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North Cascades National Park: How the Rivers Work

differential erosion
The effect of differential erosion: rivers draining across hard and soft rocks eventually erode the soft layers into valleys.

When a piece of the Earth’s crust first emerges from the sea and rain begins to fall on it, the water running off begins to erode it. As the first gullies develope into channels, the drainage of this new land gets organized by an interplay of three principle factors: differential erosion, stream capture, and base level.

Differential Erosion

As the water runs down hill, it erodes channels that become organized into a drainage pattern. As running water deepens its channel, it encounters rocks of different hardness and adjusts its channel in response. Soft layers erode faster than hard layers. This simple statement is an important principle of erosion which that applies to almost every natural scene. Eventually, hard layers or other hard parts of rocks stand out in bold relief, and softer layers or parts retreat into swales, gullies, and valleys. Geologists call the process differential erosion.

For much of their history, the major rivers may be able to maintain their courses across rocks of different hardness. Differential erosion will produce rapids or falls where the rivers cross harder rocks, but for the most part the rivers will maintain their general course. Small side streams, on the other hand, will be more quickly influenced and their courses will adjust to follow weaker layers of rock. A look at the geologic map will show that many rock bodies in the Metamorphic Core and Methow Domains in particular are aligned northwest-southeast. Structures such as bedding and metamorphic foliation are aligned in the same as direction. This structural trend has influenced most major streams and rivers tributary to the Columbia River in the North Cascade region. Rock hardness is an important factor in erosion, but a stream’s ability to cut depends also on how much debris it carries that is, the pebbles, sand, and silt which are its cutting tools and its gradient, which controls its velocity.

Stream Capture

Stream capture or piracy, is the process whereby a stream easily deepening its valley in soft rock can cut headward across a drainage divide to capture a portion of a neighboring stream that is working away slowly in harder rock. Examples of pure stream capture are hard to find in the North Cascades because of the overriding disruption of drainages by the growth and retreat of the Canadian Ice Sheet, but the overall parallel conformity of most North Cascade streams and rivers to the northwest-southeast-trending structural grain, indicates stream piracy has ruled for millions of years.The figure above, representing an idealized situation, illustrates the process.

Base Level

A fast-flowing stream with many rapids obviously has more energy for erosion than a slow, and placid, meandering stream. Therefore, a young stream on a newly uplifted mountain block will erode rapidly, cutting a deep valley into the rocks., and the lower parts of the stream or river, having with more water and plenty of cutting tools, will cut deeper faster than the headwaters. Soon, the lower reaches of the stream will have a low gradient that approaches the flatness of the sea or lake into which it flows. This lower end point is called the stream’s base level.

base levels

The profile of the stream moves towards a concave curve, flat at the lower end, curling up more steeply at the upper end. Eventually, if no geologic events interrupt the downward cutting of the stream, it reaches a state of pseudo-equilibrium, where the upper reaches are cutting very, very slowly because the rock resistance almost matches the energy of the falling water.

base levels
Graded stream profile and base levels. Stream is cutting at C, depositing sediment at F. Dashed line is ideal profile. (from Manning, 1967)

At the lower end, the stream cannot cut deeper than its base level, therefore, its energy goes into cutting its banks sideways. As a result, the stream meanders, gradually widening its valley.

Presumably, if the process continued long enough, the land would be reduced to sea level. Usually, however, the cycle is interrupted many times by geologic events, especially in mountains like the North Cascades. Rockslides into valleys may dam streams uplift of the land gives streams new energy glaciers grow and carve valleys into different profiles, or divert streams to a new locales and volcanoes grow and change the landscape altogether.

Any change in the base level will propagate up the stream in some manner to affect its flow. A rockslide damming the a river makes a new, temporary base level for the upper part of the stream. Naturally the stream deposits all the debris it carries in the lake that forms. Eventually the stream spills over the top of the dam and, falling rapidly down the face, begins to erode it away. All lakes are temporary interruptions in a stream’s course. We must enjoy them while we can.

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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