Gravity Wave

As a simple example, consider a lab tank with a vertical divider between salt and fresh water. Removing the divider causes the more dense salt water to move under the fresh water, creating a gravity wave.

The majority of the waves that one may see in a river or lake, whether produced by a passing boat or wind forcing, are gravity waves (those with wavelength under 7cm [see Kundu] are capillary or capillary-gravity waves).