Return to Coastal Currents: Waves. The highest surface part of a wave is called the crest, and the lowest part is the trough. The vertical distance between the crest and the trough is the wave height. The horizontal distance between two adjacent crests or troughs is known as the wavelength. Wave height is affected by wind speed, wind duration, or how long the wind blows, and fetch, which is the distance over water that the wind blows in a single direction. If wind speed is slow, only small waves result.