The key ingredients needed to build a dune system include a source of sand, a shoreline perpendicular to the prevailing winds and a low landscape over which dunes can migrate. In addition, plant species that are adapted to survive the drying winds and shifting sands are needed to help shape and build the dunes. Here in the Humboldt Bay area, the Mad and the Eel rivers supply most of the sand. Winter storms flood these rivers and transport sand to the ocean. Sand is carried by currents along the coast and pushed up on to the beach by gentle summer waves. Once dry, the sand is moved by the prevailing summer winds from the northwest. This dynamic process has created of a variety of dune habitats within a narrow stretch of coastline.