|Location||Bergen County, New Jersey and Rockland County, New York, United States|
|Primary inflows||Hackensack River|
|Basin countries||Newark Basin|
|Surface area||1,255 acres (5.08 km2)|
|Water volume||3,500,000,000 US gal (13,000,000 m3)|
|Surface elevation||56 ft (17 m); |
55 ft (17 m): 222
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.: 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.
The Lake Tappan reservoir covers 1,255 acres (5.1 km²),: 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. The passage of this water between reservoirs can occur in as rapidly as two to three hours. Lake Tappan has an average depth of 21' with a maximum depth of 43'.
On March 11, 2003, New Jersey Governor Jim McGreevey visited the reservoir and proposed protecting it as well as the nearby Woodcliff Lake Reservoir and their tributaries with Category 1 water purity status.
The reservoir is owned by Suez North America, a private utility.
- U.S. Geological Survey Geographic Names Information System: Lake Tappan
- Leiby, Adrian C. (1969), The Hackensack Water Company, 1869-1969, In collaboration with Nancy Wichman, Bergen County Historical Society, ASIN B0006C01Q0, OCLC 13847.
- Knight, Robert (June 14, 2012). "June 26 Hearing on Pearl River Housing Project". Rockland County Times. Retrieved 9 January 2013.
- "Lake Tappan Vacation Info".
- "McGrevey seeks purity standards for reserviors". Archived from the original on 2016-03-03. Retrieved 2004-07-15.