Work began Tuesday on the biggest dam removal project in Maryland history.

The 220-foot-wide Bloede Dam in the Patapsco Valley State Park has blocked valuable migratory fish and been a death trap to swimmers since it was built in 1908.

The removal of the dam across the Patapsco River will take as many as 20 months and cost between $13 million and $18 million.

RELATED: History of floods in Ellicott City

Workers began closing a section of the popular Grist Mill trail in the park in preparation for the relocation of a major sewer line and to give access to the dam for demolition.

At least eight illegal swimmers have died near the dam since the 1980s. The last was in 2013.

A fish ladder passage added to the dam structure in 1992 proved ineffective.

The dam has blocked the migration of shad, herring and American eels, which migrate from the Atlantic ocean to reproduce in Maryland streams. An effort to restore the shad run in the river is underway. The dam removal will open 60-miles of additional habitat in the river and its tributaries to migratory species.

Bloede dam was the first hydroelectric project in the U.S. that had generators inside the structure under the level of the water being held back. The dam has not generated electricity since the 1920s.