Flood Waters and Climate Change on the Rockaway River in Denville

 by Brian T. Lynch, MSW

A network of storm drains built over decades relies on our rivers to carry stormwater away and to prevent floods. The system is maxing out and climate change is making matters worse. Here is one example.

The recent flooding of the Rockaway River at St. Clare’s in Denville piqued my curiosity as to how much water it takes to flood the parking lots at the hospital. Starting at the Calvin L. Lawrence Memorial Bridge on Diamond Spring Road, the river at its normal level looks to be about 40 feet across and maybe two feet deep. The manmade river channel looking east from the bridge is about 100 feet wide and maybe 10 to 12 feet above the surface of the water.

Just past the bridge area the channel lowers as it widens into the natural flood plain and the natural riverbank reemerges. The right bank (northern bank) of the river is maybe 2 to 4 feet high as it cuts through the expansive flood plain, while the left bank is even lower in places. A secondary embankment leading up to the parking lot surrounds and defines the flood plain. This embankment is about 7 feet above the flood plain. From what I could see the embankment on the other side was of similar height. The flood plain stretches out more than 1,000 feet in some parts. This flood plain can hold a lot of water before spilling into the parking lot opposite the main hospital entrance. Nevertheless, this parking lot frequently floods and there are signs warning visitors to park at their own risk.

A short way further east the river turns north under a bridge across Pocono Road and the flood plain narrows once again. On the other side of this bridge the height of the riverbank remains about 2 to 3 feet high, but the eastern parking lot opposite the Emergency Room entrance slopes down almost to the river’s edge. The flood plain on the other side is about 100 feet across leading to an embankment of perhaps 10 feet in height.

Standing on the parking lot beside the river at this spot, and looking back towards the hospital, it is apparent that this lower parking lot is built within the flood plain of the Rockaway River. The lot slopes up toward an embankment on which is built the access road to the ER entrance. So, when the river fills this eastern parking lot, it is merely flowing into its natural flood plain. At least that’s how it appears to me.

With its extensive flood plain at this location, the Rockaway River can retain an enormous volume of water before spilling into residential areas and causing serious damage. It has mostly been adequate for this purpose in the past, but that that may no longer be true. As global warming produces heavier and more frequent rain events, the river’s capacity to hold stormwater runoff will be exceeded more often and to a greater extent. However, when the lower eastern parking lot fills with water, it should not be considered a flood event.




The network of storm drains in the highlands of New Jersey relies heavily on the existing flood plains, wetlands, tributaries, and rivers to hold and carry away runoff from impervious surfaces in developed areas. Through the decades, the area of impervious surfaces continues to grow while wetlands are filled in, flood plains constricted by development, and wilderness areas where rain can soak into the earth diminished. As a result, less rain is available to recharge our aquifers. More rain rushes out to sea where it isn’t available to support us or the biosphere on land. Climate change, with its near-drought conditions between episodes of heavier than normal rain events, exacerbates the consequences of excessive land development. Heavy rainfall quickly saturates the soil and leads to more runoff. Dry spells in between rainfall cause more water to evaporate from the soil, leaving less to recharge the aquifers below.

The assumptions and calculations used in the past to mitigate flooding need to be revisited. There is much that can be done to retain more of the freshwater that falls from the sky, but it requires a change of thinking and a willingness on everyone’s part to do things differently.