the debris-filled morass. The popular hike-and-bike trail was temporarily closed.

But above all, Austin’s lakes protect the city from the Colorado River by capturing water, which, according to John Hofmann, executive vice president of water at the Lower Colorado River Authority, is entering the upper end of the system at a rate of “250 Olympic swimming pools of water every minute.”

That astounding feat 0f engineering has succeeded despite mounting challenges in what the Texas Demographic Center calls one of the fastest growing regions in the United States.

“It only becomes a flood when there are people nearby” said Don Riley, former deputy commander of the Army Corps of Engineers, noting that the excess water often goes unnoticed when it washes over farmland.

The floods have already required some rescues. On Oct. 8, Jamie McDonald — who is running across the U.S. to raise $1 million for charity under the name “Adventureman” — awoke in his hotel room about 40 miles outside Austin to “loads of shouting and screaming,” the Briton said in a phone interview.

Outside on the balcony, a crowd had formed, yelling encouragement to a naked man holding onto a tree limb in the water. Soon, a helicopter came and swooped him up, depositing him on solid ground.

“It was spectacular to see how they dealt with it,” said McDonald. “It definitely put a little bit of a fear factor in me.”

That was when the water started to rise — first 10 feet one day, then 24 feet another, and finally 39 feet, said Briley Mitchell, executive director of the Llano Chamber of Commerce.

“The ground was just saturated,” he said. “That just created the perfect storm.”

Moravec reported from Austin. Sellers reported from Washington.