The Corps Water Management System (CWMS) includes four interrelated models to assist with water management for the basin:
- GeoHMS (Geospatial Hydrologic Modeling Extension)
- ResSIM (Reservoir System Simulation)
- RAS (River Analysis System)
- FIA (Flood Impact Analysis)
The Merrimack River is formed by the confluence of the Pemigewasset and Winnipesaukee Rivers at Franklin, New Hampshire. Just south of this confluence, the steeper valley of the north gives way to a much wider, flatter flood plain, which, beginning in Concord, New Hampshire, is quite heavily developed. The river flows southerly through New Hampshire into Massachusetts, then just south of the state line, it turns abruptly northeastward joining the Atlantic Ocean near Newburyport, Massachusetts, 35 miles north of Boston.
The river has a total length of 116 miles, of which the lower 22 miles downstream of Haverhill are tidal. The total length of the Merrimack and Pemigewasset Rivers, the principal tributary, is approximately 186 miles. From its headwaters in the White Mountains to the ocean, the total fall of the river is 2,700 feet, with an overall average of 14.5 feet per mile. The fall in the Pemigewasset River from its source to its confluence with the Winnipesaukee River at Franklin is 2,450 feet, an average of 34.6 feet per mile. The Merrimack River from Franklin to the ocean falls only 50 feet, or an average of 1.3 feet per mile.
The northern tributaries of the Merrimack flow from the White Mountains and are generally short and steep with narrow valleys. The principal tributaries enter from the west. These rivers originate in hills from 1,000 to 1,200 feet in elevation and generally follow slow, meandering courses to the main river. The important southern tributaries, the Nashua and Concord Rivers, have flatter gradients and consequently, are more sluggish.