With four major tributary branches that converge west of the city of Westfield, it flows 78.1 miles (125.7 km) (measured from the source of its North Branch) before its confluence with the Connecticut River at Agawam, across from the city of Springfield's Metro Center district.

The Connecticut's northern tributary, the Deerfield River, is nearly as long as the Westfield, being only 2.1 miles (3.4 km) shorter.

During the mid-20th century, the Westfield River was so polluted that it would change color based on the nature of the contaminant.

[4] It is a state and locally managed river featuring native trout fishing and rugged mountain scenery in the context of a historical mill town settlement (at Westfield).

Rising in the Berkshire Hills region of Massachusetts, it flows southeastwardly to join the Connecticut River at Agawam—directly across from Springfield's Metro Center (downtown).

The modern hydroelectric plant is also owned by Eagle Creek Renewable Energy, and is rated at 1.4 MW, with two Francis turbines.

After the 1936 flood on the Westfield and Connecticut Rivers, it was constructed by the Army Corps of Engineers from August 1939 to December 1941 for $3.3 million.

[12] The dam is credited with averting hundreds of millions of dollars in downstream flood damages.