ICE JAMS ON THE LOWER MOHAWK RIVER, NEW YORK: LESSONS FROM RECENT BREAKUP EVENTS

The principle hazard on the Mohawk River is flooding and ice jamming during breakup. In the last 170 years, about 80% of large floods have been thaw events with ice floes. In these events, flood stage elevation may be unrelated to discharge because ice jams can raise water levels rapidly, causing serious damage and prompting rapid emergency response. The January 1996 event on the Mohawk River was the fourth largest stage elevation in Schenectady in the last 100 years, and one of the top ten stage elevations in the last 170 yr. In Schenectady, the event was punctuated by a number of serious ice jams that resulted in higher than normal stage elevation and widespread flooding. Ice jamming and water backup occurred at the site of the old Burr Bridge abutments, which form a constriction in the channel. Stage elevation for the February 2000 event in Schenectady was not unusual (~3 m). This event was characterized by warm temperatures (~15° C) but the ice pack was unusually thick (380-500 mm) due very cold, snow-free weather. The ice floe successfully moved through Schenectady without incident. However, jamming of the 5-10-km-long floe occurred near Crescent, NY (~10-20 km downstream) where some 60 homes were evacuated and power was disrupted when ice snapped a utility pole. Ice scar frequency and elevation indicate that two jam levels occurred during this event. A ice jam formed at the Crescent Bridge where ice scar elevations have a maximum elevation of 2.9 m, but average ~1.2 to1.8m. An later ice jam formed at Crescent, NY, ~2 km upstream from Lock 6 where ice-scar elevations are as high as 4.4 m, but average ~2.1-2.4 m. These jams occurred due to: (a) channel constriction along a bedrock incised reach; (b) shallow channel due to sediment infilling upstream from Lock 6 which is a permanent dam.