Why the Ramapo River floods: Mahwah, Oakland, Pompton Lakes and Wayne
The Ramapo comes out of New York, runs the length of a narrow valley through Mahwah and Oakland, and ends in a lake held back by a dam in Pompton Lakes. Each of those three stretches floods in its own way, and the gauges record all three.
The Ramapo River enters New Jersey from New York at Mahwah and flows south through Oakland into Pompton Lake, whose dam sits in Pompton Lakes; below it, the Ramapo meets the Pequannock to form the Pompton. The National Weather Service counted 106 floods at the Mahwah gauge between 1903 and 2018, and Hurricane Irene set new records at both Mahwah and Pompton Lakes in August 2011. This guide covers the gauges, what each flood level means on named streets, the floodgates added to Pompton Lake Dam in 2007, and what that history means for people who live by the river.
The short version
- The NWS counted 106 floods at the Ramapo gauge near Mahwah between 1903 and 2018, about one a year.
- Irene set records at Mahwah, 15.77 feet, more than two feet above the previous record, and at Pompton Lakes, the highest peak in more than 90 years of record.
- Major flood stage is 12 feet at Mahwah, 17 feet at Oakland and 14.5 feet below the dam at Pompton Lakes.
- The Pompton Lake Dam floodgates, finished in 2007, protect about 300 homes upstream in Oakland and are not designed to reduce flooding downstream.
- FEMA records 2,535 flood claims in Pompton Lakes, with 463 from Irene alone.
From New York to Pompton Lake
The National Weather Service keeps forecast points on the Ramapo at Ramapo and at Suffern in New York before the river crosses into Bergen County. The US Geological Survey gauge near Mahwah drains 120 square miles. Downstream, the gauge at West Oakland Avenue in Oakland drains 145, and the gauges at and below Pompton Lake Dam drain 160. The river then leaves Pompton Lake. The state Passaic River Flood Advisory Commission reports that the Ramapo and the Pequannock meet about 1,600 feet below a pair of old feeder dams to form the Pompton River. One of those, the Pompton feeder dam, crosses the Ramapo inside Wayne Township. The commission describes the upper Passaic basin, which includes the Ramapo, as steep Highland ridges cut by narrow valleys that produce runoff of high velocity and high volume. That is the Ramapo valley above Oakland: a river with a large hill catchment funnelled into a narrow corridor shared with Route 202, Route 17 and the homes along them.
Mahwah: 106 floods in a century
The NWS Middle Atlantic River Forecast Center lists 106 floods at the Mahwah gauge from 1903 to 2018, measured against the 8-foot flood stage. That is roughly one a year. Current NWS categories there are action at 7.5 feet, minor at 8, moderate at 9 and major at 12. At 8 feet minor flooding begins along Route 202. At 9 feet Route 202 floods downstream, water reaches backyards, and there is minor flooding on Brakeshoe Place, Brook Street and Catherine Avenue. At 12 feet the intersection of Route 202 and Route 17 floods and is closed, with flooding along Riverview Drive, Halifax Road and Bridle Path Lane. The USGS records the Irene peak at Mahwah as 15.77 feet and 18,300 cubic feet per second on August 28, 2011, and its summary of the storm notes that the peak stage there was more than two feet above the previous record. Before Irene the highest Mahwah crests were 13.35 feet in April 1984, 12.53 feet in October 1955, 12.52 feet during Floyd in September 1999 and 12.44 feet in November 1977.
Oakland and the floodgate project
Oakland is where the Ramapo has had the most engineering. The water used to back up from Pompton Lake Dam into the upper end of the lake and into Oakland during high flows. The Army Corps of Engineers project along the 3.3 miles from Pompton Lake up to West Oakland Avenue included channel modification of 5,800 feet of the river, relocation of the Doty Road bridge, wetland creation at Potash Lake and new floodgates at the dam. The Corps reports it was completed in 2007 and is operated by NJDEP. The state commission describes it as giving about a 40-year level of protection to roughly 300 homes in Oakland. The NWS gauge at Oakland now sets action stage at 14.8 feet, minor at 15, moderate at 16 and major at 17. At 15 feet there is minor flooding on River Road near Oakland Avenue. At 16 feet water reaches backyards along River Road and flooding begins on Spring Avenue, Riverside Drive, Center Street, Park Drive and West Street. At 17 feet the NWS expects water in homes along River Road.
