That's a Huge River! Well, Not Exactly...Sediments in the Siuslaw River Estuary

In the photo above we're on a hill overlooking the Siuslaw River near Florence, Oregon. From this point of view the river looks huge and indeed the channel is navigable and leads to a working marina a mile or two upstream. But looks can be deceiving, and a careful observer will note that the "river" spends half of the time flowing the wrong direction. Like two of the last posts here on Geotripper, the Moon is part of the story...this is a tidal estuary. The Moon has the greatest influence on the intensity of the tides.

There is a Siuslaw River, of course, but it is just not quite this big. It is 120 miles long, starting at an elevation of 636 feet in the Oregon Coast Ranges, draining an area of about 773 square miles. The discharge of the river varies greatly depending on the season. The long-term average is around 3,000 cubic feet per second, but last week when I was there it was a mere 145 cubic feet per second. During the worst of flooding the river can exceed 50,000 cfs. But it is the lower part of the river that is affected most by the tides. During extreme high tides, changes in river level can be noted 26 miles upstream.

An estuary is the portion of a river influenced by tides where there is a constant mixing of salt and fresh water. Estuaries are rich in nutrients and one of the richest biomass producers on the planet. The estuary of the Siuslaw River developed at the end of the last Ice Age. With so much water locked up in glacial ice, sea level was hundreds of feet lower than it is today, and the Siuslaw River occupied a deep channel that continued for miles west of the current coastline. As the ice melted, sea level rose and flooded the river valley, but the Siuslaw River carried vast amounts of sediment to fill the flooded channel, forming the flat level valley we can see today. Additional sediment is added by wind blowing sand from the coastal dunes that line the lower channel (the Siuslaw is at the northern end of Oregon Dunes National Recreational Area).

When I was there a week or two ago, we experienced an intense low tide that exposed some rarely seen sedimentary structures in the channel. When the tides rise, water rushes upstream, and when the tides fall, the water goes out to sea. Flowing over the loose silt and sand, the flow causes the development of gigantic ripples like those in the pictures above and below.

It didn't occur to me at the time, but this is not a natural channel. Because of the ship traffic, the channel is dredged to maintain sufficient depth for the boats to pass through. There are other changes in the last century. The drainage of the Siuslaw is one of the most heavily logged regions in Oregon, and the clearcutting of timber has changed the nature of slope failure and flooding on the river. Trees and logjams used to trap sediment upstream, providing a rich breeding ground for salmon. The logjams were removed and the river scoured the channel to bedrock in many places. One of the most destructive activities was the process of "splash-dam" logging. Temporary dams were built across the river and trees were cut and floated in the reservoir. The reservoir was then dynamited and the resulting flood carried the logs downstream to the mills. The practice, needless to say, was hugely destructive of the salmon fisheries. Over the years the salmon runs declined from hundreds of thousands of fish each year to mere thousands.

Some of the sources I checked pointed out that the Siuslaw once was the second richest salmon fishery in Oregon after the Columbia River. Efforts are being made to improve the environment to build the salmon runs. They'll never be what they were a century ago given the vast changes upstream, but there is a lot of potential for growth of fish populations. In the meantime, it is an interesting place to visit if you are ever lucky enough to find yourself on the central Oregon coast (especially during the present heat wave!).

A Convergence of Wonders: Day 6, In the Land of the Great Draining

The young man was stalking an elk along the shore of the huge lake where he had lived for much of his short life. Stepping quietly through the underbrush, he was sweating, not from the heat, but from exertion. There was no heat, even now in the height of summer. The ice lands lay just a few miles to the north, and the scattered low shrubs provided little shelter from the winds pouring off the frigid glacial slopes. He had been stooping for hours, staying out of sight of the animal. He carried no bow, no atlatl, for his culture knew nothing of these things. His weapon was a spear.

He was nearly close enough, and positioned himself for the attack. The elk looked up, startled, but the man had done nothing to reveal himself. The animal started bounding up the slope, and the man started running, hoping for a lucky throw, but he stopped, for now he heard what startled the animal. The ground was rumbling. The lake, he realized, was moving, and currents of muddy water were starting to tear at the banks. He watched, stunned, as the lake level started to drop. From his vantage point on the high ridge, he could see water surging through the gap in the hills, tearing away at the soil and rock. He could hear giant boulders bouncing in the depths.

In the next four days, the lake that he and his people had known for all of their lives simply disappeared. In its place were stinking mud-filled canyons and strange forests of dead trees and stumps that had been hidden by the lake's dark waters.

