1 00:00:01,222 --> 00:00:04,643 Chad Kiel: Hey guys, it's Chad Kiel with Learning Geology and Nature Reconnection, 2 00:00:04,643 --> 00:00:07,213 and today we're learning about Alluvial Fans. 3 00:00:07,213 --> 00:00:08,897 What is an alluvial fan? 4 00:00:08,897 --> 00:00:12,500 And I'm going to tell you about some interesting data about this alluvial fan, 5 00:00:12,500 --> 00:00:15,135 and the processes that created it, because it happens to be in 6 00:00:15,135 --> 00:00:16,321 my backyard. 7 00:00:16,321 --> 00:00:19,499 Let's go check it out. 8 00:00:19,499 --> 00:00:23,520 So before we dive down the rabbit hole of alluvial fans, let's first take a 9 00:00:23,520 --> 00:00:28,058 second to talk about weathering and erosion, because those are two factors 10 00:00:28,058 --> 00:00:30,895 that are gonna come into play again and again when we're talking about this 11 00:00:30,895 --> 00:00:32,395 alluvial fan. 12 00:00:32,395 --> 00:00:36,300 Right here around 6,500 feet into the mountains, temperatures can vary from 13 00:00:36,300 --> 00:00:42,471 20 degrees each day, causing the rocks to kind of expand and contract, and this 14 00:00:42,471 --> 00:00:45,993 expanding and contracting's gonna cause fracturing. 15 00:00:45,993 --> 00:00:51,641 Now these fractures are susceptible for water to percolate down into the rock, and 16 00:00:51,641 --> 00:00:55,745 when the water freezes in the rock, we all know that ice has a larger volume than 17 00:00:55,745 --> 00:00:58,914 water, it again fractures out that rock, and these fractured blocks are deposited 18 00:00:58,914 --> 00:01:05,037 here in this active channel, and then we have the erosion factor that comes and 19 00:01:05,037 --> 00:01:07,073 transports it down mountain. 20 00:01:07,073 --> 00:01:11,844 So that's the beginning process of this whole alluvial fan story. 21 00:01:11,844 --> 00:01:15,414 Alluvial fans are triangular-shaped deposits of water transported material 22 00:01:15,414 --> 00:01:17,422 often referred to as alluvium. 23 00:01:17,422 --> 00:01:21,227 There are examples of an unconsolidated sedimentary deposit, and tend to be 24 00:01:21,227 --> 00:01:25,766 larger and more prominent in arid to semi-arid regions, like we are today. 25 00:01:25,766 --> 00:01:29,569 These alluvial fans typically from in elevated or even mountainous regions 26 00:01:29,569 --> 00:01:33,689 where there's a rapid change in the slope from a high to a low gradient. 27 00:01:33,689 --> 00:01:36,876 The river or the stream carrying the sediment flows at a relatively high 28 00:01:36,876 --> 00:01:41,031 velocity due to the high slope angle, which is why coarse material is able 29 00:01:41,031 --> 00:01:43,182 to remain in the flow. 30 00:01:43,182 --> 00:01:48,236 When the slope decreases rapidly into a relatively plane area or plateau, the 31 00:01:48,236 --> 00:01:51,466 stream loses the energy it needs to move its sediment. 32 00:01:51,466 --> 00:01:56,373 Deposition subsequently occurs, and the sediment ultimately spreads out, creating 33 00:01:56,373 --> 00:01:58,875 the alluvial fan. 34 00:01:58,875 --> 00:02:03,330 As the streams gradient decreases, it drops the coarse grain material. 35 00:02:03,330 --> 00:02:07,099 It makes swagger of the channel, and forces it to change direction, and 36 00:02:07,099 --> 00:02:11,688 gradually build up a slightly mounded or shallow conical fan shape. 37 00:02:11,688 --> 00:02:15,893 The sediment that results from the erosion in this elevated mountainous region right 38 00:02:15,893 --> 00:02:21,353 here, ultimately flows into the primary streams and the regions, where the streams 39 00:02:21,353 --> 00:02:25,257 act as a drainage system, and carry the sediment to the alluvial plane, which is 40 00:02:25,257 --> 00:02:26,540 down below us. 41 00:02:26,540 --> 00:02:30,646 Once the sediment exits the feeder stream, the sediment is no longer confined to the 42 00:02:30,646 --> 00:02:31,848 channel walls. 43 00:02:31,848 --> 00:02:36,636 With this unconfinement, the sediment begins to fan out, the alluvial fan 44 00:02:36,636 --> 00:02:40,757 becomes wider with increasing distance from the mouth of the canyon. 45 00:02:40,757 --> 00:02:44,610 The biggest natural hazard on alluvial fans are floods, and debris flows. 46 00:02:44,610 --> 00:02:47,997 Floods on alluvial fans are commonly flash floods, they occur with little to 47 00:02:47,997 --> 00:02:53,263 no warning, usually have high velocities in sediment transporting capabilities, and 48 00:02:53,263 --> 00:02:54,949 are relatively short. 49 00:02:54,949 --> 00:02:58,736 Modern flood control infrastructure is essential to stop flooding, property 50 00:02:58,736 --> 00:03:01,771 damage, and the loss of life from happening. 51 00:03:01,771 --> 00:03:06,609 Successful development can and does occur on alluvial fans, even in rural 52 00:03:06,609 --> 00:03:11,466 areas like ours, mitigation measures are utilized; building pads are elevated, 53 00:03:11,466 --> 00:03:15,537 obvious drainage paths are left unobstructed, and in some cases, 54 00:03:15,537 --> 00:03:17,291 flood walls are built. 55 00:03:17,291 --> 00:03:21,246 Floodplain management principles of alluvial fans are part of everyone's 56 00:03:21,246 --> 00:03:24,266 life that chooses to live in this special place. 57 00:03:24,266 --> 00:03:28,235 I hope this helps you understand more about alluvial fans, and the processes 58 00:03:28,235 --> 00:03:29,439 that create them. 59 00:03:29,439 --> 99:59:59,999 Until next time, I'm Chad Kiel.