How the gates at Pompton Lake Dam work
The commission report sets out the mechanics. A 90-foot section of the dam was replaced with two steel gates, each 18 feet high and 35 feet long. A computer reads the lake level every 15 minutes. When the lake is about 1.5 feet above normal the gates open 3 inches, then by 3, 6 or 12 inches at a time depending on how fast the water is rising, and close again once the flood wave has passed, which may take hours or days. The NWS gauge above the dam carries the note that automatic gate operations begin at 11 feet. The gates exist to protect upstream Oakland; they are not a flood-control dam for the towns below and were designed to have no effect on them either way. After the March 2010 flood the Army Corps estimated their operation added 214 cubic feet per second and about 0.25 feet to the peak just downstream, and shifted its timing by 1.5 to 3.25 hours, none of which it considered significant. Because local observers reported unexpected surges downstream, NJDEP later commissioned an independent hydraulic model of the gate operations.
Pompton Lakes: below the dam
The gauge on the Ramapo at Pompton Lakes, below the dam, has action stage at 10 feet, minor flood stage at 11.5, moderate at 13.5 and major at 14.5. At 10 feet there is nuisance flooding in low-lying areas near the dam. At 11.5 feet the south section of Pompton Lakes floods. At 12.5 feet several homes along Riveredge Drive take on water. At 13.5 feet the Hamburg Turnpike is closed between Route 202 and Riverview Road and many homes south of Dawes Highway flood. At 14.5 feet the NWS describes major flooding with evacuations, made worse by the confluence with the Pompton. Irene crested there at 22.62 feet in August 2011, and the USGS records the peak flow as 17,800 cubic feet per second, against 15,400 in April 1984 and 14,000 in Floyd. The USGS reported that Irene was the highest flood peak at this gauge in more than 90 years of record. Other high crests on the current NWS list are 18.05 feet in March 2010, 16.99 feet in April 2007, 16.72 feet in March 2011 and 15.87 feet in December 2023. The forecast center flags older crest heights at this gauge as read at a different site or datum, so compare floods by flow rather than by height.
What the FEMA record shows
FEMA NFIP claims data records 2,535 flood claims in Pompton Lakes, a borough of about 11,000 people by the US Census Bureau count. Its worst year was 2011, with 773 claims, and its worst single storm was Irene, with 463; the March 2010 nor'easter produced 284 and Floyd 259. FEMA lists 259 repetitive-loss properties in the borough, 111 of them severe repetitive loss. Not all of that water is the Ramapo: the Wanaque and the Pompton also flood parts of the borough, and the NWS impact statements for the Wanaque gauge name Pompton Lakes streets as well. On the Ramapo itself, the state commission listed 6 Pompton Lakes homes bought out with FEMA hazard mitigation grants before 2011, and NJDEP announced in October 2014 that 19 more Pompton Lakes homeowners had been approved for Blue Acres buyouts. The commission also listed flood projects for the Ramapo at Mahwah and Suffern among those in planning or design in 2011.
What this means if you own or rent here
The Ramapo is three rivers in one, so watch the gauge that matches where you live. In Mahwah, the gauge near Mahwah tells you when Route 202 and the Route 17 intersection will go under. In Oakland, the West Oakland Avenue gauge gives the street-by-street picture for River Road and Riverside Drive. In Pompton Lakes, watch the gauge below the dam and remember that when the Pompton is also high, the NWS expects worse flooding at the confluence. The gates at the dam do not lower downstream floods, so their opening is not a signal that the danger has passed. Keep anything valuable off basement floors, move cars before the forecast crest, and do not walk or drive through moving water. Standard homeowners and renters policies exclude flood; a separate flood policy is needed, and whether a given loss is paid is decided by the insurer under its terms. FEMA notes new flood policies normally carry a 30-day wait, and the FEMA Flood Map Service Center shows the mapped flood zone for a specific address.
Common questions
What is flood stage on the Ramapo River?
It depends on the gauge. The National Weather Service sets minor flood stage at 8 feet near Mahwah, 15 feet at Oakland and 11.5 feet at Pompton Lakes below the dam. Major flood stage is 12, 17 and 14.5 feet respectively.
What was the highest Ramapo River flood?
Hurricane Irene in August 2011. The USGS recorded 15.77 feet and 18,300 cubic feet per second near Mahwah, more than two feet above the previous record, and 17,800 cubic feet per second at Pompton Lakes, the highest peak there in more than 90 years of record.
Do the Pompton Lake Dam floodgates protect Pompton Lakes?
No. They were built to stop water backing up into Oakland, upstream of the dam, and were designed to have no effect, good or bad, on communities downstream. The Army Corps estimated that in the March 2010 flood the gates added about a quarter of a foot to the peak just below the dam.
Which streets flood first along the Ramapo?
The National Weather Service names Route 202 near Mahwah at 8 feet, River Road near Oakland Avenue in Oakland at 15 feet, and low-lying land near the dam in Pompton Lakes at 10 feet on the gauge below the dam, followed by the south section of the borough at 11.5 feet.
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