Why is flooding always so bad in North Jersey? The area was a prehistoric lake

Though a torrential downpour and 70 years of unfettered suburban development are to blame for the flooding that inundated Passaic River communities this past weekend, the problems can be traced further back. 

Way back.

The Passaic River Basin was a massive lake that formed 25,000 years ago after a glacier that covered a large portion of of North Jersey retreated, leaving debris that blocked gaps in the Watchung Mountains, according to federal and state reports.

Water collected in the low-lying, tub-like basin and formed "Lake Passaic," which existed for about 2,000 years, said Scott Stanford, a research scientist for the New Jersey Geological and Water Survey 

At its largest, the lake was 15 miles wide and stretched 30 miles, from Paterson through Morris County down to Short Hills, said Jeff Hoffman, the state geologist. 

The lake eventually drained through a rock formation in Little Falls, now called Great Notch, helping to form the modern-day Passaic River.

But Great Notch has an elevation that is 110 feet higher than the basin's previous drainage point near Short Hills, which became blocked when the Wisconsin Glacier retreated north. It caused the Passaic River to flow out of the basin much more slowly, allowing for a greater risk of flooding, which lasts to this day, Stanford said Tuesday. 

Flooding has been a problem since long before the post-World War II housing boom that saw homes and businesses built on or near Passaic River flood plains. 

The earliest mention came in 1765 from colonists living near the Passaic River. The worst flood occurred in 1903, when the Passaic crested at 17.5 feet — about 10 feet above flood stage — after 15 inches of rain fell in the Paterson region over five days. 

After more floods, the Army Corps of Engineers was charged in 1936 with finding a solution. But efforts stalled because of a lack of funding and disputes over possible remedies. 

The construction and expansion of Routes 80, 46 and 23 through the flood plain helped launch major development. Route 80 was completed in Wayne in 1969, the same year Willowbrook Mall opened in a flood plain. The mall's parking lot was designed to serve as a detention basin.

Over the past three decades, hundreds of homes in Wayne, Pequannock, Lincoln Park and other towns have been bought and razed by the state.

But a state commission in 2011 estimated that it would cost $3.4 billion to buy and demolish 6,300 properties in the 10-year flood plain in the Passaic River Basin, including $300 million for the 800 properties in the floodway.  

A long history of flooding

• 180 million years ago: A large geological basin forms in what would become the Passaic Valley with the Watchung Mountains to the east and Highlands ridges to the west. 
• 125,000 years ago: The Wisconsinan glacier advances into the basin. 
• 25,000 years ago: The glacier retreats and Lake Passaic is created.
• 23,000 years ago: Lake Passaic finishes drains through Great Notch in Little Falls, creating the Passaic River. 
• 253 years ago: The first large flood in Passaic County is reported by colonists in 1765.
• 115 years ago: The worst flood occurs in 1903, when the Passaic River crests at 17.5 feet — about 10 feet above flood stage — after 10 inches of rain fell in four days. 
• Floods in 1968, 1971, 1972, 1973, 1975 (two that year), 1984, 1992, 1999, 2001, 2005, 2010 and 2011 (from Hurricane Irene) prompted federal disaster declarations.
• 28 years ago: Congress authorized the construction of a 21-mile, $1.8 billion tunnel that would carry floodwater to Newark Bay. But support dwindled because of the cost and concerns over its environmental impact. 

More on NJ floods

State of emergency:Gov. Phil Murphy declares state of emergency in Bergen, Essex, Monmouth, Ocean, Passaic

Recent flooding:North Jersey flooding caused by almost 5 inches of rain in 50 minutes

Watch:Bogota police rescue bride from flooded car on her wedding day