Definition
In law, MOUTH has a narrow technical application in property and water law: it refers to the point where one body of water empties into another. Specifically, in states that have codified the term, the mouth of a river or creek is defined as the point where the middle of the channel of the tributary intersects the middle of the channel of the receiving waterway. This geometric definition matters for boundary surveys, land descriptions, and the allocation of water rights where a confluence serves as a legal reference point.
Outside this specialized water-law context, the term appears occasionally in legal writing in its ordinary anatomical sense — most commonly in criminal law (assault, battery, or sexual offense statutes describing bodily contact) and in medical-legal contexts — but carries no distinct legal meaning in those uses.
Common Language
Modern common usage (Wiktionary): The opening of the face through which food is taken and speech is produced; by extension, any opening or exit point, including the place where a river flows into a larger body of water.
Historical common usage (Webster's 1913): The aperture between the jaws or lips; the buccal cavity containing the tongue and teeth; also, any opening affording entrance or exit, including the opening of a vessel or the point where a river discharges into another body of water.
The common meaning and the legal meaning overlap here, but with an important distinction. In ordinary usage, "mouth of a river" is a colloquial geographic description with no fixed geometric precision. In the legal context — as codified in California and Arizona — the term is given an exact surveying definition: the intersection of the midlines of the two channels. A researcher relying on lay intuition about where a river "mouth" is located may place that point differently from where the statute locates it, with potentially significant consequences for land boundaries.
Why It Matters in Research
The primary research relevance of MOUTH is in water-law and property-law sources, particularly those involving riparian boundaries, metes-and-bounds descriptions, and disputes over land grants or ownership along waterways. A few points for corpus researchers:
First, the statutory definition noted in Black's is jurisdiction-specific. California's Political Code (§ 3908) and Arizona's Revised Statutes (par. 931) give the term a codified geometric meaning. Researchers working in other jurisdictions should not assume the same definition applies; common law and other state codes may treat the confluence point differently or leave it to survey evidence.
Second, the term is more likely to appear in historical survey records, patents, and deed descriptions than in case reporters. Searching only case law will miss most instances where this definition is operative.
Third, the intersection-of-midlines rule is a surveying standard, not just a legal one. Researchers tracing disputes involving this term will often need to consult both legal and cartographic sources to understand how the point was actually located on the ground.
Fourth, be alert to the different uses of "mouth" across legal subdisciplines. When the term appears in a criminal statute or indictment, it is anatomical, not geographic, and the water-law definition is irrelevant. Context determines which meaning is operative.
Historical Dictionary Support
Black's Law Dictionary (2nd Ed.) addresses MOUTH solely in the water-law sense and does so by direct reference to statutory authority rather than by articulating a common-law definition. This signals that the term had no settled general legal meaning at common law independent of what individual jurisdictions chose to codify. Black's cites California and Arizona as examples, implying the definition was a legislative choice rather than a universal rule.
What historical dictionaries miss: Black's does not address how courts applied the intersection-of-midlines rule in practice — for example, what happens when channel midlines are ambiguous due to seasonal variation, braided channels, or human alteration of a watercourse. Nor does it address how the term functions in federal land grants, where the governing rule might differ from state statutory definitions. Researchers requiring depth on those questions will need to go beyond lexicographic sources.
Jurisdictional Note
The intersection-of-midlines definition is a creature of statute in California and Arizona, and should not be assumed to govern in other states. Federal land survey practice and common law may locate the "mouth" of a tributary differently, particularly in states relying on metes-and-bounds or federal general land office survey traditions.