PALEOFLOOD HYDROLOGY OF THE OUTLET OF LAKE PASSAIC DURING THE LAST GLACIAL STAGE OF THE GREAT NOTCH THROUGH THE THIRD RIVER, NORTHERN NEW JERSEY, USA

Glacial Lake Passaic was one of the proglacial lakes that formed during the recession of the Laurentide Ice Sheet in northern New Jersey and existed for approximately 2000 years - 25.5-23.5 ky BP. The lake formed behind the Watchung Mountains and drained through different gaps that are today the modern Passaic River and the Raritan River waterways and their tributaries. The lake was first discovered by George Hammell Cook, NJ State Geologist, during the late 19^th Century. The lake had both glacial drainages and post-glacial drainages. The glacial drainages included the Chatham Stage, Moggy Hollow Stage and the Great Notch Stage. Our work concentrated on the latter, where the breach is located in the First Watchung Mountain, represents a lake drop of more than 24 m, and incises the headwaters of the Third River and its downstream valley.

We used the NJ 1-m resolution lidar dataset to reconstruct the topography of the Great Notch spilling into the Third River, which today is a highly urbanized landscape. However, in a small park between the cities of Clifton and Montclair, Alonzo F. Bonsal Wildlife Preserve, there is a well-preserved sluice and a large number of partially buried boulders on the left valley slopes, quite distinct from the modern channel bed gravels of the Third River below, different both in size and lithology. These boulders are presumably flood erratics, as their lithology mostly resembles that of the till in which the sluice is cut, and we interpret that they were transported during a large flood to their current resting place. The highest boulders were accurately mapped, and their lithology and size were also recorded. The data was then incorporated into a hydraulic model, where the modern hydrology served as the base elevations for generating flood profiles. Initial results show paleoflood discharges were 4000-5000 m³/s, 2-3 orders of magnitude larger than the modern largest discharges of the Third River generated by snowmelt, rain or hurricanes (8-40 m³/s). The outburst drainage of Lake Passaic flooded extensive parts of the Third River valley and beyond, all the way downstream to the present confluence with the modern Passaic River. This paleoflood reconstruction not only adds a dynamic perspective of the catastrophic lake drainage but allows assessment of glacial and post-glacial valley erosion rates and also valley soil formation processes since the end of the last ice age.