STANWOOD — More fish, birds, native plants and animals are expected to thrive in Port Susan following the removal of a dike along the bay to restore part of the Stillaguamish River estuary.

The Nature Conservancy, an environmental group that owns more than 4,000 acres of tidelands in Port Susan, has begun work on a project to remove an earthen dike that seals off the bay from 150 acres of former estuary.

The second part of the project involves rebuilding another dike to the east to serve as the new boundary between the bay and adjoining farmland.

The $4 million project, being done for the Nature Conservancy by Northwest Construction of Bellevue, is expected to be finished in October. The money is coming from state and federal environmental programs and private donations.

The area just north of the mouth of the main stem of the Stillaguamish River, also called Hatt Slough, has been shut off from tidal influence for more than 50 years, said Kat Morgan, who manages the Port Susan program for the Nature Conservancy.

Not only does the 13-foot-tall earthen dike isolate the 150 acres from salt water, but it forces the fresh water and nutrients from the river southward in the bay rather than allowing them to spread out, she said.

As a result, the northern part of Port Susan is starved of the fresh water it needs to provide the right mix for native plants and animals.

“It is degrading the habitat,” Morgan said of the outer dike.

Reinforcement of the current inner dike also will help in flood protection, Nature Conservancy officials said. Several local groups, such as the Stillaguamish Flood Control District and Snohomish Conservation District, along with larger government agencies, were consulted on the plan, Nature Conservancy spokeswoman Robin Stanton said.

The group also has worked with the flood control district to install a tidegate to help with flood relief for farms on nearby Florence Island, Stanton said.

The 1.4 mile-long outer dike near the river mouth was built in the late 1950s by farmer Menno Groeneveld, the son of a Dutch immigrant, according to the Nature Conservancy. He believed if he could keep out the salt water, he could farm the land inside the dike.

It proved to be a difficult proposition. Not only did salt water have a way of creeping into the diked-protected area, but rainwater had a tendency to collect there as well. Groeneveld set up an electrically powered pump that he had to keep running almost constantly to keep the water out, Morgan said. The only crop he succeeded in growing was a salt-tolerant type of bent grass, she said.

The Nature Conservancy began negotiating with Groeneveld in 1990 to buy his property, which included the 4,000-plus acres of tidelands. He died before the negotiations were completed, but the environmental group eventually bought the property from his estate for $2 million in 2001.

The group then studied the possibility of removing the outer dike, Morgan said.

“The research was pretty clear the dike was having an effect on the entire ecosystem,” she said.

Healthy estuaries have a variety of life forms, sometimes changing in just a few inches difference in elevation, Morgan said. This habitat needs a salt water-fresh water mix to support the variety of plant life that juvenile salmon and other sea creatures need to thrive, she said.

For example, it provides the young salmon with cool, deep channels where they can hide and slowly transition to the saltwater environment. Invertebrates in the mud flats provide food for flocks of ducks, terns and other shorebirds.

Also, migratory birds such as sandpipers, yellowlegs and dowitchers stop off at Port Susan, Morgan said.

The area is a popular site for birders. The Nature Conservancy has opened the outer dike to the public on a by-appointment basis, and will do so again after the project is finished, but it is closed now and will remain so during the work, Morgan said.

The group plans to dike off a small 10-acre area — where Groeneveld’s now-dilapidated house still sits — as a vantage point.

Dirt will be skimmed off the top of the outer dike and used to start building up the inner one.

The inner dike, which varies in height between eight and 12 feet, will be built to 14 feet, said Jenny Baker, a restoration manager for the conservancy.

When the inner dike has been sufficiently rebuilt to keep water out, the outer one will be breached and removed.

It could take five years afterward for the habitat to restore itself, Morgan said.

“It will be really interesting to watch the whole site change,” she said.

Bill Sheets: 425-339-3439; sheets@heraldnet.com.