Blog · September 30, 2026
What Is Avulsion?
Avulsion is a sudden, perceptible change in a watercourse, such as a channel that jumps in one flood. When a boundary follows the water, avulsion usually leaves the boundary where the channel used to be. The new channel does not, by itself, redraw the deed.
Platbookmapper is an Amazon Associate. If you buy through an Amazon link on this page, we earn a commission. The link is a product, not the county map.
A jump, not a creep
Accretion and erosion move a bank so slowly that the law lets an ambulatory boundary creep with them. Avulsion is the opposite pace. A river cuts a new channel in a storm, or a lake shore fails in a slump you can watch. The change is sudden and perceptible. The traditional result is that the boundary stays at the old location even though the water has left it, or even though new water covers what was dry. The old thread, the old bank, or the old ordinary high-water line, as the deed defined it, remains the line. The new river is a physical fact on top of that legal line. People standing on the new bank feel that the world moved. The deed often did not.
Proof is historical
You prove avulsion with the before-and-after: older surveys, photos, agency channel records, and testimony about the event. A single modern aerial cannot show that the change was sudden. A GIS shoreline that differs from an old plat cannot, by itself, classify the difference as avulsion or as decades of erosion. County parcel maps are especially weak here. They get updated from new photography and look authoritative. They are assessment graphics. Platbookmapper links to the county GIS and does not decide avulsion, draw lot lines, or show zip codes. If your deed runs to the thread of the stream, a surveyor can try to relate the present channel to older evidence of the former thread. That is a survey. A phone GPS track of today’s water is only today’s water.
What owners do wrong after a flood
They move fences to the new channel, or they abandon land the water left because they think the state took it. Both moves can be wrong if the event was avulsion and the boundary stayed put. They can also be wrong in the other direction if the change was actually slow erosion and the boundary did move. The classification comes first. Do not cross a neighbor’s remaining land to plant a flag at a vanished corner. Do not dig in a raw bank for an old monument. A surveyor and a title lawyer are the people who can compare the call, the event, and the grants. Until they do, leave occupation where it was and do not treat the flood photo as a new plat.
What to do next
- Separate a sudden channel change from slow erosion before you move any line.
- Collect older surveys and photos instead of relying on one new aerial.
- Keep the deed’s boundary call in the analysis. A fixed call does not follow the water at all.
- Do not restake a lot from the post-flood channel with consumer GPS.
The mix-up to avoid
Letting the new channel become the boundary because it is the water you can see. If the jump was avulsion, the boundary is probably still the old line, dry or flooded.
Questions people ask
Is every flood an avulsion?
No. A flood can deposit sediment gradually over years, which is accretion, or tear a new channel in a day, which is avulsion. The same storm can do some of each in different bends. The label is local and factual.
Does avulsion change a lot that stops at a fixed iron?
If the boundary is the iron and a recorded line, the river’s behavior does not move that line. Avulsion doctrine matters when the boundary is the water. Read the call before you apply the word.
Find the county map
The county GIS map is the practical place to start. Find it from the Platbookmapper directory, then treat the recorded plat and deed as the documents behind the picture.
Not a survey. County GIS and tax parcel maps are assessment and planning layers. They are not a substitute for a recorded plat, deed, or licensed survey.