Dune Geology

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.

Dune Ecology

Photo by Andrea Pickart The waveslope is the area of the beach that shows evidence of having been washed by waves during the last tidal cycle. Beachcombing here you can find everything from eelgrass, pacific razor clam shells, sand dollars, to the carcass of a gray whale. This is also the feeding area for a number of shore birds. They feed on small invertebrates in the sand and in the seaweeds washed on shore. The upper end of the waveslope is the strand. Here you may find plants such as sea rocket and native dune grass starting to colonize the bare sands.

The series of dunes and ridges paralleling the beach are collectively called the foredunes. In areas that are relatively undisturbed by invasive species, you can find a unique community of plants referred to as the dune mat. Here, a wide array of wildflowers adapted to the drying conditions of the dunes help stabilize the shifting sand. The dune mat is home to two federally listed endangered plant species, the Humboldt Bay wallflower and the beach layia. Take care to stay on the main trails through these areas.

Hollows/Swales form when the summer wind has removed the sand down to the water table, allowing water-loving plants to move in. During winter storms the water table rises and forms seasonal ponds in these areas. Tadpoles of the pacific tree frog and red-legged frog can be found here. Eventually forests may develop in these areas.

Few plants can survive the open sands of moving dunes. Look for mammal tracks of nocturnal animals like gray fox, skunk and raccoon crossing from the different areas. In some places, you can see that these moving dunes are slowly covering the dune forest. Large beach pine and Sitka spruce create an area that is surprisingly different and diverse. Developed soils allow for thick plant growth, with huckleberry, silk tassel, red-flowering currant and salal. The forest is also home to many species of lichens, including puffy mats of reindeer lichens, more characteristic of northern forests.

Salt marshes and estuaries are recognized by biologists as among the most productive habitats in the world. The nutrient rich waters form the basis of the salt marsh food chain. Plants such as pickleweed and salt grass are specially adapted to tolerating the salty conditions of a tidal area.

Nature & Science