What is an Alluvial Fan? EXPLAINED | Learning Geology

hey guys it's Chad Kiel with learning

geology nature reconnection and today

we're learning about alluvial fans what

is an alluvial fan and I'm going to tell

you about some interesting data about

this alluvial fan and the processes that

created it because it happens to be in

my backyard let's go check it out so

before we dive down the rabbit hole of

alluvial fans let's first take a second

to talk about weathering and erosion

because those are two factors they're

gonna come into play again and again

when we're talking about this alluvial

fan right here around 6500 feet into the

mountains and temperatures can vary from

20 degrees each day causing the rocks to

kind of expand and contract and this

expanding and contracting is gonna cause

fracturing now these fractures are

susceptible for water to percolate down

into the rock and when the water freezes

in the rock we all know that ice has a

larger volume than water it again

fractures out that rock then these

fractured blocks are deposited here in

this active Channel and then we have the

erosion factor that comes and transports

it down mountain so that's the beginning

process of this whole alluvial fan story

alluvial fans are triangular shaped

deposits of water transported material

often referred to as alluvium there are

examples of an unconsolidated

sedimentary deposit and tend to be

larger and more prominent in arid to

semi-arid regions like we are today

these alluvial fans typically form and

elevated or even mountainous regions

where there's a rapid change in the

slope from a high to a low gradient the

river of the stream carrying the

sediment flows at a relatively high

velocity due to the high slope angle

which is why course material is able to

remain in the flow when the slope

decreases rapidly into a relatively

plain or area or plateau the stream

loses the energy it needs to move its

sediment deposition subsequently occurs

and the sediment ultimately spreads out

creating the alluvial fan as the streams

gradient decreases it drops the coarse

grain material it makes Swaggart of the

channel and forces it to change

direction and gradually build up a

slightly mounded or shallow conical fan

shape the sediment that results from the


this elevated mountainous region right

here ultimately flows into the primary

streams in the regions where the streams

act as a drainage system and carry the

sediment to the alluvial plain which is

down below us once the sediment exits

the feeder stream the sediment is no

longer confined to the channel walls

with this unconfined meant the sediment

begins to fan out the alluvial fan

becomes wider with increasing distance

from the mouth of the canyon the biggest

natural hazard on alluvial fans are

floods and debris flows floods on

alluvial fans are commonly flash floods

they occur with little to no warning

usually have high velocities and

sediment transport aiding capabilities

and are relatively short modern flood

control infrastructure is essential to

stop flooding property damage and the

loss of life from happening successful

development can and does occur on

alluvial fans even in rural areas like

ours mitigation measures are utilized

building pads are elevated obvious

drainage paths are left unobstructed and

in some cases flood walls are built

floodplain management principles of

alluvial fans are part of everyone's

life that chooses to live in this

special place I hope this helps you

understand more about alluvial fans and

the processes that create them until

next time I'm Chad keel