[2]: 221–223  Within New Jersey, the lake traverses the border separating the municipalities of River Vale and Old Tappan in Bergen County, while extending northward across the New York state line into the town of Orangetown in Rockand County.

The reservoir was formed in 1967, flooding areas that included the former CAPROC Field Civil Air Patrol airstrip.

[3] The Lake Tappan reservoir covers 1,255 acres (5.1 km2),[2]: 222  the majority of its area being within New Jersey, and it contains 3.5 billion US gallons (13,000,000 m3) of water, with up to 12 to 13 million US gallons (45,000 to 49,000 m3) released downstream daily into the Oradell Reservoir, which lies entirely within Bergen County in New Jersey.

Lake Tappan is a local fishing spot as well, serving as a habitat for bluegill, bass, perch, catfish and carp.

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