A visit to Prichard State Park is not exactly a life-changing event. The half acre includes a grassy swale, a couple of trees, a small parking area, and no facilities at all. There's a single sign identifying the site. There's no interpretive signage, and little information on the internet about the history or genesis of the park. I'm guessing it was some land that was donated to the state, and the officials that be couldn't really decide what to do with it.
Looking at the park, I was reminded of something that I tell my students on the opening day of every class: geology is where you find it, and every place you find has a geological story. At first glance I was hard put to imagine an interesting geological story for this place. But a moment's reflection proved the opposite.
Let's take a look at the setting of the park. It is a grassy slope that is situated along the Siuslaw River, which from this angle looks like a huge body of water. But it isn't technically a river. It's a tidal estuary, and the wide body of water flows either downstream towards the sea or upstream in response to the daily tides. In a few hours the entire mudflat in these pictures will be covered with water. The area of tidal influence extends 26 miles inland, which is nearly 20% of the entire length of the river. Upstream of the tidal influence, the river is modest, with an average flow of about 2,000 cubic feet per second. That's less than a third of the more familiar Rogue River.
The story of any part of the Earth starts with the rocks that underlie the site. A quick look at the geological map of the park region shows that the "bedrock" is composed of "Qal", translated as 'Quaternary alluvium". Alluvium is our term for any of the loose sediment that covers the more solid rock underneath. "Quaternary" is the last little gasp of geologic time, encompassing the last 2 million years. The unit might include the mud of the estuary, river and or gravel, or dune sand. The Qal at Prichard is most likely slightly consolidated dune sands, which is a material that underlies most of the coastal areas in the vicinity of Florence. Sand carried along the coast by wave transport is blown inland by persistent onshore winds, forming the dunes for which Oregon Dunes National Recreation Area is justly famous.
The picture above shows the coastline just north of Florence. The region has undergone a significant change in the last hundred years or so. Migrating dunes can certainly be a problem in developed areas, so a European species of beach grass was introduced in the 1920s in an effort to stabilize the dunes. The grass worked too well, and sand has been trapped in the area adjacent to the shoreline, building into an ever higher ridge of grass-covered sand. The area inland has been starved of sand, so it has blown farther inland, leaving a low area called the deflation plain, a region now covered by small ponds and scrubby forests.
Sand is a famously unstable foundation for buildings (it's even in the Bible), but dunes that have been stable for centuries or millennia will sometimes be consolidated by calcium carbonate or other minerals in the groundwater. That is a much firmer surface to work with, and thus the development of the Florence area has been possible.
Going back in geological time often leads to strange changes in the appearance of the landscape. For instance, during the ice ages of the last 2 million years this little park would have looked far, far different than it does today. It's not because there was any ice; the glaciers that were present in Oregon were many miles away up in the Cascades. The big different was sea level. With so much ice locked up in the vast continental glaciers that covered almost all of Canada and northern Europe, sea level dropped to around 300-400 feet lower than today. Prichard State Park a few tens of thousands of years ago would have been perched on a terrace above a Siuslaw River ensconced in a deep river gorge hundreds of feet below. The outlet of the river would have been miles to the west of where it is today.
The other very serious threat is that of tsunamis. These destructive surges of water could be developed by a massive earthquake on the Cascadia Subduction Zone that runs parallel to the coast. A quake has now been documented as having reached magnitude 9 in 1700. Such an earthquake is thirty times more powerful than the magnitude 8 quake that devastated San Francisco in 1906, and around a thousand times more powerful than the 1988 Loma Prieta earthquake (just short of magnitude 7). Tsunamis can also be generated thousands of miles away in places like Japan or Alaska. Whether local or distant, Prichard State Park would be a dangerous spot if a tsunami ever hit. There would be no "breaking wave" as is shown in many photoshopped images. The water instead arrives as a surge moving rapidly upstream at high speed. In moments, the park would be inundated to a depth of several tens of feet. The water would eventually recede, but then a second, third, and maybe fourth wave will follow.