The Westfield River basin is located in Berkshire, Franklin, Hampshire, and Hampden Counties of west-central Massachusetts, and includes all or part of 29 cities and towns, including the cities of Westfield and Holyoke; the Farmington River basin is located in Berkshire and Hampden Counties, and includes all or part of 10 towns. Together, these basins comprise about one quarter of the drainage area of the Connecticut River in the State. The basins are largely forested and sparsely populated, with most of the population concentrated in the southeastern corner of the Westfield River basin in the cities of Holyoke and Westfield.

Elevations in the Westfield River basin range from 2,300 ft above sea level along the northwestern drainage divide in Windsor to 50 ft above sea level at the confluence of the Westfield and Connecticut Rivers. In the Farmington River basin, elevations range from 2,050 ft along the northwestern drainage divide to 650 ft on the West Branch Farmington River at the Connecticut State line. The following description of the water resources of the Westfield and Farmington River basins is based on Hydrologic Investigations Atlas 716 (Maevsky and Johnson, 1988).