THE GEOMORPHIC HISTORY OF THE CENTRAL PASSAIC BASIN, INSIGHTS FROM LIDAR DEM

The Passaic River Basin, located in northeastern New Jersey, is a 935 square mile watershed that covers much of the northern counties of New Jersey and two counties in New York. The Passaic River is fed by the Rockaway, Whippany, Pompton, and Saddle Rivers before flowing northeast along the second Watchung Mountain and turning abruptly south to disgorge into Newark Bay. The Passaic River Basin has varied topography. The river’s low gradient course and wide, frequently inundated floodplains contrast the Northern tributaries, which have higher gradient narrow bedrock channels and flashy runoff. The Passaic’s shallow gradient is attributed to blockage of the river’s original course by glacial ice and moraine deposits. Advancing ice during the Late Wisconsinan Glaciation (22,000-18,000 years B.P.) led to the creation of Glacial Lake Passaic that left buried valleys of lake deposits within the surrounding bedrock. Isostatic rebound after the retreat of the glacier caused the basin to rise to a gradient of 0.4m/km toward N28°E, likely contributing to the shallow gradient of this northward flowing river.

One of the Central Passaic Basin’s largest wetlands, the Great Piece Meadows, rests on a buried valley. While aerial imagery of this floodplain shows the pronounced oxbow lakes and cutoffs that mark former paths of the central Passaic, more subtle features are hidden by vegetation. Lidar Digital Elevation Modeling (DEM) strips away the vegetation, giving a better view of the river’s abandoned channels, meanders, and other floodplain features. A core ~ 210 cm in length was taken from each of three prominent oxbow cutoffs within the floodplain. The sediment analysis and dating methods used on these cores, combined with the Lidar data, enables a detailed interpretation of the post-glacial central Passaic River’s changing course over time. This study determined that the westernmost oxbow furthest upstream was cut off from the channel around 9-9.5 ka B.P., while the oxbows downstream to the East are thought to have existed simultaneously, separating from the main channel around 3.5-4 ka B.P. Deposits within the upper 50 cm of the cores show changes in sediments and reduced sedimentation rates. Isostatic rebound and climate likely influenced changes in sedimentation and river morphology.