Legal Definitions - ad medium filum aquae

Ad medium filum aquae is a Latin legal principle that translates to "to the middle thread of the water." It refers to a common law presumption used to determine the boundary of land that borders a non-navigable river or stream. Under this principle, unless a property deed or other legal document specifies otherwise, the owner of land adjacent to such a watercourse is presumed to own the land beneath the water up to the centerline or midway point of the watercourse. This presumption applies to both sides of the watercourse, meaning two landowners on opposite banks would each own up to the middle.

Here are some examples illustrating this principle:

• Property Sale Along a Creek: Imagine a rural property owner selling a parcel of land that runs alongside a small, non-navigable creek. When the land is sold, the new owner, by default, is presumed to own not just the dry land but also the creek bed up to the middle of the creek. This presumption holds true unless the deed explicitly states a different boundary, such as "to the bank of the creek."

  This illustrates "ad medium filum aquae" because the property boundary is extended to the midpoint of the watercourse, granting the new owner rights and responsibilities over that portion of the creek bed and water.

• Dispute Over Stream Access: Consider two neighbors whose properties are separated by a narrow, non-navigable stream. One neighbor decides to build a small, decorative footbridge that extends partway into the stream from their bank. The other neighbor objects, claiming the bridge encroaches on their property.

  In this scenario, if their property deeds are silent on the stream boundary, the principle of "ad medium filum aquae" would apply. Each neighbor would legally own the stream bed and the water rights above it up to the imaginary line running down the middle of the stream. The legality of the footbridge would depend on whether it extends beyond the builder's presumed halfway point of the stream, subject to any local environmental regulations.

• Maintenance of a Riverbank: A homeowner owns land bordering a small, non-navigable river. Over time, erosion begins to destabilize the riverbank on their property. The homeowner wishes to reinforce the bank by placing natural stones and planting vegetation within the river itself to prevent further erosion.

  The homeowner's right to undertake such work, which involves altering the riverbed and bank, is supported by the "ad medium filum aquae" principle. They are presumed to own the land under the river up to its midpoint, giving them certain rights and responsibilities for that portion of the watercourse, provided they comply with all relevant environmental and permitting regulations.