Here's what to know as the scope of damage from Hawaii's floods becomes clearer

HONOLULU (AP) — The worst flooding to hit Hawaii in two decades swept homes off their foundations, floated cars out of driveways and left floors, walls and counters covered in thick, reddish volcanic mud.

Crews continued to assess the destruction Monday, but authorities said hundreds of homes had been damaged, along with some schools and a hospital.

No deaths have been reported, but more than 230 people had to be rescued.

Here's what to know as residents begin to clean up.

This was Hawaii's worst flooding since 2004

Gov. Josh Green said the cost of the storm could top $1 billion, including damage to airports, schools, roads, homes and a Maui hospital in Kula. He called it the state’s most serious since flooding since 2004, when floods in Manoa inundated homes and a University of Hawaii library.

On Oahu's North Shore, famed for big wave surfing, the waters rose quickly after midnight Friday as heavy rains fell on soil already saturated by downpours from a winter storm a week earlier. Raging waters lifted homes and cars. The storm prompted evaluation orders for 5,500 people north of Honolulu — though they were later lifted — and more than 230 people were rescued from the rising waters.

Some residents fled on surfboards as water reached waist or chest high.

Winter storm systems brought severe deluges

Officials blamed some of the devastation on the sheer amount of rain that fell in a short amount of time.

Parts of Oahu received 8 to 12 inches (20 to 30 centimeters), the National Weather Service said. That was on top of another recent storm that had dumped vast amounts days earlier. Kaala, the island’s highest peak, got nearly 16 inches (40 centimeters) late last week, on top of 26.6 inches (67.6 centimeters) between March 10 and 16.

Winter storm systems known as “Kona lows,” which feature southerly or southwesterly winds that bring in moisture-laden air, have been responsible for the deluges in the past two weeks. The intensity and frequency of heavy rains in Hawaii have increased amid human-caused global warming, experts say.

The worst of the storms appeared to be over by Sunday afternoon.

Volunteers show up to clean debris and thick, red mud

Crews are still assessing the extent of the damage, but by Monday Oahu's Department of Emergency Management had received more than 400 reports of damaged or destroyed homes, spokesperson Molly Pierce said.

In some neighborhoods, residents have had to tramp through silt up to their knees or deeper to reach their properties, she said.

Along with its consistency, the rusty hue of the mud, which comes from oxidized iron in the volcanic soil, makes it a nightmare to clean up.

"When it's dry, the dust stains, and when it's wet, the mud clings," Pierce said Monday. She compared it to heavy, wet snow.

Volunteers have been showing up from across Oahu and even from other islands to help clear away muck and debris, Pierce said.

Residents and officials fret over an aging dam

As the waters rose Friday, officials warned that the 120-year-old Wahiawa dam, north of Honolulu, was “at risk of imminent failure.” The dam has long been vulnerable, but worries eased as the water subsided.

The earthen structure was built in 1906 to increase sugar production for the Waialua Agricultural Co., which eventually became a subsidiary of Dole Food Co. It was reconstructed following a collapse in 1921.

The state has said Wahiawa dam has “high hazard potential” and a failure “will result in probable loss of human life.”

It has sent Dole four notices of deficiency about the dam since 2009, and five years ago it fined the company $20,000 for failing to address safety deficiencies on time, according to records.

Afterward, Dole proposed to donate the dam, reservoir and ditch system to the state in exchange for an agreement to repair the spillway to meet and maintain dam safety standards.

“The dam continues to operate as designed with no indications of damage,” Dole said in a statement.

How to help

Green on Sunday recommended that those wanting to support impacted households donate to the Hawaii Community Foundation, which has activated its Stronger Hawaii Fund to deploy resources across all affected islands.

The nonprofit Hawaiian Council also launched the Kako’o Oahu initiative to help individuals and families through housing and financial assistance as well as other support. The council matched the first $100,000 in donations and is continuing to fundraise.

“So many families have been displaced, homes have been damaged, and communities are facing immediate needs,” said Hawaiian Council CEO Kuhio Lewis. “The path to recovery begins now — it cannot wait.”

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Johnson reported from Seattle. Associated Press writer Gabriela Aoun Angueira contributed to this report from San Diego.