In what ways did the establishment of mill towns
in the northeastern United States rely on earlier economic and technological
developments?

There are different kinds of growth.  The ways did the establishment of mill towns  in the northeastern United States rely on earlier economic and technological  developments was through;

The industry and trade grow rapidly in the Northeast during the 19th century because the rivers provides transportation routes for trade and other economic activities.

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Establishment of mill towns  in the northeastern United States relied on economic and technological  developments that happened earlier.

Explanation:

The establishment of mill towns  in the northeastern United States  happened due to the availability and mass production of mills in the country.

IT was the mill that had kicked off the industrial revolution in England and it was as instrumental in the US too.

Thus the advancement of technology by the building of this machine is why the towns could come into the picture and become industrial powerhouses they eventually became.