WEBVTT

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Chad Kiel: Hey guys, it's Chad Kiel with 
Learning Geology and Nature Reconnection,

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and today we're learning about Alluvial
Fans.

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What is an alluvial fan?

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And I'm going to tell you about some 
interesting data about this alluvial fan,

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and the processes that created it,
because it happens to be in

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my backyard.

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Let's go check it out.

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So before we dive down the rabbit hole
of alluvial fans, let's first take a

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second to talk about weathering and 
erosion, because those are two factors

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that are gonna come into play again and
again when we're talking about this

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alluvial fan.

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Right here around 6,500 feet into the
mountains, temperatures can vary from

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20 degrees each day, causing the rocks to
kind of expand and contract, and this

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expanding and contracting's gonna
cause fracturing.

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Now these fractures are susceptible for 
water to percolate down into the rock, and

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when the water freezes in the rock, we all
know that ice has a larger volume than

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water, it again fractures out that rock,
and these fractured blocks are deposited

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here in this active channel, and then we
have the erosion factor that comes and

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transports it down mountain.

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So that's the beginning process of this
whole alluvial fan story.

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Alluvial fans are triangular-shaped
deposits of water transported material

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often referred to as alluvium.

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There are examples of an unconsolidated 
sedimentary deposit, and tend to be

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larger and more prominent in arid to 
semi-arid regions, like we are today.

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These alluvial fans typically from in 
elevated or even mountainous regions

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where there's a rapid change in the slope
from a high to a low gradient.

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The river or the stream carrying the 
sediment flows at a relatively high

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velocity due to the high slope angle, 
which is why coarse material is able

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to remain in the flow.

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When the slope decreases rapidly into
a relatively plane area or plateau, the

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stream loses the energy it needs to move
its sediment.

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Deposition subsequently occurs, and the
sediment ultimately spreads out, creating

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the alluvial fan.

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As the streams gradient decreases, it 
drops the coarse grain material.

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It makes swagger of the channel, and
forces it to change direction, and

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gradually build up a slightly mounded or
shallow conical fan shape.

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The sediment that results from the erosion
in this elevated mountainous region right

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here, ultimately flows into the primary 
streams and the regions, where the streams

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act as a drainage system, and carry the 
sediment to the alluvial plane, which is

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down below us.

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Once the sediment exits the feeder stream,
the sediment is no longer confined to the

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channel walls.

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With this unconfinement, the sediment
begins to fan out, the alluvial fan

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becomes wider with increasing distance
from the mouth of the canyon.

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The biggest natural hazard on alluvial
fans are floods, and debris flows.

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Floods on alluvial fans are commonly 
flash floods, they occur with little to

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no warning, usually have high velocities 
in sediment transporting capabilities, and

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are relatively short.

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Modern flood control infrastructure is 
essential to stop flooding, property

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damage, and the loss of life from
happening.

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Successful development can and does 
occur on alluvial fans, even in rural

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areas like ours, mitigation measures are
utilized; building pads are elevated,

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obvious drainage paths are left 
unobstructed, and in some cases,

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flood walls are built.

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Floodplain management principles of
alluvial fans are part of everyone's

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life that chooses to live in this special
place.

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I hope this helps you understand more 
about alluvial fans, and the processes

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that create them.

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Until next time, I'm Chad Kiel.