Not all currents occur at such a large scale. Individual beaches may have rip currents that are dangerous to swimmers. Rip currents are strong, narrow, seaward flows of water that extend from close to the shoreline to outside of the surf zone. They are found on almost any beach with breaking waves and act as “rivers of the sea,” moving sand, marine organisms, and other material offshore. Rip currents are formed when there are alongshore variations in wave breaking. In particular, rip currents tend to form in regions with less wave breaking sandwiched between regions of greater wave breaking. This can occur when there are gaps in sand bars nearshore, from structures like piers or jetties, or from natural variations in how waves are breaking.