Surface water

The Millers River drains a total area of about 390 mi2, about 313 mi2 of which are in Massachusetts. About 78 percent of the basin is forested, 11 percent is open land, 8 percent is wetland, and 3 percent contains urban areas. 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 10 acres or more. Only one lake, Lake Monomonock (Lake Monomonac) in Winchendon (592 acres), is larger than 500 acres.

The Millers River formed at the end of the last glacial period when several glacial lakes joined and, eventually, drained into the Connecticut River. The river's headwaters are in the White Mountains of New Hampshire and in Ashburnham and Winchendon, Massachusetts.

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 the Otter, which enters the Millers River in Winchendon, and the Tully River, which enters the Millers River in Athol. Both of these tributaries largely flow through wetlands.Overall, the Millers River has a moderate gradient, averaging about 18 ft/mi from the headwaters area to the USGS streamflow-gaging station at Erving, a distance of about 43 river mi. However, a 5-mi reach of the Millers River through a wooded area between South Royalston and Athol has an average gradient of about 43 ft/mi, which is about five times the average for rivers in Massachusetts. The gradient of the tributary Otter River averages about 18 ft/mi for a distance of about 11.5 river mi, and that of the East Branch Tully River, the major tributary of the Tully River, averages about 52 ft/mi over a distance of about 13 river mi.