Wednesday, July 29, 2015

Vagabonding on Dangerous Ground: A Geologist Walks Onto a Bar in Cascadia...

A very strange-looking sandbar at Big Lagoon in Humboldt Lagoons State Park on California's north coast.
A geologist walks INTO a bar. She may get hammered, vulcanized, laminated, stoned, cemented, bombed, or petrified. And all her drinks will be on the rocks...

But when that geologist walks ONTO a bar, she just gets sand on her feet.

Yeah, yeah, I know, shut up and stick with the science...

We're traveling north on a journey through the Cascadia Subduction Zone, exploring this unique region with an eye to the geology, and the beauty, of the region. We've explored the Redwood forests of the Eel River, and the Lost Coast where the Cascadia zone begins. Today we are looking at a unique state park along the coast north of Eureka, California, and south of Crescent City. It's called the Humboldt Lagoons State Park, and the three lagoons found there are bounded by stunning examples of baymouth bars.
Big Lagoon at Humboldt Lagoons State Park (source: Google Earth)
One of the reasons that the already infamous Cascadia Subduction Zone earthquake might not be as bad as it could be is that so few people actually live on the coast of Washington, Oregon, or Northern California. Why is this? If one looks at a map, one notes the paucity of flat lands along the coast. There are sometimes some coastal terraces and a number of small natural harbors, but for most of the distance between Vancouver Island and Cape Mendocino, the mountains rise from the sea. There's no place to build. That's not to minimize the tragedy. There will be horrible results, but the largest cities like Seattle and Portland are inland, behind the coastal ranges, and they will be spared the worst of the tsunami damage, if not the shaking. There are certainly a number of towns along the coast, but there are also long stretches with few people. One of the lightly populated stretches of coast is between Eureka and Crescent City.
Big Lagoon at Humboldt Lagoons State Park
Along Cascadia's mountainous coast, wave action is violent and constant, and cliffs are quickly worn back, forming a relatively straight coastline. But there are also a number of drowned river valleys and coves, many caused by the rise of sea level after the last ice age. There are four small examples of these valleys at Humboldt Lagoons (one of which was filled in to allow some farming). But the coastline is very straight. Why?

Intense rainfall sends sediments down the many rivers and streams, and vast amounts are added to the coastal waters. When waves encounter the coast at an angle, the swash and backwash of the water causes sediment to be transported along the coast, a process called longshore drift. There is a tremendous amount of sediment in this coastal system.
Humboldt Lagoon also include a county park at the south end of Big Lagoon. It's a nice place to see coastal erosion!
When waves hit rocky coasts, they expend their energy wearing away at the rocks. When those same waves reach a cove of open water, the wave energy is dissipated as the waves spread out. When sand is being transported around the headland and into a cove, the declining energy causes the sand to settle into a curving sandbar, called a hooked spit. The spit may grow large enough to close off the mouth of the bay, becoming a baymouth bar.

In most bays along the Cascadia coast, rivers are large enough to keep the bays open, but at Humboldt Lagoons only small streams are present. Water can simply seep through the sandbar rather than flowing out. The bays are breached only during the wettest, most intense storms. The baymouth bars are miles long, and incredibly straight (see the Google Earth image above). They don't look natural, and yet they are.
Smaller Stone Lagoon is just north of Big Lagoon.
The lagoons are a fascinating spot along the north coast, and they get a bit less attention than Redwoods National Park and the state parks. They are an important link in the ecosystem of the region, and are marvelous spots for birdwatching and looking for other animals. If you ever travel that way, the state park (and associated county park) is well worth a visit.

The next stop on our vagabonding tour of the Cascadia Subduction Zone is Crescent City at the far north end of California. We'll have some things to learn about tsunamis in California.

1 comment:

Celia Lewis said...

Wonderful description and photos, Garry. I've been down the coast several times years ago, but never turned off to Humboldt Park. Thanks for sharing your information with us all. Baymouth bars - I'll know them now!