Gully: A channel cut into the soil that is formed by running water.
In the North Carolina Piedmont, gullies are a very common erosional feature. They are formed by running water and—according to Spell and Johnson—can be an important indicator of poor land management practices.
A gully forms when water runs downslope over a landscape and creates small grooves called “rills” in the soil. Over time, these rills join together and increase in depth until they become “gullies.” Although gullies are formed by water, they only contain water after major rain events—thus most of the time, gullies are dry.
Any process that disrupts the soil or encourages soil erosion can enhance gully formation. Many man-made factors contribute to this as well, including deforestation, forest fires, road construction, and hiking trails. Deforestation increases erosion by destroying the underground tree root networks that provide vital structure and water capacity storage to healthy soils. Fires also prove to be a major contributing factor to soil erosion as they burn through undergrowth and leaf litter, effectively drying out the soil and exposing it to the elements. Furthermore, when roads are constructed, land is cleared often destroying vegetation (which ideally should be replanted on either side). However, if this is not done or sufficient drainage systems are not installed, gullies can form on one—or both—sides of the road. Finally, man-made roads and trails compact the soil and reduce its water capacity, leading to further gully formation.
Of course, there are natural influences that can cause soil erosion and gully formation as well. These natural factors include precipitation, topography, soil type and vegetation cover. High levels of precipitation—such as intense, long rain seasons—can increase surface run-off, followed by erosion and gullying. Topography can also be a major factor in gully formation as steep slopes will see an increase in water velocity and erosive power. Soil type is important because as the particle size increases, the infiltration rate increases and resistance against erosion decreases. Therefore, sandier soils are more susceptible to erosion, while clay soils are more resistant. Finally, vegetation cover is an important factor in gully formation—and remediation—as strong roots can stabilize soil. Without this root system, soil is unstable and can be eroded easily, ultimately causing gullying.
In the North Carolina Piedmont we see lots of gullies because of our severe lack of topsoil. The B horizon is composed of mainly clay, which as we learned before, has a very small particle size and is therefore more resistant to erosion. Because of this, water from rain events rushes over the land surface at a high velocity and is redirected into rills. These rills eventually erode to form gullies over time.
Another reason for gully formation in the Piedmont is because land use was historically dominated by agriculture. According to Johnson & Spell, “agriculture techniques are linked to erosion, nutrient loss, and valley-bottom aggradation.” And as we learned earlier, gullies are more common in areas that are susceptible to erosion.
Below, is a video of Dr. Johnson, an esteemed geologist and soil scientist, explaining some of the common characteristics we see in gullies in the Piedmont.
References
- https://study.com/academy/lesson/hillslope-erosion-process-formation-of-rills-gullies.html
- http://www.fao.org/3/ad082e/AD082e01.htm#:~:text=A%20gully%20develops%20in%20three,according%20to%20these%20developmet
- https://tennesseerivervalleygeotourism.org/entries/great-smoky-mountains-national-park/101a7742-107f-477d-90f5-9ede2bde74fa
- https://www.dictionary.com/browse/gully#:~:text=noun%2C%20plural%20gul%C2%B7lies.,drainageway%20after%20prolonged%20heavy%20rains
- https://www.wbir.com/article/news/local/national-park-service-dont-move-the-rocks-in-smokies-streams/51-8b724068-7043-4731-9315-0831d89c3e56
- https://www.tandfonline.com/doi/abs/10.1080/02723646.2019.1574145
- https://alanhamr.weebly.com/uploads/1/8/6/0/18607614/3788646_orig.jpg?395
- https://www.sciencedirect.com/topics/earth-and-planetary-sciences/gully#:~:text=Gullies%20are%20permanent%20erosional%20forms,over%20by%20tillage%20any%20more
- https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98WR02513
- https://files.nc.gov/ncdeq/Energy%20Mineral%20and%20Land%20Resources/Erosion%20and%20Sediment%20Control/esc_education/erosion_patrol-student_packet_1-31-19.pdf
- https://deq.nc.gov/about/divisions/energy-mineral-land-resources/erosion-sediment-control