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.
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The east and south side of the mountain drains into the Souhegan River watershed, to the Merrimack River thence the Atlantic Ocean; the west and north sides drain into the Millers River watershed, to the Connecticut River, thence into Long Island Sound.