POSTGLACIAL DRAINAGE EVOLUTION OF THE ADIRONDACK NORTH-FLOWING TRIBUTARIES OF THE ST. LAWRENCE RIVER, NORTHERN NEW YORK

Four adjacent, north-flowing rivers, Oswegatchie, Grass, Raquette, and St. Regis serve as models for deciphering the postglacial evolution of drainage patterns from the Adirondack Highlands across the St. Lawrence Lowlands to the St. Lawrence River. Five phases of drainage development are related to the primary events of deglaciation from the LGM including: 1) high (roughly 350 to 400+ m) proglacial lake formation; 2) Glacial Lake Iroquois formation; 3) Champlain Sea invasion; 4) rebound and Champlain Sea drainage; 5) modern, neotectonic adjustment of lower reaches tributary to the St. Lawrence River. In phase 1, drainage routes and elevations in the Adirondack uplands were to the SW and developed sequentially as the Wisconsinan ice mass wasted northward in the St. Lawrence Lowland as documented by several workers. Preglacial bedrock valleys most strongly influenced phase 1 drainage. Delta positions document initiation of northward flow into Lake Iroquois and later into the Champlain Sea in phases 2 and 3.

Numerous paleochannels drained stagnant ice meltwater and high proglacial lakes through valleys now over sized for their respective streams. Late in phase 1, the Raquette R. flowed westward near Sevey and cut a large valley that is now occupied by underfit Windfall Bk. and Dead Ck. and connected with the (present) South Branch of the Grass R. and then the Oswegatchie. The Raquette has since been pirated as it now abruptly turns northward at Moody Falls where it once flowed westward. Leonard Bk., a Grass R. tributary, lies in a vastly over sized valley that begins at a plunge pool where water once poured over a bedrock sill from the Raquette R. 2.4 km south of Colton. The large valley extends westward, where it now holds the underfit Grannis Bk. and Little R. Today the sill is dry, and the captured flow is northward. Rebound or neotectonism reversed the drainage direction during phase 4 or 5, causing flow to the NE. An over sized valley containing Indian Ck. and Upper and Lower Lakes represents a paleochannel of the Oswegatchie R. to the Grass R. from Rensselaer Falls to Canton active in phase 4. All flow has since been captured by the Oswegatchie as it drains to the north, meeting the St. Lawrence R. in Ogdensburg. Further effects of neotectonism are demonstrated by convergence of the Grass, Raquette, and St. Regis Rivers at Massena, NY.