Definition
A weir is a structure — typically a fence, enclosure, or low dam — placed across a watercourse. In legal contexts, the term carries two related but distinct senses:
1. FISH WEIR: A fence or enclosure of twigs, stakes, or netting set in a stream or river to trap or catch fish. This is the primary legal sense reflected in the historical dictionaries and in early American statutory law.
2. WATER CONTROL WEIR: A fixed or adjustable barrier built across a river or stream to raise the water level upstream, regulate flow, or divert water. This sense predominates in modern water law, property law, and riparian rights disputes.
Common Language
Modern common usage (Wiktionary): An adjustable dam placed across a river to regulate the flow of water downstream; also, a fence placed across a river to catch fish; also, seaweed.
Historical common usage (Webster's 1913): A dam in a river to stop and raise the water, for the purpose of conducting it to a mill, creating a fishpond, or the like; also, a fence of stakes, brush, or the like, set in a stream for catching fish.
The common and legal meanings are closely aligned on the surface, but the legal significance of a weir lies in its intersection with riparian rights, navigability doctrine, fishing rights, and public access. A weir is not merely a physical structure — its lawful construction and operation implicates the rights of upstream and downstream landowners, navigation easements, and public fishery rights. Common usage treats a weir as a neutral engineering feature; legal usage treats it as a potential interference with a bundle of property and public rights.
Recognized Forms
/SUBTYPES
FISH WEIR: The older legal form. A passive trap constructed in a watercourse to intercept fish. Common in early colonial and English law regulating fisheries. Subject to licensing, abatement as nuisance, or prohibition where they obstruct navigation.
MILL WEIR: A barrier raising water level to power a mill. Frequently litigated in nineteenth-century riparian rights cases involving flowage, backwater damage to upstream landowners, and interference with downstream mills.
FLOOD CONTROL / FLOW REGULATION WEIR: The modern engineering form. Subject to permitting under water resources law, environmental statutes, and navigational servitude doctrine.
Why It Matters in Research
Researchers will encounter "weir" most frequently in three bodies of law: fishery rights, riparian property disputes, and nuisance/public obstruction cases.
In fishery rights, the fish weir appears early in American and English law as a structure that private parties attempted to claim exclusive rights over in navigable or semi-navigable waters. Courts frequently had to determine whether a weir obstructed the public right of navigation or interfered with common fishing rights. Statutory materials are essential here — the citation in both Black's editions to the Massachusetts Public Statutes reflects that weir construction and operation was a matter of positive legislative control, not purely common law.
In riparian disputes, the mill weir is the dominant form in nineteenth-century American case law. Flowage disputes — where a weir raises water levels and floods upstream land — generated extensive litigation, particularly in New England and the mid-Atlantic states. The line between a lawful weir exercising riparian rights and an unlawful obstruction causing actionable flooding was highly fact-specific and jurisdiction-dependent.
Researchers using historical sources should be alert to spelling variations: "wear," "were," and "ware" all appear in older English and colonial records referring to the same structure. The 2nd edition of Black's itself contains a typographical rendering ("fenew vi an inclosure of wigs") that reflects the scanning and transcription artifacts common in digitized historical legal dictionaries — the intended text is "fence or an inclosure of twigs."
Navigability is a critical threshold question for any weir dispute. On navigable waters, weir construction requires public authorization and is subject to navigational servitude; on non-navigable streams, the analysis shifts to purely private riparian law.
Historical Dictionary Support
Both Black's editions define weir narrowly as a fish trap — a fence or enclosure of twigs set in a stream to catch fish — and both cite the Massachusetts Public Statutes as authority. The 2nd edition adds a Maine case citation (Treat v. Chipman, 35 Me. 38), indicating that by the second edition there was enough case law to warrant a judicial citation alongside the statutory one. Neither edition addresses the water-control or mill weir function, which is a meaningful omission given how prominently the mill weir featured in nineteenth-century American property law. Researchers relying solely on Black's for this term will get the fishery definition but will miss the broader riparian and water law context entirely.
The narrow focus of the historical dictionaries likely reflects the drafting choices of the Black's editors rather than the full legal landscape of the period. For the mill weir and flowage law context, treatises on riparian rights and American water law are more reliable guides than the dictionary definitions.
Jurisdictional Note
Weir law varies significantly between states with riparian water rights systems (primarily eastern states) and those with prior appropriation systems (primarily western states). In riparian jurisdictions, weir construction and its effects on water flow are evaluated under common law riparian principles modified by statute. In prior appropriation states, any structure altering streamflow must generally conform to the terms of a water right permit, and unlicensed weirs may interfere with senior appropriators' rights.