Lake Tappan is a reservoir impounded by the Tappan Dam on the Hackensack River, straddling the border between the U.S. states of New Jersey and New York. 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 Lake Tappan reservoir covers 5.1 km2, the majority of its area being within New Jersey, and it contains 13,000,000 m3 of water, with up to 45,000 to 49,000 m3 released downstream daily into the Oradell Reservoir, which lies entirely within Bergen County in New Jersey.
