Z Wikipedie: "Groynes (often referred to as "spur dikes" or "wing dykes"), are often constructed (nearly) perpendicular to the river banks, beginning at the riverbank with a root and ending at the regulation line with a head. They serve to maintain a desired channel for the purpose of preventing ice jamming, improved navigation and erosion control over lateral erosion, that would form Meanders. Groynes have a major impact on the river morphology; generally speaking, they cause autonomous degradation of the river. The areas between the groynes are referred to as groyne fields." http://en.wikipedia.org/wiki/Groyne
