Glossary of "Environmental Science: Earth as a Living Planet (2nd ed.)" by Botkin & Keller (1998)
- The point on a rive where there is an abrupt drop in elevation of the land and where numerous waterfalls occur. The line is typically located where streams pass from harder to softer rocks.
- Botkin, D. B. & Keller, E. A. 1998. Environmental Science: Earth as a Living Planet, 2nd ed. -xxxxii + 649 pp. John Wiley & Sons, Inc., New York.