The Passaic rises in the center of Mendham, in southern Morris County. Initially, it generally flows south, through Morristown National Historical Park, and forms the boundary between Morris and Somerset counties. In its current path, it passes along the western edge of the Great Swamp, which it drains through several small tributaries. The river passes through a gorge in Millington and then turns abruptly northeast, flowing through the valley between Long Hill to the west and the Watchung Mountains to the east.
Near Chatham it turns north, forming the boundary between Morris and Essex counties. It passes Livingston and West Caldwell, where it flows through the Hatfield Swamp and is joined by the Rockaway River. Southwest of Lincoln Park it passes through the Great Piece Meadows, where it turns abruptly eastward and is joined at Two Bridges by its major tributary, the Pompton River.
The river flows northeast into the city of Paterson, where it encounters the Great Falls of the Passaic, below which it becomes navigable. On the north end of Paterson, it turns abruptly south, flowing between Paterson on the west and Fair Lawn, Elmwood Park, Garfield on the east, next through the city of Passaic, then passing Rutherford, Lyndhurst, Nutley, Belleville, and North Arlington, New Jersey.
In its lowest reaches, it flows along the northeast portion of the city of Newark, passing Kearny and Harrison, New Jersey on the opposite bank. Near downtown Newark it makes an abrupt northwesterly bend, then south, joining the Hackensack River at the northern end of Newark Bay estuary, a back bay of New York Harbor.
The Passaic River formed as a result of drainage from a massive proglacial lake that formed in Northern New Jersey at the end of the last ice age, approximately 13,000 years ago. That prehistoric lake is now known as Glacial Lake Passaic and was centered in the present lowland swamps of Morris County, forming because of a blockage of the normal drainage path. Eventually the lake level rose high enough that the water flowed out of a new outlet. The Passaic River found a new path to the ocean via the Millington Gorge and the Paterson Falls as the glacier that covered the area retreated northward and the lake drained. As a result, the river as we now know it was born.
The river was highly significant in the early industrial development of New Jersey. It provided a navigable route connected by canals to the Delaware River starting in the late eighteenth century. It also was an early source of hydropower at the Great Falls of the Passaic in Paterson, resulting in the early emergence of the area as the center of industrial mills.
Much of the lower river suffered severe pollution during the nineteenth and twentieth centuries because of the development. Although the health of the river has improved due to environmental legislation and regulation and the decline of industry along the river, it still suffers from substantial degradation of water quality. The sediment at the mouth of the river near Newark Bay remains contaminated by such pollutants as dioxin which largely was produced at the Diamond Shamrock Chemical Plant in Newark as a waste product resulting from the production of the agent orange defoliation chemical used during the Vietnam War. The cleanup of the dioxin contamination on the bottom of the Passaic River is the subject of a major environmental lawsuit regarding the responsibility for the cleanup, which has been ongoing for decades without resolution.
The decline of manufacturing on the lower river has left a post-industrial landscape of abandoned and disused factories and other facilities. In particular, the stretch of the river along downtown Newark came to be regarded in the later decades of the twentieth century as particularly wretched. Starting in the 1990s, the lower river became the subject of federal and state urban restoration efforts, which have resulted in new construction along the riverfront, including a regional headquarters of the FBI.
While there has been a decline in the industrial use of the river, recreational use has increased since the early 1990s. There has been a long tradition of high school rowing by Kearny, Belleville, and Nutley High Schools and, in 1990, the historic Nereid Boat Club (originally founded in 1868) was revived, broadening participation in the sport of rowing on the Passaic River. In 1999, the Passaic River Rowing Association became the second rowing club along the banks of the Lower Passaic River. Today, the rowing community is very active through the two Rowing Clubs (Nereid Boat Club & Passaic River Rowing Association) and eight high school crews that include Kearny, Belleville, Nutley, Don Bosco Prep, Montclair, Ridgewood, Teaneck, and Westfield.
The Passaic River can be accessed via a number of county parks. One notable park is called Stanley Park between Summit and Chatham. Other parks along the river are located in Passaic County. As part of the ongoing Newark revitalization effort by the city government, parkland is proposed along the banks of the river.
The Passaic River generally is free of industrialization until it reaches the Summit and Chatham border. The upper portion of the river, above Summit and Chatham, are more natural in appearance and the river has more of a young river character in places. The middle portion of the river flows through natural marsh lands and forested areas in Essex County, which are generally inaccessible and then through heavily populated areas of Passaic County where it is accessible via parallel roads and parks. Lower portions of the river, south of Paterson are wider, more industrialized, and more mature in nature. The banks of the lower few miles of the river mainly are industrialized.
The Passaic River is known for chronic flooding problems during periods of heavy rainfall or snow-melt, especially where the Pompton River joins the Passaic River in Wayne, New Jersey. The two rivers form a sizable flood plain in this area. Building has been allowed in the flood plain and during extreme weather events that occur on a fairly regular basis, homes and businesses in the flood plain flood. A plan has been proposed to build the massive 20 mile (32 km) structure, the Passaic River Flood Tunnel, to divert the periodic flood waters southeastly into Newark Bay, thus relieving these flooding problems upstream. Some residents have accepted buy-outs from the federal government while the concept of constructing a flood tunnel is debated, however, many residents still live within the flood plain and flooding appears to be growing worse as the land in the Passaic River basin continues to be developed.
The Little Falls Flood Gauge along the Passaic River is located just south of the junction with the Pompton River, in an area that frequently floods. Flood stage is at this location.
(As encountered travelling upstream to its source):