The elders had spoken of times like these, when the gods removed the lake as punishment for their indiscretions. The young man had no reason to disbelieve the stories, but neither had he ever thought that he would live to see the punishments carried out. His people prepared to migrate south in hopes of better hunting...

Before my archaeologist friends jump all over me, I can state that my story of people in Montana 15,000-20,000 years ago is based in no way on known archaeological data. It's just that I have read the story of the Spokane Floods from the point of view of imaginary witnesses downstream, but not from people who could have been living at the source of the great floods.

The story of J Harlan Bretz and the Spokane Floods is one of the legends of the geological sciences. Bretz had amassed a great deal of evidence supporting his contention that vast amounts of water had swept across Washington in a series of massive floods, but he could not pinpoint a source of the waters. Joseph Pardee, a U.S. Geological Survey scientist could. He had been studying odd terraces on the hills above Missoula Montana, and eventually outlined the boundaries of a huge lake in western Montana that had been hundreds of feet deep, with enough water to fill one of the Great Lakes. It was formed by a glacier that had blocked the westward drainage of the lake near Lake Pend Oreille in Idaho. The floods had come when the unstable dams had collapsed, sending cubic miles of water surging over the Columbia Plateau in Washington.

It was day 6 of our Pacific Northwest expedition, and after exploring the effects of the flood in Washington the previous day, we were having a look at the evidence for the presence of glacial Lake Missoula. Our day would end with our arrival at Glacier National Park.

We didn't stop there, but we couldn't help but pull off the highway and drive through the business district of Wallace, Idaho (below). If that elevated freeway looks somehow familiar, you have seen the movie Dante's Peak. Wallace was the stand-in for the town that was destroyed by the giant volcanic eruption while Pierce Brosnan ran around saving Linda Hamilton, and watching irritating grandma die in the acid lake. We looked and looked, but there just wasn't a huge volcano looming over the town...

The freeway is elevated because town residents fought to keep their historical district from being bulldozed for the highway. There isn't a lot of room in these narrow canyons. The towns grew along with logging and mining (this is a big silver and lead mining district), and there wasn't a lot of level ground to work with.

After stops at the Montana state border for a discussion of mining, and at St. Regis for a discussion of whether to have breakfast or lunch, we set off for the Camas Prairie, the setting for some of the most dramatic evidence of Lake Missoula and the Spokane Floods. The prairie is a bit unusual for western Montana due to a lack of forest cover. There are no through-going rivers to speak of, and a rain shadow effect that keeps precipitation low. The grasslands expose some fascinating features.

As we looked around, we could see that the valley floor was not flat, but had an undulating surface. The strange low hills are hard to visualize from ground level, but are linear ridges hundreds of feet long and as high as 35 feet. The ridges are around 300 feet apart from each other.

The drying grass highlights the ridges. The best view is from above (see this USGS page for a good shot), but we had a pretty good angle from near the top of Markle Pass. These ridges are ripple marks! Just like the kind you can see in the silt of any river. Of course, in a river the ripples may be an inch high, and on the Camas Prairie, the ripples are 30-35 feet high. How? In short, the ripple size is determined in large part by velocity. Water was moving over the surface of this prairie at speeds of 50-60 mph, in water that was hundreds of feet deep! It is astounding to stand at the crest of Markle Pass and imagine the scene during the collapse of the ice dam...

 Before we left, I couldn't help snapping a few pictures of the many flowers littering the valley...

 I figure the first is some kind of clover, but I don't know the other. ID is welcome!

As we headed towards Kalispell and Glacier National Park, we could see strand lines from glacial Lake Missoula on the surrounding hillsides. They are subtle in the picture below, but the horizontal benches record multiple shorelines as the lake level flucuated.

We arrived at the western edge of Glacier National Park late in the afternoon. The afternoon storm was breaking up over Lake McDonald, making a spectacular play of clouds and shadows (see the photo at the top of the page), but my attention was also drawn to the pebbles on the shore. The rocks are a literal rainbow of bright colors.

 There was also a real rainbow along the lakeshore...I love those things!

As the sun came out, we headed south towards Marias Pass, because the Going to the Sun Highway was still buried under as much as 35 feet of snow. It is expected to finally open next week, making it the latest opening ever. We finally arrived at our camp at Lake St. Mary late in the evening. There were a few changes to our camp protocol...we were in bear country now.

The next day we had to decide whether we would flee the country or not...