When a fast flowing body of water, like the Winooski River, meets a slow moving body of water, like Lake Champlain, stream velocity slows and following the Hjulstrom Chart, larger sediments (gravels) are deposited at the mouth of the river. As the water continues to slow, smaller and smaller sized sediments settle out of the water (silts and sands). Storm after storm, year after year, topset and foreset beds prograde out into the lake, forming a fan-shaped lobe of sediments sorted by size.
As our large east-to-west draining rivers – like the Lamoille, Winooski, andÂ Missisquoi Rivers – drained glacial sediments off the Green Mountains down into the Champlain Valley, they reached still water at 620′ in elevation, creating deltas at that elevation. The same process happens today, but at about 100′ in elevation where these rivers now reach still water (Lake Champlain). Looking at the surficial geology map of Vermont, we see exactly this. At around 620′ there are deltaic sediments deposited in large lobes around these rivers.
Surficial geology map of Vermont (1970). Available at: http://dec.vermont.gov/sites/dec/files/geo/StatewidePubs/SurfMapopt.pdf
The map above shows surficial deposits in the Burlington area east to Waterbury, with the Winooski and Lamoille Rivers. The salmon pink represents glacial till, dark pink is exposed bedrock. The yellow are river deposits and orange are deltaic sediments from Glacial Lake Vermont. Lower in elevation, just west of the Glacial Lake Vermont deltas are the Champlain Sea deltas.