HOUSTON — One of two major flood-control reservoirs in the Houston area began spilling over for the first time in history, despite efforts to prevent such “uncontrolled” overflow the day before, officials said.

The U.S. Army Corps of Engineers confirmed Tuesday morning that water was spilling from the north end of the Addicks Reservoir, which has been overwhelmed by extreme rainfall from Hurricane Harvey. Officials said they expect the Barker Reservoir, to the south of Addicks, to begin overflowing similarly at some point Wednesday.

A Harris County Flood Control District meteorologist said the overflow from the reservoirs would eventually flow into downtown Houston.

The reservoirs, which flank Interstate 10 on the west side of Houston, feed into the Buffalo Bayou and are surrounded by parks and residential areas. Water levels in the two reservoirs had already reached record levels Monday evening, measuring 105 feet at Addicks and 99 feet at Barker.

Engineers were unable to measure water levels at the Barker Reservoir on Tuesday because its gauge was flooded overnight, said Jeff Lindner, the Harris County flood control meteorologist.

The overflow did not represent a “failure” of the dam, stressed Richard K. Long, a natural resource management specialist with the Army Corps of Engineers.

“These are not your typical dams; these are unique because of the type of terrain we have,” Long said, referring to Houston’s relatively flat plain. The Addicks and Barker reservoirs each have a main spillway and two auxiliary spillways. Water hadn’t breached any of those spillways, but instead was overflowing through a slightly lower point on the north end of the Addicks Reservoir.

“But again, we don’t know what Mother Nature’s going to give us,” Long told The Post on Tuesday.

And either way, he added, “it’s going to take quite a while for us to get rid of all this water.”

Hurricane Harvey struck Southeast Texas as a Category 4 storm Aug. 25. Texans now face catastrophic flooding, which is expected to worsen. (Elyse Samuels, Zoeann Murphy, Whitney Leaming, Kurt Kuykendall/The Washington Post)

Officials had hoped to prevent just such a spillover by releasing water — slowly, at first — from both the Addicks and Barker dams, starting early Monday morning. Water levels in both reservoirs had “increased dramatically” late Sunday night, rising more than half a foot per hour, leaving engineers with two choices: to begin releasing water through the dam gates earlier than expected — or risk it spilling out suddenly around the ends of the dams.

“If we don’t begin releasing now, the volume of uncontrolled water around the dams will be higher and have a greater impact on the surrounding communities,” Col. Lars Zetterstrom, the U.S. Army Corps of Engineers Galveston District commander, said in a statement about 2:30 a.m. Monday. “ … It’s going to be better to release the water through the gates directly into Buffalo Bayou as opposed to letting it go around the end and through additional neighborhoods and ultimately into the bayou.”

The Corps planned Monday to release water at 4,000 cubic feet per second from each reservoir over a six- to 10-hour period. Officials said thousands of homes along the reservoirs could be affected, and by midmorning Monday, streets and houses in some surrounding neighborhoods had already begun flooding. (A list of subdivisions adjacent to the reservoirs is available here.)

Officials had originally predicted the storm water levels in both reservoirs would not threaten nongovernment land until early Monday (for the area around the Addicks Reservoir) and Wednesday (for the Barker Reservoir).

Because of that, the release from Addicks Reservoir had been slated to start at 2 a.m. Monday, and from Barker Reservoir a day later.

Late Sunday night, local officials issued voluntary evacuation notices for residents around the reservoirs — but urged them to wait until daylight to leave the area, if they chose to do so. About 2:30 a.m. Monday, however, the rapidly rising water levels prompted the Army Corps to begin releasing water from both reservoirs.

A few hours later, residents in adjacent neighborhoods — including Canyon Gate at Cinco Ranch to the southwest of Barker Reservoir and Bear Creek Village near Addicks Reservoir — were reporting rising water levels in their streets and approaching their homes.

On Tuesday afternoon, helicopters and boats roamed the upscale subdivisions southwest of the Barker Reservoir, attempting rescues of families.

Jason Mckey, who lives in the Canyon Gate neighborhood and works to restore wetlands so there won’t be flooding, had spent morning rescuing neighbors in his black duck-hunting mud boat as the controlled releases from the Barker Reservoir affected the area.

“It’s mostly elderly people in just horrible way,” Mckey said as he put on his duck-hunting waders.

People in the Lakemont neighborhood, just south of Canyon Gate, were streaming out of their homes, huddled under umbrellas near a Walgreens. Fountains in the neighborhood’s man-made lakes were still running strong and had started to overflow.

“We never flood,” said Gloria Strayhorn, a retired interior designer who was out for a walk her husband. They were in raincoats and shook their heads at an overflowing lake, suggesting the man-made bodies of water were only adding to the problem.

“They were just refurbished this spring and they spent so much money setting up benches — and now look,” Strayhorn said.

Much of the relatively new development is built on former rice farmland and cow pastures, the Strayhorns said. Wetlands are normally a natural drainage system, but new subdivisions, with all their pavement, have left the water with nowhere to go.

“We couldn’t sleep when we heard the reservoirs would be released,” said Jessica Wang, 34, who lives in a neighborhood called Grand Ridge Crossing in Katy, just southwest of the Barker Reservoir.

Wang said her family had taken in another family from a flood-prone area — resulting in eight people under one roof — because they thought their area wouldn’t flood. Now, on Tuesday, they were watching rescue missions from the road and said that homes nearby had water up to their chests.

“It’s hard to say what’s gonna happen now,” Wang said. “Every time I say, ‘No, it won’t happen,’ it happens.”

To complicate any evacuation efforts, several major roadways that run through both reservoirs are underwater, including portions of State Highway 6, Barker-Cypress Road, Clay Road and Westheimer Parkway. Officials expect those routes to remain impassable for “several weeks to several months.”

The Addicks and Barker dams and reservoirs were both authorized by the Rivers and Harbors Act of 1938 and completed in the 1940s to prevent the flooding of downtown Houston and the Houston Ship Channel. The U.S. Army Corps of Engineers owns and operates the reservoirs.

Addicks Dam is more than 11 miles long, while Barker Dam is more than 13 miles long.

The Harris County Flood Control District noted the structures “have protected greater Houston area residents against loss of life and property over the last 70 years” and that there were no signs of structural issues with the dams.

At a news conference Monday morning, Lindner, the Harris County Flood Control District meteorologist, sought to clarify why neighborhoods to the west of the reservoirs were being affected by rising water levels, if the dams were releasing into the Buffalo Bayou to the east. There was, he said, simply a higher amount of water flowing into the reservoirs from its feeder creeks in Waller, Fort Bend and western Harris counties.

“You have a dam around the front of the reservoir, and the back of the reservoir acts as a bathtub,” he said. “So as the pool rises, it will back up to the west, like the water would back up to your bathtub … How far west it goes will be determined by the coordination with the Corps of Engineers and releases of the water managing those upstream inflows.”

Lindner added: “Of course, the wild card to all this is additional rainfall.”

Record rainfall levels have drenched Houston over the past four days. August is now the wettest month of all time for the city, surpassing June 2001, when Tropical Storm Allison battered the area, according to the National Weather Service. A flash flood watch for the Houston area is in effect through Wednesday, the Weather Service said.

A Periscope video by storm chaser Jeff Piotrowski about 8:30 a.m. Monday showed water reaching about the midpoint of the Baker Reservoir, leaving only the tops of trees visible.

Capital Weather Gang's Jason Samenow shares what's next for the battered Texas coast and tracks Harvey's path toward Southwestern Louisiana. (Claritza Jimenez, Jason Samenow/The Washington Post)

Wang reported from Washington. This post has been updated.

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