There were at least 3 proposals for Lake Hackensack. I believe the first one dated to the 1920's or 1930's. The County purchased two properties for the project. Both sat as County-owned for decades, but not officially as parkland. One is now a County-owned preserved woodland along the river in New Milford, and the other is now Hackensack River County Park in Hackensack. The latter got Green Acres funding for improvements around 1990, and soon after officially became a County Park.
The proposal included walking and bicycle trails up and down both sides of the river. These were scaled back in the later versions due to development along the river, esp. in New Milford.
My recollection is that it was determined that the proposed lake would NOT BE SWIMMABLE, due to the amount of urban runoff entering into it, and that was as big a deal-killer as the potential environmental damage south of the proposed dam at the Susquehanna railroad bridge.
I have maps and documents somewhere in my files. I was involved for years around 1991 to 1994 with the Hackensack River Pathway Committee. We had a committee that met monthly with active representatives from 7 towns. The County committed to the project and that backed off when one town, Oradell, decided they didn't want the pathway on County land behind people's yards.