Illegal dumping of green waste and years of accumulated silt contributed to the overflow of Waikane Stream on Sunday night, which flooded Kamehameha Highway and blocked Windward Oahu traffic for hours, officials said

Waikane Stream is usually 3 to 4 feet deep, according to the U.S. Geological Survey. But at about 6 p.m. Sunday the water rose to between 6 and 10 feet — reaching a flood alert threshold and causing the stream to overflow its banks, officials said.

That level of flooding occurs at least every other year at Waikane Stream, said Ronald Rickman, USGS assistant director for hydrologic data in Hawaii. "It’s a big peak but it’s not unusually big," Rickman said.

Waikane area residents said upstream debris, including tree branches, contributed to the flooding.

Marinda Pacyau, whose 90-year-old family house is near the stream, said flooding-related road closures have become more frequent in recent years because of an increase in runoff at the Waikane Stream Bridge on Kamehameha Highway.

She said the stream is not as deep as it once was because of an accumulation of silt, and maintains that it needs to be dredged. "The mud sediment is above my knees, almost hip-high," Pacyau said.

State transportation spokesman Tim Sakahara said that about three days before the flooding, crews cleared the area of debris, including tree trunks and branches. But more debris came down later, he and residents said.

"A lot of debris appears to have come from dumping green waste upstream," Sakahara said. "There is a flood zone, so it’s susceptible to flooding."

Traffic was halted from 8:45 to 11:30 p.m. Sunday as water covered the highway, Honolulu police said.