Why the Rockaway River floods: Dover, Denville, Boonton, Montville and Parsippany-Troy Hills
The Rockaway runs out of the Highlands, through a string of Morris County town centres, into Jersey City’s reservoir at Boonton and on to the Passaic. The reservoir does less to stop floods than many people assume.
The Rockaway River rises in Jefferson Township, flows through Dover, Rockaway and Denville into the Boonton Reservoir, and then runs through Montville and Parsippany-Troy Hills to meet the Passaic at Pine Brook. Below the reservoir it drains 119 square miles. The National Weather Service counts 84 floods at that gauge since 1903, and Irene in August 2011 set a record 9.73-foot crest. This guide sets out what the gauges, the Morris County hazard plan and FEMA claims show, what the reservoir does and does not do, and what to watch if you live along the river.
The short version
- The Rockaway drains 119 square miles at Boonton below the reservoir, where the National Weather Service counts 84 floods since 1903.
- Irene set the record at that gauge, 9.73 feet in August 2011, against a major flood stage of 8 feet and a previous high of 8.3 feet.
- At 8 feet the weather service says the river backs up through storm drains into six named streets in Montville.
- Jersey City’s Boonton Reservoir is a water supply; the weather service states that its spilling does not cause flooding below the dam, and floods still occur below it.
- The Morris County plan lists 818 flood claims in Denville, 637 in Parsippany-Troy Hills and 572 in Dover, from FEMA data current to 2019.
A Highlands river with town centres along it
The Morris County hazard mitigation plan, drawing on the FEMA Flood Insurance Study, traces the Rockaway from its source in Jefferson Township southwest and then east into the Boonton Reservoir. On the way it passes through the Townships of Jefferson, Rockaway, Denville, Randolph and Boonton, the Boroughs of Wharton and Rockaway, and the Towns of Dover and Boonton, and below the dam it continues through Montville and Parsippany-Troy Hills. The plan names Boonton, Denville, Jefferson, Parsippany-Troy Hills and Rockaway Township as municipalities that have flooded from the river. Near its end the Whippany River joins it in Hatfield Swamp, and the combined flow meets the Passaic at Pine Brook, at the southeast end of Hook Mountain. From there, the plan notes, the Passaic turns between Fairfield and Lincoln Park toward Two Bridges, where the Pompton joins. The 2011 state flood commission lists the Rockaway as one of the seven major tributaries of the Passaic.
What the Boonton gauge records
The weather service forecast point with the longest record is the Rockaway River at Boonton below the reservoir, where the US Geological Survey gives a drainage area of 119 square miles. Action stage is 4.5 feet, minor flood stage 5 feet, moderate 7 feet and major 8 feet. Against the 5-foot flood stage the weather service counts 84 floods since 1903. The highest crest is 9.73 feet in late August 2011, during Irene. The next highest are 8.3 feet on January 25, 1979, 8.2 feet on May 29, 1968, 8.17 feet on April 6, 1984, 8.12 feet on March 14, 2010, 8.09 feet on April 16, 2007 and 8.03 feet on March 12, 1936. Irene was nearly a foot and a half higher than any of them, and higher than the 7.8 feet recorded in the great flood of 1903. Floods below the dam are not rare: the record lists a run of minor floods in almost every year from 2003 to 2011, and the river crested at 6.03 feet on December 28, 2023, according to the weather service flood summary for that event.
What each level means in Montville and Parsippany
The weather service ties the Boonton readings to specific roads below the dam. At 6 feet Vail Road in Montville floods, and the road heading into Parsippany is subject to closures. At 7.7 feet Change Bridge Road in Montville, north of Woodmont Road and south of Stiles Lane, floods, as does River Road near the garden centre, and both are closed. At 8 feet, the major flood stage, high water on the Rockaway backs up through storm drains and floods parts of Lancaster Avenue, Normandy Drive, Buckingham Circle, Sheffield Place, Manchester Drive and Konner Avenue in Montville, and backs up Hatfield Creek, adding to the flooding on Lancaster Avenue. That last statement matters for anyone in those streets. The water does not have to come over the bank to reach a house; it can come up through the drainage system that normally carries rain away. In Parsippany-Troy Hills the county plan describes a related pattern: the Lake Hiawatha neighbourhood and parts of the township south of Route 46 have flooded from the Whippany River, Troy Brook and Eastmans Brook, which flow through Troy Meadows into the Rockaway.
Upstream: Dover, Rockaway and Denville
Above the reservoir a newer gauge at Main Street in Boonton, installed in 2012 to replace an older one, carries the weather service impact list for the upstream towns. Minor flood stage there is 4.5 feet, moderate 6 and major 7. At 5 feet Savage Road and other roads around Gardner Park in Denville flood. At 6 feet Main Street and Jackson Avenue in Rockaway are flooded, water backs up and comes through the sewer system, and Main Street in Denville begins to flood. At 7 feet the Park’s Lake area in Denville is flooded and residential areas in Dover next to the river see significant flooding. At 8 feet Interstate 80 in Denville floods, and at 10 feet Main Street businesses in Denville have several feet of water. The county plan adds that flooding in Denville occurs along virtually the whole length of the river in the township, with the worst damage along Riverside Drive and near the Diamond Spring Road bridge, and that Den Brook and the Rockaway together have inundated Route 46 and Denville Center more than once. At Dover the weather service lists crests of 9.4 feet in August 2011 and 8.9 feet in September 1999.
