2. Tectonic Estuaries are caused by the folding or faulting of land surfaces. These estuaries are found along major fault lines, like the San Francisco Bay area in California.

3. Bar-built Estuaries form when a shallow lagoon or bay is protected from the ocean by a sand bar or barrier island. Examples of these are found along the Eastern Seaboard and the Gulf Coast of North America.

4. Fjords are U-shaped valleys formed by glacial action. Fjords are found in areas with long histories of glacier activity, like northern Europe, Alaska and Canada.

Most of today's estuaries formed because the sea level has slowly risen during the last 18,000 years, drowning river valleys and filling in glacial troughs.