North Park Lake will become a muddy hole by the end of the year, Allegheny County and federal officials say.

"Drawing down," or emptying, the lake is the first step in restoring the silt-choked body of water to something close to its original condition.

Draining of the lake will begin this month. Officials hope the work will be completed and the lake refilled no later than Memorial Day 2011.

Plans include relocating fish as the lake level drops.

Long a popular spot for fishing and boating, North Park Lake was created in 1935 when the county built a dam to hold water from Pine Creek and the North Fork of Pine Creek.

Over the decades, the man-made lake has lost half its depth as sediment carried from upstream has settled on the bottom. In addition, the surface area of the horseshoe-shaped lake has shrunk from 75 acres to as few as 60 acres. The lake is one of the main attractions of 3,000-acre North Park, which covers portions of Hampton, Pine and McCandless.

Allegheny County and the U.S. Army Corps of Engineers have agreed on a two-phase plan to remove sediment from what soon will become the exposed lake bottom and truck it to a disposal area near Wildwood Road.

The corps soon will select a contractor to undertake work in the northern arm of the lake, which is fed by the North Fork of Pine Creek. Federal funds will cover 65 percent of the $7.7 million cost of what is called an "aquatic ecosystem restoration."

That project will cover about 33 acres of lake bottom. It calls for removal of about 104,000 cubic yards of sediment, according to project manager Kathleen Anderson, of the Army Corps of Engineers Pittsburgh District.

While that work is under way, the county plans to hire a separate contractor for the $4.5 million second phase of the project. It calls for removing material from the southern arm of the lake, which is fed by Pine Creek.

Both dredging crews will work across the lake bottom, moving toward the dam at the eastern end of the lake. Babcock Boulevard runs across the top of the dam.

The lake restoration project also includes a $3.38 million effort to prepare a storage site near Wildwood Road for the sediment, according to Steve Smallhoover, project manager for Allegheny County's Department of Public Works.

The combination of emptying the lake and dredging the bottom offers the most cost-effective way to carry out a large-scale project, Ms. Anderson and Mr. Smallhoover said.

Other elements of the project are designed to protect adjoining wetlands, which will help keep new sediment from reaching the lake, and to improve water quality and habitats for fish and other aquatic life, officials said.

As North Park Lake has grown more shallow, water lilies and a non-native plant called Eurasian water milfoil have been flourishing, reducing fish habitats and making boating difficult. Draining the lake will make it easier to eliminate excess plants and keep them under control, officials said.

The restored lake also will have new habitats and nesting areas for more species of fish, Ms. Anderson said.

"It will be muddy when the water level is brought down, but it will dry out," she said. "And there will be some unwelcome smells associated with that."

Those smells and potential environmental damage have outdoors organizations concerned. "If they drain the whole lake, I think it is really going to smell bad," said Jim Rossmiller, president of the Allison Park Sportsmen's Club. He also said he was worried about excessive fish kills.

The state Fish and Boat Commission will seek public help to rescue aquatic life, according to the Army Corps of Engineers Web site.

Plans to restore North Park Lake date back almost 30 years. Aided by a federal grant from the Environmental Protection Agency, Allegheny County began underwater dredging efforts in 1980, removing a slurry of sediment and water from the lake. Nine years later, the county had spent almost $900,000 and removed only 31,000 cubic yards of silt.

The county ended the effort when officials realized that the material was collecting faster than dredges could remove it.

At the request of the county, the Army Corps of Engineers began planning a new effort to tackle the sediment problem in 1999.