Irene and the record
Irene is the flood against which everything on this river is now measured. The county plan records that in August 2011 the Rockaway set a new flood record, carved out a section of Interstate 287 in Boonton, where northbound lanes were sheared away, and caused two major slope failures in the Town of Boonton. The old USGS gauge above the reservoir recorded a maximum discharge of 8,210 cubic feet per second and a gauge height of 9.31 feet during the storm, and was discontinued that month. The plan also cites a FEMA analysis of that gauge’s record from 1938 to 2011, in 20-year increments, which shows the average annual peak flow increasing by 84 per cent. For Boonton Township the plan points to urbanisation, soils of low permeability and a lack of adequate storm sewers, and names areas between Oak and Green Streets, the Sheep Hill section, ground near Boonton High School and behind Kanouse Street, and land beside the river from Greenback Road to the Montville line along Vreeland Avenue.
What the Boonton Reservoir does and does not do
The Boonton Reservoir is a water supply, not a flood control dam. The City of Jersey City states that it owns the Boonton and Splitrock reservoirs and finished the Boonton dam in 1904. The weather service sets the reservoir’s spill level at 305.25 feet and explains that a spillway at the top of the pool releases water when the reservoir is at or near capacity, that spilling can go on for weeks or months, and that spilling does not cause flooding below the dam. The flood record below the dam shows the other half of the story. Floods at the Boonton gauge happen with the reservoir in place, and Irene produced the record there more than a century after the dam was built. None of the sources reviewed for this guide describes the reservoir being operated to hold back floods, so it is safest not to count on it.
The insurance record, town by town
The Morris County plan reproduces FEMA flood insurance figures, with policies and claims current to September 2019 and repetitive-loss counts to April 2019, combining policies written directly by the NFIP and by private insurers on its behalf. On that basis Denville has 818 flood claims and 76 repetitive or severe repetitive-loss properties, Parsippany-Troy Hills 637 claims and 50 such properties, and Dover 572 claims and 45. Montville has 125 claims and 5, Rockaway Borough 82 and 9, Rockaway Township 71 and 6, Boonton Township 56 and 10, and the Town of Boonton 14 and none. These figures include every flood source in each town, not just the Rockaway, and in Parsippany the Whippany and its brooks play a large part. For comparison, Lincoln Park, which sits beside the Passaic and Pompton just downstream of the confluence, has 3,263 claims and 364 repetitive-loss properties. The plan also estimates that about 1,013 Denville residents, 6.0 per cent of the township, live in the 1 per cent annual chance floodplain, against 457 in Montville and 1,294 in Parsippany-Troy Hills.
What this means if you own or rent here
Know which gauge is yours. Above the reservoir, from Dover through Rockaway and Denville to the Town of Boonton, the Main Street gauge at Boonton carries the impact list. Below it, in Montville and Parsippany-Troy Hills, the gauge below the reservoir is the one to watch, and its 8-foot statement about storm drains is the reason a backflow valve or a plug for a floor drain deserves a look in the low streets it names. Treat a full reservoir as no protection. In a big storm, move cars and anything valuable off basement floors early, never walk or drive into moving water, and keep away from electrics if water reaches them. Flood cover is a separate policy from homeowners or renters insurance, and FEMA states that a new policy normally takes effect 30 days after purchase. Damage from water backing up through drains is treated differently by different policies, and whether a loss is paid is decided by your insurer and your policy wording. For the zone at a specific address use the FEMA Flood Map Service Center.
Common questions
Does the Boonton Reservoir protect towns from Rockaway River flooding?
It is not a flood control dam. Jersey City owns it as a water supply, completed in 1904. The National Weather Service says spilling over the spillway does not cause flooding below the dam, but the gauge below it has still recorded 84 floods since 1903, including the record 9.73-foot crest during Irene in 2011.
What is flood stage on the Rockaway River at Boonton?
Below the reservoir the National Weather Service uses 4.5 feet as action stage, 5 feet as minor flood stage, 7 feet as moderate and 8 feet as major. At the Main Street gauge above the reservoir minor flood stage is 4.5 feet, moderate 6 and major 7.
Which Montville streets flood from the Rockaway?
The weather service lists Vail Road at 6 feet on the Boonton gauge below the reservoir, Change Bridge Road and River Road at 7.7 feet, and at 8 feet Lancaster Avenue, Normandy Drive, Buckingham Circle, Sheffield Place, Manchester Drive and Konner Avenue, where water backs up through storm drains and Hatfield Creek.
Where does the Rockaway River meet the Passaic?
The Morris County hazard plan places the confluence at Pine Brook, at the southeast end of Hook Mountain, just below where the Whippany joins the Rockaway in Hatfield Swamp. The Passaic then flows between Fairfield and Lincoln Park to Two Bridges, where the Pompton joins it.
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