Mean Low Water Explained

If you’re a buyer interested in coastal waterfront property, you may have heard or seen the term “mean low water” or “MLW.” You may also have heard about “mean high water” or “MHW.” These water level indicators are relevant when looking for property along any body of water affected by the ebb and flow of tides. But what do they mean?

The definition of mean low water or MLW is pretty straightforward: it refers to the average water level at low tide. Conversely, mean high water is the average height of the high tide waters. But when you’re looking at MLW and how it affects property lines, it’s a little more complicated.

What causes fluctuations in MLW?

Mean low water and mean high water are affected by the changing tides, the periodic rise and fall of seawater in relation to a fixed reference. Most of us know this phenomenon is caused by the gravitational pull of the sun and the moon over Earth’s waters. However, many variations can occur based on the actual positions of these celestial bodies in relation to Earth and other environmental factors, like wind and weather. The mean high tide and mean low tide are determined using the average of these levels in a given location over time, known as the National Tidal Datum Epoch — a time span of 19 years.

Why 19 years, you ask? The moon's relative position to the Earth follows a specific course that spans an 18.6-year cycle that covers the full range of lunar positions. So, we rounded up to 19 years. Using this period flattens the variations and makes it possible to create reliable measures you can use to establish standards to determine property lines, environmental zones, high seas boundaries, boating safety, and flood prediction.

Why is MLW important to property owners?

The special rules related to property owned adjacent to a standing body of water like an ocean, bay, delta, sea, or lake are known as littoral rights. The rules related to flowing water like streams or rivers are riparian rights.

In the United States, littoral rights relate to shoreline property, like those in Annapolis. Unlike land-locked real estate, where you can easily plot the property line with the help of a land surveyor, littoral properties are subject to the changing tides.
Usually, waterfront property lines end at the “mean high-water mark,” which is the average line where the water advances during high tide. But this isn’t always the case, especially in older properties that have established the property line in-between the mean high-water mark and the mean low-water line. These measures are critical in determining the size of a property, but it poses unique challenges since it’s a moving target.

And regardless of whether the mean high water mark, mean low water mark, or average between the two is used, when the water levels fluctuate—or rise permanently—it’s a serious issue for waterfront property owners.

Rising tides and property lines

The reason for shifting sea levels is up for debate, but there’s no question that these levels are rising, which poses a problem for coastal real estate, like Annapolis waterfront properties. When land is entirely and permanently submerged under tidal waters, homeowners may literally lose ground. Although some homeowners may build structures to protect littoral property from encroaching sea levels, some designs are limited by state and local regulations. Who officially owns the diminishing property can vary significantly between states — and so can their options when it comes to protection.

Know before you buy

Waterfront properties are beautiful, but they have some unique issues for potential home buyers. A well-informed buyer can avoid any surprises by knowing what rules apply to the specific waterfront property they’re like to purchase. In addition to making sure you’re well-versed in local tide levels and littoral rights, some other things to consider when looking at waterfront properties include:
  • Access—Make sure the property you’re interested in has access rights and the extent of that access.

  • Environmental restrictions—If the desired property is near tidal waters or wetlands, there may be some restrictions on landscaping or building plans. Be sure to do your research.

  • Water depth—In addition to knowing where property begins and ends, water depth is also necessary to know for boating options, dock placement, or other shoreline construction.

  • Flooding—Request a copy of the property owner’s elevation certificate and run it by your insurance company to determine risk and insurance expense.
Depending on your state, the land adjacent to a body of water typically carries certain rights relative to that body of water. A buyer should always consult someone well-versed in waterfront properties like the professionals at The Waterfront Group, who specialize exclusively in waterfront properties. The experienced realtors at The Waterfront Group can help you navigate the process to find the perfect Annapolis waterfront property.

To recap:

  • Mean low water or MLW: The average water level at low tide
  • Mean high water or MHW: The average water level at low tide
  • Littoral rights: The special rules that relate to property owned adjacent to static water like an ocean, bay, delta, sea, or lake
  • Riparian rights: The special rules that relate to property owned adjacent to flowing water like streams or rivers
  • National Tidal Datum Epoch: 19 years (rounded up from 18.6 years)  that span the entire celestial course the moon follows in relation to the earth
Are you ready to start looking at Annapolis waterfront properties? Reach out to knowledgeable local agent Patrick Deleonibus for expert guidance. Patrick and his team at The Waterfront Group are experts in waterfront properties and will help you navigate your next real estate transaction with the best service and guidance along the Chesapeake Bay.


The Waterfront Group was a natural decision to form. This blended the waterfront lifestyle that Patrick lives to the marketing knowledge and experience he has gained in the industry. The Waterfront Group now serves those looking to buy or sell waterfront across the region with a trusted advisor and marketing expert.