Lake Tappan was formed by a dam placed on the Hackensack River in 1967. It straddles the border between River Vale and Old Tappan, New Jersey. It extends north past the New York state border and into Orangetown, New York.

The Lake Tappan reservoir covers 1,255 acres (5.1 km²) and contains 3.5 billion gallons (13,000,000 m³) of water, with up to 12 million to 13 million US gallons (45,000 to 50,000 m³) released per day down stream to the Oradell Reservoir. Travel time for the passage of water can be as little as 2 to 3 hours.

On March 11, 2003 New Jersey Governor Jim McGreevey visited the reservoir and proposed protecting it, Woodcliff Lake (a neighboring reservoir) and their tributaries with Category 1 water purity status. [1]

The reservoir is owned by United Water , a private utility.