Inland waters are any bodies of water located on the landward side of a country’s coastal baseline, plus all freshwater and saline water systems found entirely within a nation’s land territory. This includes lakes, rivers, streams, reservoirs, ponds, wetlands, estuaries, harbors, and even groundwater. The term carries slightly different meanings depending on context: geography, international law, and navigation rules each draw the boundaries a little differently.
Inland Waters in Geographic Terms
In the simplest sense, inland waters are water bodies found on land rather than in the open ocean. These include lakes, reservoirs, rivers, streams, wetlands, ponds, estuaries, and groundwater systems. They can be freshwater (with salinity below 1 gram per liter) or saline (above that threshold). Saltwater lakes, desert springs, and brackish estuaries all count as inland waters alongside the obvious rivers and freshwater lakes.
These systems are enormously important ecologically. Many contain unique species found nowhere else on Earth, with single lakes or river drainages sometimes hosting entire assemblages of species that evolved in isolation. They also provide drinking water, irrigation, transportation, and recreation to populations worldwide.
The Legal Definition Under International Law
Under the United Nations Convention on the Law of the Sea (UNCLOS), internal waters are defined as all waters on the landward side of the baseline of the territorial sea. That baseline is the key dividing line: everything shoreward of it is internal water, and the territorial sea extends 12 nautical miles seaward from it.
This matters because a nation has complete sovereignty over its internal waters, the same way it controls its land territory. Foreign vessels have no automatic right to pass through. That’s a significant legal difference from the territorial sea, where ships of all nations enjoy what’s called “innocent passage,” meaning they can transit peacefully without needing permission.
There is one exception. When a country draws a straight baseline (connecting headlands along a jagged coast) and that line encloses areas that weren’t previously considered internal waters, foreign ships retain the right of innocent passage through those newly enclosed areas.
How Baselines Determine the Boundary
The baseline is the foundation of all maritime boundaries, and understanding it explains where inland waters end and the territorial sea begins. There are two main methods for drawing one.
A normal baseline follows the low-water line along the coast as marked on official nautical charts. The United States uses this method. NOAA charts mark the baseline using mean lower low water, the average of the lowest daily tides recorded over time. The U.S. specifically chose normal baselines over straight baselines to preserve broader freedom of navigation in waters along its coast.
A straight baseline connects prominent points along deeply indented or island-fringed coastlines, creating a simplified outer boundary. Countries like Norway use this method because their fjord-heavy coastlines would produce an impractical zigzag if a normal baseline were applied. The trade-off is that straight baselines can enclose large swaths of water as “internal,” which is why the innocent passage exception exists for newly enclosed areas.
Bays get special treatment. A closing line can be drawn across the mouth of a bay, and everything behind that line becomes internal water. Rivers also have closing lines drawn across their mouths where they meet the sea. These closing lines, combined with the low-water line, form the complete baseline from which all other maritime zones are measured.
U.S. Navigation Rules and Inland Waters
For boaters in the United States, “inland waters” has a very specific practical meaning. The U.S. Coast Guard defines inland waters as the navigable waters of the United States shoreward of official demarcation lines that divide the high seas from harbors, rivers, and other inland waters. The Great Lakes on the U.S. side of the international boundary with Canada are also included.
This distinction matters because different navigation rules apply on each side of the line. Offshore, vessels follow the International Regulations for Preventing Collisions at Sea (COLREGs). Once you cross shoreward of the demarcation line into inland waters, U.S. Inland Navigation Rules take over. The two sets of rules are similar but differ in specifics like whistle signals, light requirements, and right-of-way procedures. If you operate a boat, knowing which set of rules applies to your location is essential.
The inland rules also specifically reference “Western Rivers,” which include the Mississippi River, its tributaries, and connected waterways. These rivers have their own additional provisions because of their strong currents and heavy commercial traffic.
The Great Lakes: A Unique Case
The Great Lakes illustrate how the definition of inland waters can shift depending on which law you’re applying. Under international law as reflected in UNCLOS, the Great Lakes are considered internal waters because they sit landward of the baseline from which the territorial sea is measured. For navigation purposes, the Coast Guard classifies them as inland waters.
But several U.S. federal court decisions have treated the Great Lakes as “high seas” for purposes of admiralty, maritime jurisdiction, and federal criminal law. Federal statute specifically defines the Great Lakes, the waters connecting them, and the St. Lawrence River as part of the “special maritime and territorial jurisdiction of the United States” for criminal cases. Eight U.S. states border the Great Lakes, and under the Submerged Lands Act, each state’s jurisdiction extends to the international maritime boundary with Canada.
So the Great Lakes are simultaneously internal waters, inland waters, and functionally “high seas” depending on the legal question being asked.
Where Inland Waters End on Shore
Just as the baseline marks the seaward edge of inland waters, there’s also a landward limit. For non-tidal waters in the United States, that boundary is the ordinary high water mark (OHWM), the line on the shore where water levels have been high enough, long enough, to leave a visible mark on the landscape through changes in soil, vegetation, or debris patterns.
The OHWM defines federal jurisdiction under the Clean Water Act for non-tidal waters when there are no adjacent wetlands. It also sets the lateral limits of jurisdiction under the Rivers and Harbors Act of 1899 for non-tidal navigable waters. In practical terms, activities like filling, dredging, or building structures below the ordinary high water mark typically require federal permits, while activities above it generally fall under state and local authority alone.