STREAM TERRACE EVOLUTION DRIVEN BY DESCENDING PROGLACIAL LAKE LEVELS IN CENTRAL VERMONT

Mapping in the lower Jail Branch basin of central Vermont reveals two prominent sets of post-glacial, paired stream terraces. Jail Branch flows northwest into Stevens Branch, a Winooski River tributary, with a confluence at 166 m/544 ft. Jail Branch basin is filled with tens of meters of Late Wisconsinan glacio-lacustrine sediments capped by till.

Both strath terraces are eroded into fine-grained glacio-lacustrine rhythmites. Relative to the modern stream gradient, T2 is steeper in its upper reach and less in its lower. T2 is found 18-4 m above stream level and is dissected in its upper position but prominent in the lower reach. It is unconformably overlain by 0-15 cm of fluvial cobble gravel. Terrace 1 (T1) has a steeper gradient than the modern stream and it is found 5-2 m above stream level. It is well developed in its upper position but is dissected and merges with the floodplain in its lower reach. T1 is unconformably overlain by ~ 2 m of alluvium.

Terrace formation is tied to down-dropping base level during glacial lake evolution in the Winooski River basin, which was a function of ice margin position. Drainage development commenced ca. 14 ka when glacier retreat drained a small, sediment-filled proglacial lake at 360 m/1181 ft elevation. At this time, regional Glacial Lake Winooski was established (285 m/935 ft), 75 m lower. Jail Br. fluvial incision into unconsolidated lake deposits must have been rapid until it graded to the Lake Winooski water plane. No fluvial deposits from this period remain due to continued erosion.

Continued glacier retreat resulted in a new, lower lake level, Glacial Lake Mansfield I at 191 m/627 ft, but Jail Br. first graded to the surface of exposed Lake Winooski’s bottom deposits, which lie at 220 m/722 ft. At this time, T2 was formed. Knickpoint migration eventually captured Jail Br., which abandoned T2 and established T1. T1 is graded to a local bedrock position but also projects to the Mansfield water plane. Eventually, T1 was abandoned for the present stream position, perhaps due to an increase in stream power.

In summary, terrace development is a function of stream adjustment to down-stepping proglacial lake levels, as has been demonstrated for the western Winooski basin by previous studies. Perhaps uniquely, in the eastern basin a former lake bottom also served as a local grade control.