By Neva Knott
It’s a crisp and sunny Saturday in late September. Tillamook Head looms a short distance to the south. It’s a quick pull-off from Highway 101 to park and walk down to the estuary on Neawanna Point in Seaside, Oregon. It’s National Estuary Week, and I’m walking the land with the North Coast Land Conservancy.
Neawanna Point is the land mass of the Necanicum Estuary. It is covered in Sitka spruce dunes, coastal prairie, and tidal marsh. The topic of our walk was the formation and characteristics of the various elevations of terraces across the point, and what these variations tell us. Such changes in elevation are indicated by vegetation change, so we learned some plants along the way. The goal of the information was to help those of us on the walk understand why conservation of this estuary is important. Geologist Tom Horning proved an engaging and informative host.
Estuaries are unique ecosystems. They are the places where rivers meet the ocean, where fresh water flows into salt water and where the two mix as tides flux. Estuaries are places of outflow and inflow. They are the endpoints of watersheds. This estuary is classified as a coastal plain, formed by sea level rise. The sea level here has risen about 20 feet in the last 4500 years. Horning explains that, in simple terms, geology is a process of oceans rising, land forms sinking or building up. Here, the land mass is building up at the same rate the sea is rising. Within this equation, the amount of sand in the soil tells about the force and wave energy hitting the shore.
Geology and the mix of fresh and salt water are the features used to classify estuaries. This mixed water is called brackish. The salinity levels of estuary water change repeatedly with tide change, from what comes downstream through the watershed, and because of weather. NOAA, the National Oceanic and Atmospheric Administration, continuously monitors estuary water quality. Automatic data loggers record temperature, depth, salinity, turbidity, and pH. These are the aqua-indicators of estuary health.
As we pause along the creek-side, looking out to the shoreline, Horning states that geology creates habitat. With a sweep of his arm, he explains that everything we’re looking at is created through an exchange of carbon and oxygen. The Sitkas fall and become driftwood, and will eventually sink to the bottom and degrade into cellulose. This matter becomes food for inhabitants on the ocean floor. On the landscape level the same process occurs—trees fall, degrade, and are eaten by bugs, or continue to degrade and become peat. Estuaries are very rich from the constant pulse of waters coming in. The sediment deposited carries nutrients, but also pollutants. This creates a dual function for the coastal plain—it is at once a feeding station for its inhabitants and a cleansing agent for its waters. During times of flooding, estuaries act as sponges, with the peat soil absorbing extra water.
It’s a flat but subtly varied landscape. The growth of the estuary land mass is marked in terraces. The lowest terraces have spots that are naturally barren; they are underwater too much of the time to grow vegetation. The lower terraces are also populated with salt-tolerant plants, those that can function when saturated by incoming salt water. The dominant species here is commonly called pickle weed. It is edible, tastes salty, and is foraged as food. Pickle weed signifies newer terrace formations at the lowest level—these terraces are about 13 years old. Sea barley marks the next change in elevation, just barely higher than that of the pickle grass. Plants that seek salt are known as halophytes. Plants that can adapt to the ongoing changes in salinity in their habitats are known as euryhaline. There are relatively few species of euryhalines because it takes an exceptional amount of energy to constantly adapt to varying degrees of salt intake. There’s a slight distinction here, and an important one in using plants as salt-level indicators—pickle weed is halophylic—it likes salt. Sea barley can tolerate salt, but doesn’t seek it. This little difference is how scientists like Horning are able to mark changes in the landscape by looking at the vegetation. As I look down, I notice the sea barley is on slightly higher ground and a bit farther away from the shore.
Halophytes and euryhalines can regulate how much salt comes into their systems. This is done with changing root and membrane structure. This tissue is constantly adapting to allow for more or less salt from the water.
As our guide explains how the vegetation marks the elevation, age, and saturation of the terraces, he also explains other details of processes that go on here. The vegetation traps debris from the incoming water, cleaning the water as it flows inland across the marsh. The amount of sand found in the soil mix tells of the high wave energy coming in from the ocean. The amount of shell debris tells of sand spit erosion and disappearance. Sand bars are not static, as is often assumed, but fluctuate rapidly and regularly. And those buried logs didn’t just fall into the sand; they are there because of a volcanic event or tsunami. Some of them may have floated in as driftwood; the root position explains if the tree was originally growing here or not.
We walk on. The next level is determined by location farther in from the tide line and by Pacific Beach Grass. Here we find, in about three inches of soil, embedded logs. Sitka spruce cannot live in the salty water-logged soil of the marsh. The placement of these embedded logs tells us that we’ve reached the terrace where the water that flows across the land is most filtered and fresh rather than salty. These logs are estimated to be from about 1700 AD, giving a marker of landform change over time here. A few yards further, the marsh turns to prairie and then to dune, very old Sitka spruce stand tall as a forest.
Entering the Sitka forest is like going down the path into the fairy tale world of Hansel and Gretel. The trees are old and moss-covered. We learn this was the site of a Clatsop Indian village about 2500 years ago. The ground on which we stand isn’t really soil, but is a five-foot deep driftwood pit, in various stages of decompositions—trees slowly turning to soil. Our guide points out the pattern of the shoreline that protects this stand of trees; he calls it a spit hook, a configuration that forms from the interplay of sand and wind. Because of this combination of landscape features, there is an unusual salmon run here at the mouth of the river. The fish run just about New Year’s, most likely one of the reasons the Clatsop held a village here.
Just outside the forest, there is a place at the dune’s edge that Horning is most intrigued by. It’s an extension of a gravel ridge that runs from Tillamook Head that sits several miles down shore, and was created about 2,500 years ago, then buried by sand. This gravel ridge, made up of Columbia basalt, was tossed up into the dunes by the wave action of a tsunami. Horning speculates that this could have been the cause for the Clatsop relocation.
While the terraces tell the story of land build up from sediment deposits, the driftwood piles and cobble berm give evidence tsunami activity. This small segment of coastline has been battered by such big waves in 1700, 1899, possibly the 1930s, and in 1964.
As we’re making our way back out of the forest across the basalt cobbles and back onto sand, several crows fly out of the trees and circle, clearly checking our activity. Another scientist with us points to a sandbar. Several gulls gather there, and are engaged in what he calls salt-cropping. Birds are apparently found of salt so gather at low tide to eat it up.
The group of us pause to take it all in. In two hours we’ve identified ten species of plants, learned a little Indian lore, seen how waves toss sand and rocks upon the shore, and now what all this means in terms of landscape ecology. As a wrap-up, Horning points out a couple of human influences on the estuary. Though this property is under conservancy, it is considered anthropogenic. Simple uses such as the harvesting of driftwood for stove fuel has caused sterility to the world underwater in terms of available food. Development upstream has caused changes in creek flow and levels. Tide gates, once thought to be protection from floods and for fish are now known to increase floodwater levels and to disallow fish passage.
Better understanding of human and non-human use of such aspects of an estuary is the goal of conservancy. With the historic overlay, ecology and geological information given by Horning the interdependency between humans and landscapes as they shift over time is clear.