Twice a day, high tides now flood a 50-acre wetland that had been cut off from the Columbia River estuary system.
Nature and workers breached one of the dikes that kept tidal water out of most of South Clatsop Slough.
After a 100-year absence, tidal water has returned to South Clatsop Slough at Lewis and Clark National Historical Park.
The work - which began Aug. 22 - is nearly complete.
Scott Stonum, the acting superintendent at the park, said the parcel is small compared to the entire estuary system.
More than 70 percent of estuary marshes have been diked and drained - or "reclaimed" - for farming or other human uses.
The national park is managing the lands, which have been used for different purposes in the past. Stonum said the dike project exceeded the capabilities of the park service, so it reached out to the Astoria-based Columbia River Estuary Study Taskforce - which had experience acquiring the necessary grants and funding for the project.
"A hundred years ago, men reclaimed the land for pasture: Now we're reclaiming it for the estuary," said Allan Whiting, the CREST program manager for the South Clatsop Slough.
He said states, nongovernmental organizations, the private sector and Bonneville Power Administration all played important roles in funding the project.
"This is, in part, to offset some of the impacts of dams," Whiting said. "Bonneville Power is a big player in estuaries these days."
The parcel was a pasture for a farmer's cattle, with reed canary grass and chamomile covering the ground. Stagnant water was a breeding pool for mosquitoes.
The first step for the reclamation project was to remove cattle from the pasture, which had been drained for agriculture 100 years ago. Within a week of removing the cattle, park employees saw Roosevelt elk from two nearby herds grazing the property.
When a tide gate broke away from the dike during a storm last year, the park service chose not to replace it. The project was almost under way, and the gate would eventually have been removed. Since that time, tides have flooded the pasture twice a day.
Large channels - hidden by the tall grasses for 100 years - allow tidal water to rush in and out of the reclaimed wetland.
"The daily influx of tidal water brings in nutrients and seeds: Every day the tide comes in, it will float in seeds from the rest of the estuary," Stonum said.
Park service personnel inspect the slough every day, looking for plants they don't want to see.
The bright-green reed canary grass - which had been thriving and suppressing the native vegetation - doesn't grow in salty water and is beginning to die, opening large channels that kayakers may soon enjoy.
Plant and animal species, which haven't been seen in the slough for 100 years, are showing up.
Sedges, soft-stem bull rushes and bunch grass - emergent estuary and marsh plants - are growing.
Northern red-legged frogs hop through the bunch grasses.
Saltwater tides flush out the estuary twice a day, leaving no standing mosquito pools.
Fort Clatsop Road acted as a dike for the pasture over the past 100 years. Sixty years ago, when the road was moved and the dike rebuilt in accordance with the Flood Control Act of 1936, the pasture between the old and new dikes was left to return to its natural state.
If a person were to stand in the new road, on the west side - where the re-emerging slough is - they'd see a quiet grassland.
But on the other side they'd see and hear a thriving wetland with buzzing insects, croaking frogs and chirping birds. Asters line the cattail-covered marshland. Dragonflies and purple martins zip through the air.
The east side of the road has returned to marshland. It supports native insects, birds, mammals and fish. Salmonids use its reeds for cover as they feed and grow in the nutrient-rich water.
Whiting said estuaries produce more carbon - biologic energy and mass - than almost any other habitat in the world.
South Clatsop Slough has already begun to change from a macrodetritus system - a system with large organic matter, like leaves that provide few nutrients to the water and suppress plant and animal growth. It is becoming a microdetritus system - which infuses the water with tiny organic materials, suspended in the water.
"This is putting the 'micro' back in," Whiting said.