WESTBROOK — Piece by piece, the Presumpscot Regional Land Trust is trying to fashion together a corridor of protection for a small brook that does a big job.

The Mill Brook Preserve received another 30-acre parcel that was donated by two landowners, adding to the 70 acres the land trust already had protected along the brook in Westbrook. The land trust began protecting the land along Mill Brook two years ago so that it could keep the brook clear of debris to help alewife migration runs. The wooded preserve that now has access from a road, thanks to the new acquisition, also will be a spot to explore woods trails.

Mill Brook runs from Highland Lake in Westbrook into the Presumpscot River, site of a significant alewife spawning run. While the alewife runs in the Presumpscot River are small compared to others in the Gulf of Maine, the Presumpscot River run is the largest in Casco Bay, said Karen Wilson, an environmental science professor at the University of Southern Maine who studies the alewife runs.

Alewives are an ocean fish that migrate up rivers to spawn in freshwater lakes. It is an important sea-run bait fish because so many species of fish and wildlife feed on it. The spawning runs in the Presumpscot River occur in the spring and again in the fall, and when they do, the thousands of fish can be seen piled up, pushing up or down the rivers.

It makes for an exciting viewing opportunity.

“One of the things that is very exciting for us, is the chance to educate the public about this brook now. There was no public access (to Mill Brook) before this parcel,” said Rachelle Curran Apse, the land trust’s executive director.

The parcel donated two weeks ago was given by landowners Ralph and Marilyn Hatt. Ralph Hatt had lived beside the swath of forestland for most of his 72 years. So when it came time to consider how to best pass on the land, Hatt felt the only reasonable choice was to protect it.

The first seven acres of the parcel belonged to Ralph Hatt’s grandparents, who used it as a summer retreat back when Greater Portland was sprawling farmland. Hatt began acquiring another 23 acres 40 years ago.

The land across from the Hatt house remains pristine and wild, with tall pines and deep ravines. An old town road that was abandoned in 1877 still runs through this cathedral of trees.

The steep slopes drop down to stream banks littered with glacial erratics. The five small brooks here are criss-crossed with sheets of rock, acting like nature’s viewing platforms.

Here, Hatt still hikes daily with his sturdy walking stick, flicking branches off the trail with his pole as he goes, making it obvious he loves this land.

“We used to swim down by the river here when I was a boy. I’ve watched the alewives runs here now for years,” Hatt said.

In the ravines, the only sounds are of running brooks, waterfalls, birds and branches.

“This will be the first major trail network in Westbrook that doesn’t go along a road,” Apse said.

The effort by the land trust to protect the land along Mill Brook only began two years ago after it was identified as an important part of the Presumpscot watershed.

“It’s important ecologically because these are native fish that were once very abundant in the watershed. They are part of the ecological story of our coastline,” said Wilson, the University of Southern Maine professor.

Now with public access along Mill Brook, the land trust can help keep the brook clean of debris, which is particularly important when beavers build dams. Keeping it clear will aid the alewives runs, Wilson said.

Fish counts directed by USM that have taken place at Highland Lake the past several years have counted as many as 50,000 migrating alewives and as few as 6,000, Wilson said. But she said the poor runs are not necessarily an indication of a drop in population, but more likely because of blocked passage in certain years.

Wilson said much of the undeveloped land along Mill Brook is teeming with wildlife, including beavers. But the 100-acre corridor owned by the land trust at the moment is all the protected land, Wilson said.

“We had game cameras there last summer. There was a parade of wildlife. A lot of animals are eating the alewife in the brook,” Wilson said.

The public can view the alewife runs up Mill Brook on May 21 when the Presumpscot Regional Land Trust leads a walk on the newly protected parcel.