DISCHARGE RATES AND FLOOD VOLUMES FOR GLACIAL LAKE PASSAIC, NEW JERSEY

Glacial Lake Passaic occupied the upper Passaic River basin, in northeastern New Jersey, during the late Wisconsinan glaciation, between 25 and 22 cal kyr BP. It included an advance stage (Chatham stage, spillway elevation 88 m), a maximum stage (Moggy Hollow stage, spillway elevation 104 m), and a retreat stage (Great Notch stage, spillway elevation 93 m) (Salisbury and Kummel, 1895; Stone and others, 2002). Sluiceways for each stage are cut into bedrock or till and are well preserved. Discharge through each sluiceway was estimated from channel dimensions and slope using the Manning equation and a roughness value of 0.045. Flood volumes released during the drop from the Moggy Hollow to the Great Notch stages, and during drainage of the Great Notch stage, were calculated from isostatically adjusted lakeshore elevation and bathymetry.

Bankfull discharge for the Chatham sluiceway is 1,800 m³/s, at a velocity of 3 m/s, and for the Moggy Hollow sluiceway it is 1,500 m³/s, at a velocity of 8 m/s. The Great Notch sluiceway was initially cut by flood discharge during the 24-m drop from the Moggy Hollow to the Great Notch stage, which released 10.4 km³ of water. Bankfull discharge for this channel is 3,500 m³/s, at a velocity of 7 m/s. At this rate, the flood would take about a month to drain. Drainage of the Great Notch stage released 5 km³ through the Weasel Brook sluiceway. Bankfull discharge in this channel is 5,000 m³/s, at a velocity of 5 m/s. At this rate, the flood would drain in 11 days.

The 1903 flood of record for the lake basin, which is equivalent to the modern Passaic River basin above Paterson, discharged 960 m³/s, so the maximum glacial discharge was nearly double the maximum meteoric discharge. At the glacier's terminal position, drainage from 38 km of ice front exited the basin through Moggy Hollow. The bankfull discharge of 1,500 m³/s implies a maximum melt rate of 3,800 m³/day of ice per meter of ice front, assuming no meteoric contribution. This melt rate converts to 0.13 m/day of ice-surface lowering over the 30-km length of glacier in the basin, assuming stagnant ice with no resupply. This lowering yields an ice-front retreat rate of 45-50 m/yr. In contrast, ice-margin chronology gives a retreat rate of about 30 m/yr during deglaciation in Lake Passaic. This discrepancy shows that the glacier was not stagnant.