Tuesday, January 6, 2009

Guest column: In Cook Inlet, change stays the same

A number of years ago I spent an evening perched 25 feet up on the cold, windy Cook Inlet bluff south of Deep Creek, watching the tide come in.

A brisk wind was coming out of the southwest and every four seconds a muddy, gray wave crashed onto the beach and dissipated into spray, bubbles and foam. As the water slithered back out, it rattled the beach cobbles menacingly like a shaman working mysterious magic with a puffin-beak rattle. The shaman-waves broke again and again, rhythmically chanting their ancient geological feast‑mantra, “Closer...closer...closer.”

The 20‑foot tide was rising fast, and the sediment‑crazed surf groped for the base of the bluff until finally it nibbled at the talus below my perch. During the 40 minutes of slack, high water, the hungry waves grazed on Tertiary sediments in a 100-mile long feeding frenzy. Eventually the ebbing tide pulled the ravenous water away from the bluff, but not until 600 breakers had been fed.

From Point Possession to Homer, Cook Inlet had taken another long bite out of the bluff. Tikahtnu was satiated — but only until the next big tide.

Most of Cook Inlet’s waves break harmlessly onto the beach, expending their energy on mundane tasks like rearranging sand, polishing stones and erasing footprints. But when the highest tides lift the waves to the base of the bluff, a set of events begin that result in a slowly changing coastline.

When tides reach the base of the bluff, the breaking waves wash out a notch and carry the sediments out to sea. Gravity likes her angles, and this disruption of the bluff’s angle of repose is not to her liking. In time, sand and gravel from above the notch fall down and fill it in. Eventually, the former angle is re-established, gravity is once again content, and the coastline has moved inland a little bit.

The usual process is for individual particles to fall in a slow trickle of downslope movement. However, if the area is saturated with water, a large part of the bluff may surge down in an earth flow and, like the Dow on a bad day, the property owner finds himself a few thousand dollars poorer. Unlike the stock market, however, there will be no recovery.

Usually only a few inches of bluff are removed at a time, but a big tide accompanied by strong onshore winds can take out as much as 5 feet. Over the years, landowners have tried everything imaginable to shore up their investment. Breakwaters have been constructed of pilings sunk into the beach fortified with everything from cement-filled barrels, to old car bodies, to the kitchen sink. Sometimes the erosion has been retarded for a few years, but in the end, the inlet always wins.

Nothing will stop the erosion of the bluff, with the possible exception of encasing the Cook Inlet basin in fiberglass.

Contrary to what one might expect, erosion of the west coast of the peninsula from Point Possession to Kachemak Bay is not due to a rise in sea level. It is due to the exact opposite — the land is rising, like Neptune coming out of the sea, causing a relative lowering of sea level.

Why should the western half of the Kenai Peninsula be rising? According to geologist Richard Reger, the reason is crustal rebound. During the Pleistocene, millions of tons of glacial ice covered the peninsula, depressing the thin crust into the less viscous mantle below. As the ice receded, the weight has been removed and the crust is slowly rising to its former state. The process has almost run its course, but over the past 10,000 years, waves have constantly cut into this rising coastline, making a new bluff.

Today the bluff has stabilized in several places but, in the long run, this, too, is only momentary. The next big earthquake or global climate change will revise the land-sea boundary and again our coastline will be changed.

When the tide lowered, I moved from my perch down to the now-exposed beach. As I picked my way down, a bit of siltstone broke away from the cliff high above me and the particles rained down around me, destined for the notch below recently washed out by the high tide. The siltstone had been layered there for 5 million years, and now, thanks to rebound, tides, waves and the pull of gravity, those particles were starting another trip on their journey through the geologic cycle.

Nothing, it seems, is forever.

Alan Boraas is a professor of anthropology at Kenai Peninsula College’s Kenai River Campus.

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