Flood Zones in Merrimack County, New Hampshire: The Army Corps Dam Network

Three Army Corps flood-control projects, built after the 1936 flood

The spring 1936 flood devastated the Merrimack River watershed, submerging parts of Manchester and Nashua and damaging mills the length of the river into Massachusetts. In response, Congress authorized the U.S. Army Corps of Engineers to build a network of flood-control dams across the Merrimack's New Hampshire tributaries, several of which sit squarely in Merrimack County. Franklin Falls Dam, on the Pemigewasset River in Franklin, was built between 1939 and 1943; Blackwater Dam, on the Blackwater River in Webster, was built in 1941; and the Hopkinton-Everett Lake system, spanning the Contoocook and Piscataquog Rivers across Henniker, Hopkinton, and Dunbarton (plus Weare, just over the line in Hillsborough County), was built between 1959 and 1963. All three are still active, federally owned and operated flood-control infrastructure -- not historical curiosities.

All three share the same basic design: each is a "dry dam," meaning there is no permanent lake behind it in ordinary conditions. The river flows through or around the structure unimpeded most of the year, and the reservoir area behind the dam functions as ordinary land -- forest, fields, trails, and in some cases private property -- until a major flood event, at which point the dam's gates close and the reservoir fills to hold back floodwater before releasing it downstream at a controlled rate.

Franklin Falls Dam and Blackwater Dam: real capacity, real numbers

Franklin Falls Dam controls drainage from roughly 1,000 square miles of watershed on the Pemigewasset River. Its permanent pool covers about 440 acres, but its total flood-storage capacity, when the gates close during a major flood, is roughly 2,800 acres -- a footprint that reaches well beyond the river's ordinary banks into surrounding low-lying land in and around Franklin. The Franklin Falls Recreational Area around it spans more than 2,500 acres and is managed by the Army Corps for flood control, ecological stewardship, and public recreation between flood events.

Blackwater Dam, an earthen dam 69 feet high and 1,150 feet long at its crest, impounds the Blackwater River in Webster. Its seasonal flood-control reservoir has a maximum capacity of 93,400 acre-feet but, like Franklin Falls, is normally dry apart from the river's ordinary flow. It is one of five related Army Corps flood-control projects in the Merrimack River basin, and it significantly reduces flood risk in downstream Merrimack County communities on the Blackwater and Contoocook Rivers, specifically named by the Corps as Webster, Hopkinton, and Boscawen.

Hopkinton-Everett Lake: New England's largest flood-control project, and a real flowage easement over private land

The Hopkinton-Everett Lake system is described by the Army Corps as the largest flood-control project in New England, and its footprint is the most consequential of the three for a Merrimack County land buyer. It combines the Hopkinton Dam on the Contoocook River (76 feet high, 790 feet long, in the town of Hopkinton) with the Everett Dam on the Piscataquog River two miles away in Weare (across the county line in Hillsborough County), plus connecting canals, and spans the towns of Henniker, Hopkinton, and Dunbarton in Merrimack County. During a major flood, water is diverted between the two lakes to create a combined flood-storage capacity of roughly 91,500 acre-feet, with discharge downstream regulated to a controlled maximum of about 1,500 cubic feet per second rather than released all at once.

The detail that matters most for buying land here: the Army Corps holds flowage easements -- not fee ownership, but a real, perpetual property interest -- over privately owned land within the project's flood-storage footprint. A flowage easement gives the federal government the right to overflow, flood, and submerge the land during project operation, the right to prohibit any structure intended for human habitation on it, and the right to require its prior written approval before an owner builds any other structure, or fills or excavates the land, within the easement area. That land can be privately owned, locally assessed, farmed, or used for recreation in the meantime -- but a buyer planning to build on or substantially alter a parcel inside the Henniker/Hopkinton/Dunbarton flood pool (or, on the Hillsborough County side, the Weare flood pool near Everett Lake) needs Army Corps sign-off in addition to, not instead of, the town's own permitting.

How this differs from an ordinary FEMA flood-zone determination

A parcel's FEMA National Flood Insurance Program flood-zone designation is a separate question from whether it sits inside one of these Army Corps flood-storage footprints, and land inside a dry-dam reservoir area is often mapped into a FEMA special flood hazard area (Zone A) precisely because it is engineered to flood on purpose during extreme events -- but the FEMA map alone won't tell you whether the federal government holds a recorded flowage easement over the specific parcel, restricting what you can build there even outside of an active flood. Both questions need to be checked independently: the town floodplain administrator for FEMA zone status, and the Army Corps of Engineers' New England District (which operates all three projects) for flowage-easement status on a specific parcel.

What to check before buying near a Merrimack County flood-control reservoir

• If the parcel is in Franklin, Webster, Hopkinton, Henniker, Dunbarton, or (in neighboring Hillsborough County) Weare, ask the town assessor or the Army Corps of Engineers New England District directly whether the parcel falls within a mapped flood-storage footprint or carries a recorded flowage easement.
• Don't assume a parcel that looks like ordinary dry forest or field is unrestricted -- a dry-dam flood-storage area looks exactly like that between flood events by design.
• If a flowage easement applies, confirm with USACE what it permits: existing structures may be grandfathered, but new construction, fill, or excavation generally requires prior written USACE approval on top of the town's building permit.
• Check the parcel's separate FEMA flood-zone status with the town floodplain administrator -- this is an independent question from flowage-easement status, and a parcel can be affected by either, both, or neither.
• For recreational or agricultural use of land within one of these flood-storage footprints, confirm with the Army Corps' project office (Franklin Falls, Blackwater, or Hopkinton-Everett) what uses are compatible with an active flood-control easement before investing in improvements.