How surveyors track high water marks throughout Maryland after flooding — and what that data is used for

Karl Winters traipses through the woods lining the banks of the Little Patuxent River in Savage, looking for subtle signs that prove the river reached a height of more than 16 feet Sunday, spilling over its banks during a storm that caused flooding throughout the region.

Treading softly in his hiking boots, Winters winds through the thickets with his eyes peeled for lines of seeds on a tree trunk, or piles of debris deposited far from the water’s edge.

“Haven’t found anything yet that’s worthy of writing home to mama about,” said Winters, a hydrologist for the U.S. Geological Survey.

Straying further from the river, he finds the evidence he’s looking for: small seeds clinging to the trunk of a bush. He flags the shrub with neon pink tape, and repeats the process at several spots along a 500-foot stretch of the river where he finds other markers.

Armed with hip-waders, hammers and a bucket of stakes, two other USGS surveyors, Michael Geissel and Shane Mizelle, take the other side of the river. Later the team will return with survey tools to chart the watermarks’ elevations.

The data will help the USGS compute the volume and speed of water flowing through the river — a measurement known as flood peak discharge — during Sunday’s storms that dumped nearly 10 inches of rain on parts of Maryland and ravaged historic Ellicott City. The measurement will bolster statistics that help predict the chances flooding like Sunday’s will occur in the future.

Over time, the data can show whether an area’s hydrology — how water moves on land — is changing. Combined with information about development, impervious surfaces and retention pools, those variables could help determine how flooding will play out in areas like Howard County in the years ahead.

“Those go all in the mix and we try to use that stuff to kind of predict if you’ve got certain development within a basin, how might that either exacerbate flooding or mitigate possible flooding,” Winters said.

As Winters and his crew surveyed parts of the Little Patuxent in Savage, similar surveys played out at other points across the region Wednesday, including in Ellicott City. There, the USGS identified high watermarks at the Hudson Branch, the Tiber Branch and the New Cut Branch — streams that converge in the Patapsco River at the base of Main Street.

“We had pretty good luck at all three,” said Jonathan Dillow, a supervisory hydrologist for USGS. All the sites had higher marks than after the 2016 flood in Ellicott City, he said. Once surveys are complete, the USGS will calculate the flood peak discharge at each site in the coming weeks. The figures will help shore up existing USGS data that shows the relationship between the height of rivers like the Little Patuxent and the Potomac and the volume of water flowing through them when the water runs higher than usual.

“It’s not going to open new horizons, particularly for people, but it’s going to verify or improve the quality of the data USGS is already providing,” said Dillow, data chief for the USGS Maryland-Delaware-D.C.Water Science Center.

USGS teams are working to survey eight to 10 more sites in the area, including spots in White Marsh, Baltimore and Columbia, before more rain falls on the region Thursday, Dillow said.

“It’s super important that we get out there right after the flood because a little bit of wind, a little bit of rain and all that evidence goes away in short order,” Winters said.

Dillow’s office has about 90 stations throughout Maryland that track the heights of rivers and streams around the clock via satellite. But after big storms like the one Sunday, surveyors go out in person to confirm that the numbers captured by sensors are accurate.

The high water in the Little Patuxent on Sunday wasn’t unprecedented, data from the USGS shows.

“It’s been up that high three or four times in the least 15 years or so,” Winters said. “What does that mean? We don’t know, we can’t say, and that’s really why we continue to collect the data.”

In Ellicott City, the data could play into decisions about city planning and flood mitigation.

“I hope and believe that it will be germane to the management and planning discussions that were already going on,“ Dilllow said

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