In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.
Fluvial processes
Fluvial processes include the motion of sediment and erosion or deposition on the river bed.
The movement of water across the stream bed exerts a shear stress directly onto the bed. If the cohesive strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow. In addition, if the river carries significant quantities of sediment, this material can act as tools to enhance wear of the bed (abrasion). At the same time the fragments themselves are ground down, becoming smaller and more rounded (attrition).
Sediment in rivers is transported as either bedload (the coarser fragments which move close to the bed) or suspended load (finer fragments carried in the water). There is also a component carried as dissolved material.
For each grain size there is a specific flow velocity at which the grains start to move, called entrainment velocity. However the grains will continue to be transported even if the velocity falls below the entrainment velocity due to the reduced (or removed) friction between the grains and the river bed. Eventually the velocity will fall low enough for the grains to be deposited.
A river is continually picking up and dropping solid particles of rock and soil from its bed throughout its length. Where the river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium.
Even small streams make alluvial deposits, but it is in floodplains and deltas of large rivers that large, geologically-significant alluvial deposits are found.
The amount of matter carried by a large river is enormous. It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea, the Yellow River 796 million tons, and the Po River in Italy 67 million tons. The names of many rivers derive from the color that the transported matter gives the water. For example, the Yellow River (Huang He) in China is named after the hue of the sediment it carries, and the White Nile is named for the clay it carries.
Erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.
Waterfall
A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf.
Waterfalls can be formed in several ways, but the most common method of formation is that a river courses over a top layer of resistant bedrock before falling on to softer rock, which erodes faster, leading to an increasingly high fall. Waterfalls have been studied for their impact on species living in and around them.
Waterfalls are commonly formed in the upper course of a river where lakes flow into valleys in steep mountains.
A river sometimes flows over a large step in the rocks that may have been formed by a fault line. Waterfalls can occur along the edge of a glacial trough, where a stream or river flowing into a glacier continues to flow into a valley after the glacier has receded or melted. The large waterfalls in Yosemite Valley are examples of this phenomenon, which is referred to as a hanging valley. Another reason hanging valleys may form is where two rivers join and one is flowing faster than the other.
Caprock model
The caprock model of waterfall formation states that the river courses over resistant bedrock, erosion happens slowly and is dominated by impacts of water-borne sediment on the rock, while downstream the erosion occurs more rapidly. As the watercourse increases its velocity at the edge of the waterfall, it may pluck material from the riverbed, if the bed is fractured or otherwise more erodible. Hydraulic jets and hydraulic jumps at the toe of a falls can generate large forces to erode the bed, especially when forces are amplified by water-borne sediment. Horseshoe-shaped falls focus the erosion to a central point, also enhancing riverbed change below a waterfalls.
A process known as "potholing" involves local erosion of a potentially deep hole in bedrock due to turbulent whirlpools spinning stones around on the bed, drilling it out. Sand and stones carried by the watercourse therefore increase erosion capacity.This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it. The rate of retreat for a waterfall can be as high as one-and-a-half metres per year.
Often, the rock stratum just below the more resistant shelf will be of a softer type, meaning that undercutting due to splashback will occur here to form a shallow cave-like formation known as a rock shelter under and behind the waterfall. Eventually, the outcropping, more resistant cap rock will collapse under pressure to add blocks of rock to the base of the waterfall. These blocks of rock are then broken down into smaller boulders by attrition as they collide with each other, and they also erode the base of the waterfall by abrasion, creating a deep plunge pool in the gorge downstream.
Streams can become wider and shallower just above waterfalls due to flowing over the rock shelf, and there is usually a deep area just below the waterfall because of the kinetic energy of the water hitting the bottom. However, a study of waterfalls systematics reported that waterfalls can be wider or narrower above or below a falls, so almost anything is possible given the right geological and hydrological setting. Waterfalls normally form in a rocky area due to erosion. After a long period of being fully formed, the water falling off the ledge will retreat, causing a horizontal pit parallel to the waterfall wall. Eventually, as the pit grows deeper, the waterfall collapses to be replaced by a steeply sloping stretch of river bed. In addition to gradual processes such as erosion, earth movement caused by earthquakes or landslides or volcanoes can lead to the formation of waterfalls.
By User:Cradel - Derived from File:Waterfall formation23.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5960898
(Sources: Wikipedia)
Standing at the listed coordinates you are facing the Millomeris waterfall. It is approx. 15m high and until recently, the Millomeri waterfall was not known because its location was not accessible. By now the waterfall attracts many visitors and tourists with its natural beauty.
Please take a look at the waterfall. You can clearly oserve the fluvial process.
Tasks to claim this EarthCache as a found:
- There is an obvious difference in the fluvial process/erosion in the upper and lower part oft he waterfall. What are the reasons?
- Is there an undercutting visible? Why or why not?
Go back over the first „bridge“
- At your new position please take a look at the riverbed. Are sediments that have been transported down the waterfall deposited here? Why or why not? Will the fluvial erosion continue?
- Optional (and appreciated): Take a picture of you or your GPSr at GZ
Please send the answers in english or german to: teamjustt@gmail.com or via message center.
According to the guidelines you may log the EarthCache AFTER sending the answers.
Enjoy your stay and respect the nature. Take your trash with you an CITO. Thank you.