Part of a watershed
As water flows it changes in relation to the landscape around it. The physical properties of flowing water determine the life that inhabits it.
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The area that a stream drains is called its watershed. It is also called a drainage basin, or catchment area. Watersheds are often named by the largest body of water. Our watershed would be the Skykomish watershed. Smaller watersheds flow into the Skykomish, so our school is located in the French Creek watershed. Water flows downhill. If you were to stand next to a stream at the bottom of a valley, all the land you see that slopes downward toward your river is part of the river's watershed. Some watersheds are so big that you can't even see where their downhill flows begin. For instance, if you stood by the Skykomish River, you might not be able to see the mountains where the river begins, Yet, rain that falls up towards Stevens Pass ends up passing by you as you stand next to the river, maybe fifty or more miles downstream. |
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Part of a watershed |
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In the Pacific Northwest, we are surrounded by mountains and hills. Water flows from the tops of these peaks to the lowest points. A large river such as the Skykomish River starts as several smaller little flows, which join to become increasingly larger ones. Streams are classified by their size and by how many streams have joined into them. Typically flowing water is classified in four sizes. The smallest streams that have water flowing in them year around and have no other streams flowing into them are called first order streams. When two first order streams meet together they form a second order stream. When two second order streams meet, they form a third order stream. As streams go up in order, they get bigger, so the largest classification would be where two third order steams join to form a fourth order. The Skykomish River and the Snoqualmie River - both fourth order rivers - meet west of Monroe and become the Snohomish River, also a fourth order river. If you could trace a drop of water in the Skykomish River back to the smallest point it might have come from, where would you end up? |
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Stream order shows size and position downstream |
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The steeper the land that water flows down, the faster it will flow. The faster water flows, the more energy it has to carry things. A fast flowing river will carry lots more dirt and bigger particles than a slow moving stream. Sections of streams are called reaches. In the mountains, the upper reaches of a stream have steep slopes and high energy. These waters tend to travel in straight stretches and erode deep, V-shaped valleys. There are lots of large, boulder sized rocks in the stream bed and often a series of pools and flows. In the middle reaches, the rivers run through valleys and curve back and forth like a snake. There might still be strong flows but the bottom is more sand and small rocks than big rocks. In the lower river reach, the energy is very low, the river moves slowly and the river bottom is small gravel and mostly silt. In our area, the lower reaches are affected by the tides of Puget sound and in their natural state, these lower reaches break into many different channels. |
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A high energy upper reach |
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A meandering middle reach |
The lower reach, where a river meets Puget Sound |
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When water rolls down the slopes of a watershed, it carries things
with it. It dissolves chemicals and carries them. If the river starts at a glacier, it will carry glacial
flour, which is sediment the glacier has made by grinding the
rock beneath it finely, making the water look almost milky. And it
carries organic matter: tiny bits of leaves, bacteria, and a lot of
other things too small to see. As it gets larger, it carries larger bits of
matter. By the time the water gets all the way down to the lower reaches, it is
full of whatever was on (and in) the land around it. The river can carry
sticks, leaves, logs, brush, and even sand, pebbles, rocks, and
boulders.
Natural events alter the river's ecology by changing the type, size, and amount of things carried. Mudslides, heavy rainfall, and floods can make drastic changes. In our area, winter floods create short periods of large volumes of fast moving water which dramatically increases the water energy in all of the reaches. The power of flood waters is enough to move house-sized boulders and the banks can erode causing trees to fall into the water which become battering rams pounding the banks even more.
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Water full of sediment |
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Flood water with lots of dirt in it |
A mudslide |
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When the water is flowing fast, like in the upper reaches, it has
more energy and the water
can carry heavier things. As the water gets into the middle and lower
reaches, or goes around a corner, the water slows down and loses energy and things
drop out of the water. For example, fine sediment like silt is
very light and can be carried further downstream into slower water than small rocks. So moving water deposits
materials based on size in relation to how fast the water moves. If you
find a bunch of big rocks in a pile on the bottom of a stream, you know
that this was where the energy of the water was no longer strong enough
to continue to move the rocks so they stopped at that point. When water moves
around a corner, like a bend in the river, the energy of the water
is strongest at the outside bank, and so on that side of the bend the
water will often form a deep pool. On the inside bank, the water moves
much slower, and any gravel or sand that is being carried can be deposited
and form a point bar, made of sand or gravel. So the energy of
the water will erode the far bank, and deposit material on the near
bank.
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Patterns of silt, sand and gravel deposit |
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A river bend showing a point bar and a pool |
A pile of logs carried by flood water |
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As a river goes around bends and forms bars and pools, the growth
of the bar pushes the current even further into the pool, increasing its
erosion. Over the time the river bend will move as the bar grows and the
bank with the pool erodes. Sometimes a river bend will eventually
become such a strong angle that the energy of the river will cut through
the point created and the river will change shape again. When
rivers cut through a point, they leave behind an arc of the old river
called an oxbow. Often you can find places that are now
covered by forest which were once part of a river.
The high water and energy of floods changes the patterns of where materials might get dropped and in some places huge piles of logs, rocks, even parts of bridges and houses can stack up and then form barriers - causing the water to flow in new directions. Sometimes a single flood will redirect the course of a river into a completely new area, leaving the old river bed to go completely dry. More often the river may split into two or more channels. |
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An old river channel that is now a forest
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A flood redirected a river through this house |
An oxbow |
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A stream has different types of habitats which are used by different types of organisms in different ways. A cascade is a fast moving steep part of water that tumbles noisily down a slope, over larger rocks or logs. A plunge pool is a deep pocket in a cascade. Cascades and plunge pools are found in the upper reaches of streams. Riffles are shallow fast moving white water, often with medium to small sized rocks that can be seen. Pools are deeper areas of slow water. Runs are fast flowing stretches that are deeper than riffles so you can not easily see the bottom and do not have white water. Glides are deep slow moving water. |
