The Millers River drains a total area of about, about of which are in Massachusetts, with the remainder in New Hampshire. About 78 percent of the basin is forested, 11 percent is open land, 8 percent is wetland, and 3 percent contains urban areas. The watershed is made up of all or part of 17 municipalities, with a population of approximately 87,000 people. Population centers within the watershed are concentrated around Winchendon, Gardner, Athol, and Orange. Many wetlands in the basin indicate areas of former shallow lakes and ponds that have gradually been filled. There are a total of 107 lakes and ponds in the basin, 72 of which have an area of or more. Only one lake, Lake Monomonac in Winchendon, Massachusetts, and Rindge, New Hampshire, is larger than.
The North Branch and the main stem of the river join in Winchendon, and the river flows westward to the Connecticut River. Major tributaries of the Millers River are Tarbell Brook, which enters about two miles west of Winchendon Center, the Otter River, which enters in Winchendon in Otter River State Forest, and the Tully River, which enters in Athol. These tributaries flow mainly through wetlands. Overall, the Millers River has a moderate gradient, averaging about 18 feet/mile from the headwaters area to the USGS streamflow-gaging station at Erving, a stream distance of about. However, a reach of the Millers River through a wooded area between South Royalston and Athol has an average gradient of about 43 feet/mile, which is about five times the average for rivers in Massachusetts, and some of the most rugged and steep terrain of the state's central upland. The gradient of the tributary Otter River averages about 18 feet/mile for a distance of about, and that of the East Branch Tully River, the major tributary of the Tully River, averages about 52 feet/mile over a distance of about.
Flood control
The Birch Hill Dam is a flood control dam located on the Millers River in South Royalston, Massachusetts. It is a part of a network of flood control dams on tributaries of the Connecticut River. Completed in 1941 at a cost of US$4 million, Birch Hill Dam was one of the first dams the Army Corps of Engineers built in New England to prevent floods like those that devastated Athol and Orange in 1936 and 1938. It has a storage capacity of. The Reservoir Regulation Team is the "nerve center" for the New England flood control dams such as Birch Hill Dam. Using radio and satellite communications, RRT constantly monitors river levels and weather conditions that influence flood control decisions. Corps personnel, in conjunction with RRT, regulate the amount of water released downstream by raising or lowering the four by gates located in the gatehouse at the dam. In a time of high water, the gates are lowered in order to hold back the water, only to be released when downstream river conditions begin to recede. In April 1987, two storms dropped over of rain, raising the water level to over at the dam, utilizing 80% of the storage capacity. It is estimated that Birch Hill Dam prevented over $9 million in damages to downstream property from this one storm.
