The Merrimack River basin's physical setting is comprised of two extremes, mountainous terrain to the north and coastal plain at the river's end.

Headwaters

The headwaters of the Merrimack rise in the White Mountains. Glacial erosion has carved a rugged terrain, with peaks reaching elevations of 4,000 to 5,000 feet above steepwalled river valleys. Mount Lafayette, located at the northernmost tip of the basin, is the highest of its peaks at 5,249 feet. The Pemigewasset River falls approximately 1,200 feet in its upper 10 miles through narrow valleys of exposed bedrock.

Northern New England Upland

At the confluence of the Pemigewasset and Mad Rivers, the topography begins to change to the Northern New England Upland province. The terrain in this area is rough and steep, with a topographic relief of 400 to 800 feet and maximum elevations of 2,000 feet.
In the northeastern section of this area, Lakes Winnipesaukee, Squam, and Winnisquam occupy a major depression carved out of the land surface by glacial action.
The main stem, beginning in Franklin, descends southward through a more moderate terrain with relief varying between 200 and 300 feet. The river valley gradually broadens in this area forming a wide corridor through the center of New Hampshire. The Contoocook, Soucook, Suncook, and Piscataquog Rivers drain the southern half of this area.

Southern New England Upland

In the Southern New England Upland area the terrain becomes hilly to gently rolling. Tributaries meandering across relatively flat areas of glacial outwash have cut broad valleys and flow generally in a northeasterly direction to the Merrimack main stem.
The Nashua is the major tributary in this area, flowing out of the southwest corner of the basin to its confluence with the main stem at Nashua, NH.
The Merrimack turns eastward just south of the MA state boundary and flows toward the coast. The river is tidal for the 22 miles below Haverhill with a mean tidal range of 7.9 feet at the mouth and 5.1 feet at Haverhill.
The river widens as it passes through the estuarine zone at Newburyport and finally flows through a narrow channel between Plum Island and Salisbury Beach into the Atlantic Ocean.

Seaboard Lowland

To the north and south of the lower main stem are areas of Seaboard Lowland. These areas tend to be gently rolling or level with a few, low rough hills occurring at inland locations.
The tributaries in these areas, notably the SuAsCo Rivers, are sluggish and in some instances falling only one inch per mile. Fresh and saltwater wetlands are common features in this landscape.
The Shawsheen River has the most extensive wetlands of the Merrimack.

Wisconsin Glacier

The topography of the Merrimack Valley was created in the recession of the Wisconsin glacier.
The most prominent result of this process is what geologists call drumlins; a hill made of till (a mixture of sand, clay, and boulders) left by a glacier.Tower and Prospect Hills in Lawrence are examples of drumlins in this area.
Even today, the land of the Merrimack Valley is reacting to the glacier, freed thousands of years ago from the immense weight of ice 5,000 feet thick, the land is still rebounding. The adjustment this causes in the earth's crust may explain the earthquakes that have been recorded here since the 17th century.

Boundaries

The Merrimack River watershed is bounded by the Connecticut River basin on the west and northwest, the Saco and Piscataqua River basins on the northeast and east, the New Hampshire and Massachusetts coastal areas on the east and southeast, and the Blackstone River basin on the south.

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