Fetch is the distance that wind has traveled over open water without encountering an obstruction. In normal circumstances, long fetches create larger, more powerful waves than short fetches.
As wind travels over water, it pushes the surface of the water in the same direction, creating waves. Because smaller waves travel faster than larger waves, the smaller waves overtake and merge with the larger waves. This causes the larger waves to gain in height, so more water is exposed to the wind. This causes more friction and gives the wave more energy. The longer the fetch, the more times this process repeats until the wave crashes against an object, such as a beach, and depletes its energy.
Both the speed and direction of the wind are important in estimating the size of waves along a fetch. Constant, high-speed wind along a long fetch creates the largest waves. For example, Land's End, Cornwall, along the south-western coast of Britain, is at the receiving end of a fetch originating from near the South American coast. This area routinely sees very large waves. The British town of Dover, in Kent, has a much smaller fetch. Although fetch is an important determination of wave size, extreme winds can cause high waves even along short fetches.