In a sense that is an oversimplification, because erosion was a major factor before the glaciers came, but almost all evidence of prior landforming aside for the obvious uplifiting was removed by the galciations, at least in the lowlands - but it was a crucial factor that erosion of the uplands continued DURING the glaciations and that will prove to be a really major thread as this this discussion of erosion unfolds.
Tabor and Cady decoded the matrix of rock and explained how the rivers consistently follow the easy routes to lower elevations, following and downcutting through the softer rock and spreading out into gravel banks when the encounter layers too hard to cut. You just heard something important, that you already know intuitively: the velocity of water in a river is a function of the gradient and the channel width. For a given rate of flow (governed by runoff from rainfall, snowmelt or simply the intersection of the stream channel with the water table) the velocity is clearly a function of the area of the channel.
You know this empirically, from holding a garden hose: obstruct the flow with your thumb and the velocity increases spectacularly.
In fact, the title to this piece is a multilevel pun, integrating erosion (carburundum) with Bernoulli's law and the "carburator" on the car you used to drive, that was based on Bernoulli's physics and used a "venturi" (the equivalent of the Dungeness River Bridge) and a throttle (the equivalent of your thumb on the hose) to neck down the airflowing into the manifold and thus increase its velocity in different areas to selectively pull fuel from different metering jets.
What you may not know is how particles or cobbles or boulders move downstream in a channel. The process is generally called "sediment transport" and in streams, "bedload transport" and the mechanism is called "saltation". John Downing, the author of one of the textbnooks I point to for this part of the discussion is an oceanographer and instrument designer, who also happens to be one of the world's great experts in this field. He designs and builds tools used by scientists all over the world who are studying sediment transport and monitoring sediment produicing projects.
I have had the great pleasure of arguing the actual mechanics of this with John, drawing the same force vectors shown in his drawing at the whiteboard in his office, and we have been sponsored by USDA to do fundamental research in this area of science.
This discussion of mechanisms is essential because we have inherited a landscape that is dominated by what was left behind when the ice retreated, and almost everything we do, and the long-term consequences of what we do, are directly influenced by this.
Some of it is easy to recognize at the landscape scale - at least after some one points it out to you once or twice:
As you drive west out of WRIA 17, headed toward Sequim and you look up to the southwest, across Bell Hill, you can see a series of meadowed and forested terraces that end abruptly in steeper hillsides. These are best understood as reminders of the dozens of enormous debris flows that left criss-crossed mudflows as the fjords and glacial lakes that were once perched above the ice collapsed, over the thousands of years between glacial stades.
Here is a map from Easterbrook that shows how thick the ice was .at maximum extent of the glacier.
This map is significantly enhanced by the detail found in W.A. Long's field notebooks:
I want to call your attention to the three major river systems on the northeastern peninsula, the Dungeness, the Quilcene and the Skokomish, all draining the Olympic mountains. They were still doing that when a mile of ice was covering the Puget Sound lowlands, just about like they do now.
The big Quilcene River actually changed its course significantly over the millenia that ice choked the sound: It was eventually deflected Northward along the edge of the ice - but it apparently used to discharge to saltwater on the other side of Mt Walker. Look at the Dungeness, flowing North: nearly half of that river's length is running through the area that was at least once a glacial moriane, a river of water running through another river, a river of stone. Same for the Skokomish, and the lower reach of the Quilcene. Strip back the thn layer of vegatation covering most of the landscape an this is what you'd see:
The generallly repeated fiction is that native Amrericans walked here either across remnants of this sea of ice or walked across a land bridge that appeared as a result of dramatic fluctuations in sea level (hundreds of feet) associated with the ice and the rebounding crust. Regardless of the path they followed, there is plenty of archeological evidence to place them here when a lot of the landscape still looked very much like the last pair of images in the next series.
25,000 years ago 20,000 years ago 18,000 years ago 16,000 years ago 6000 years ago
What you have to remember from this is that the glaciers left behind an almost incomprehensible amount of gravel, distributed along the margins of the ice, and pockets of clay that slowly developed from weathering of the terrestrial rocks above the ice, that accumulated in lake bottoms (remember: Puget Sound was a landlocked lake for thousands of years).
These clay layers, interspersed with gravel from glacial moraines and streamchannels formed during the retreat of the icesheet provide the basis for many of our aquifers.
The glacial landscape also extends into the saltwater regions in our bays and estuaries: our beaches and sandspits are continually formed and reformed according to predictable dynamics, governed by the same fundamental sediment transport processes that apply to our rivers and streams.
John Downing offers some profound advice to developers in his book The Coast of Puget Sound, its processes and development that applies every bit as well to terrestrial development and work around streams as to shorelines:
"In the past, siting and construction decisions have been based upon local knowledge of the sedimentation patterns in a coastal
area and quite frequently they were correct. New developments, however, now occur in areas of Puget Sound which are inappropriate for a
planned usage or where local knowledge is unreliable simply because existing information covers a short time span. Key project decisions in
the future will necessitate more complex engineering evaluations than ever before. The basis for these evaluations consists largely of ideas
about sediment transport acquired from studies conducted in other parts of the world and must be adapted to the Puget Sound region."
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