Identifying Fluvial Landmarks
Through the Identification of fluvial landmarks, we are able to understand many things about a fluvial system. From understanding the current setting of a fluvial system, the erosional energy of a river along its course and the deposition nature of system.
Erosional Landforms
V shaped Valley
Large V shaped valley known as the Most common Landform on the planet
Most notably located in the upper course of a river system, usually at higher regions above sea level as the this landform is less pronunced in the mid course of the river.
Formed as a result of the high level of vertical erosion from a river system as the water aims to reach sea level. As a result of this vertical erosion a steep sided river valley is formed. The high level of vertical erosion leaves behind steep banks that are eventually weathered away overtime before Mass movements of the land deposits the surrounding material into the channel which is then transported downstream. This Removal of material from the surrounding Hillside creates the ‘V’ shape used to describe these valleys.
Timescales vary on the formation of these landmarks as the erosional energy is highly dependent on the relationship between the source of the river and sea level as the larger the difference between the two the more erosional energy a system has as it carves the land to reach sea level.
A presence of a V-shaped valley indicates that the area is dominated by fluvial erosion showing that there is minimal glacial erosion taking place in this region as glacial erosion leads to the formation of a U shaped valley or trough
Formed during the Youthful stage of a river
Waterfalls/Gorges
A Waterfall is an erosional feature that is formed as a result of erosion from a river system in areas with an underlying geology of a hard (less erodible rock) with a layer of softer underneath it (more erodible surface, often sedimentary).
Usually located within the upper course of a river but can be present along the course of the river differing in size from a few metres all the way to 979m on land (Angel Falls, Venezuela).
Formed as the softer underlying geology is eroded away leaving a cliff as water travels over the edge it continues to erode the softer material creating a plunge pool as well as undercutting the harder rock surface. Over a period of time the undercutting creates an unstable overhang that will collapse as a result of gravity. With the cycle of erosion continuing the Waterfall retreats upstream leaving behind a scar in the landscape identified as a gorge.
Depending on the size and scale of a waterfall they can form over a period of 10-10,000+ years.
The presence of a waterfall indicates not only high levels of vertical erosion but also the underlying geology of the region being dominated by a overlying hard geology such as an igneous rock with an underlying sedimentary geology.
River Terrace
River terraces are an example of previous river systems that predate the current setting of a river system. A terrace is in fact the previous floodplain of a river that has been abandoned as a result of the erosional regime of a river, with changes in the amount of vertical erosion often caused as a result of sea level change leading to a river eroding downward and abandoning its previous setting.
Often found in series allowing for identification of climatic and or tectonic change in an area over large timescales. This is possible as the highest river terrace within a river system is an indicator of the original position of a river system and its floodplain before changes in the setting of the river occurred such as tectonic uplift changing the relationship with sea level impacting the erosional regime of the river system or as a result of glaciation.
River terraces can be located along the middle and lower course of a river system where a Floodplain would form.
Meanders/Ox-Bow lakes
Meanders are bends in a river system formed as a river flows along the middle and lower course of the river where erosional power of a river changes from high levels of vertical erosion too an erosional regime of dominant horizontal erosion and minimal vertical erosion as the river is closer to sea level.
As the river erodes along a horizontal plain from left to right as the flow of the water within the river circulates in a pattern that leads to the erosion of one bank creating a river cliff due to the erosion of the faster flowing water while on the opposite side slower moving water erodes the bank less creating an area of deposition known as a slip off slope.
Over time the meanders erode in such a fashion that meander bends converge to with the river opting to flow along the quickest course, which is the path of least resistance leaving behind the riverbend separated from the river system as a result of deposition along the banks.
Meanders and Ox-bow lakes provide evidence of a river system's current setting and the previous course of a river in the case of an Ox Bow lake. Meanders indicate that you are located within the middle or lower course of a system as it shows the change in the erosional power of a river system.
Depositional Landforms
Floodplain
A floodplain is the area that is adjacent to a river system that is prone to flooding. During the event of a flood the material and nutrients that is transported by the river is deposited along the area surrounding the river.
The floodplain extends from the banks of the river to the base of the enclosing valley walls.
A floodplain is an large flat area of highly fertile land around the area of river often used for agricultural purposes due to the deposited river sediments and organic material from the fluvial system
Levee
A levee is formed along the banks of a river and forms as a result of repeated flooding and recession of a river system during a flood event. Leading to the deposition of river sediments along the banks. Over long periods of time these deposits build up forming
Identifiable as they are low, linear ridges parallel to each other on both sides of the river banks. The larger sedimentary material is deposited along the ridges due to the transportational energy required to move these being so great. As such, when a river floods the smaller material is transported further away being deposited on the floodplain while the larger material remains around the banks of the river.
A natural levee was the basis for the fluvial flood defence called Levee (Artificial Levee) that do the same job as natural levees by raising the banks of the river meaning that in order to flood the river must exceed a larger bankfull area than a natural river
Braided Channels
A braided channel is a river channel that is divided into smaller channels by small islands in the stream known as eyots. This creates multiple braided streams that flow through an area usually home to a single river channel.
These channels form if a river has a significantly high sedimentary load within the channel, a steep long profile as well as being in a system with fluctuating river discharge.
As such braided channels allow for you to understand the setting of a fluvial system as they do not form unless these conditions are present.
River Deltas
A river delta is a depositional landform formed at the mouth of a river system where there is a sudden change in the energy of a river system leading to the deposition of the suspended sediment within the river system forming the landform within the channel.
Deltas can form anywhere a river enters another body of water which has a lower transportational energy/discharge than the river and cannot suspend the material. As such they are present in Oceans, Seas, Estuaries, lakes, reservoirs and potentially other rivers that are unable to transport the suspended sediment within the channel.
3 main types of Deltas are:
Arcuate or Fan-shaped - land arches around the mouth of a river system and the river system splits many times on the way to the sea creating a fan-like shape. An example of an Arcuate delta is the Niger Delta.
Cuspate - The delta forms in an arrow-like shape around the mouth of the river protruding into the open water for example the Ebro Delta, Spain
Bird’s foot - the river splits into many channels with deltas depositing around each of the channels creating an interlinked weblike delta connecting the river systems together looking like a bird’s foot