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The Fluvial System in a Piedmont Zone
Fluvial and glacial geology are two of the foremost important systems and concepts in geomorphology. Both systems work in a variety of different ways to sculpt landscapes all around the world. The book The Fluvial System by Stanley Schumm focuses on those aspects of the fluvial system and how they have changed the world around us over the past few centuries. Schumm has spent the majority of his life studying and experimenting with the processes of fluvial erosion and transport and with the amount of data and historical evidence provided it seems his hypothesis and theories are correct until proven otherwise.
In order to understand the fluvial system three things must be know. The quantity and type of sediment, the manner water is supplied from the source area, and the geologic and climatic processes that effect the sediment and water supply. Another key point is to be aware of upstream effects of downstream channel behavior and changes of base levels. The entire fluvial system is separated in three zones pointing towards the downstream direction. Figure 1-1 shows how each are divided and the direction of flow. Zone 1 is the uppermost zone and refers to the production, or drainage basin, watershed, and sediment-source zone. This is where the sediment is derived and produced. Many watershed scientist and hydrologist focus on this zone in order to understand the evolution and growth of drainage systems. Zone 2 is the transfer zone; in a stable channel the deposition of sediment would equal the output. Hydraulic and river-control engineers study this zone to understand river-channel morphology through time. Zone 3 is the deposition
sink or area of deposition. The deposition zone may appear as an alluvial fan, alluvial plain, delta, or may be deposited into deeper waters. This zone is mostly studied by sediment geologist and coastal engineers because of the internal structure, stratigraphy, and morphology of deposition.
In 1969 Thornbury created a list with 10 concepts of geomorphology and fluvial systems. Stanley Schumm used a collapsed version which consists of 3 concepts. The first principle is Uniformity, which means the laws of Newtonian physics and chemistry controlled the operation of past erosional and depositional processes as they do today. Simply the present is the key to the past. The second principle is that within the geological structure there is a determinable sequence of landscape evolution over time. The basis of this principle is the Davis model of erosion which focuses on speed and complexity of landscape evolution, progressive landscape evolution, and process-response systems. The third and final principle is complexity, both landscape and earth histories are complex. This is related to the external influences of climate change and diastrophism.
Because this book covers all aspects of fluvial geology I decided to focus on Zone 3, more specifically in the Piedmont environment. The piedmont is located favorably for the rapid deposition of sediment from an adjacent source area. Typically characteristics of a piedmont are alluvial fans but the sedimentologic and morphologic features depend on the nature and environment of Zone 1. The geology, geomorphology, and hydrology of Zone 1 will change the outcome of Zone 3 downstream. Although fans are created in a variety of environments the main prerequisite is a large sediment supply which is transferred to a lowland area. Fans can be of two
types, either dry or mudflow fans, or wet fans. The dry fan is more common in todayâ€™s environment although in the geologic past it seems the wet fan was much more prevalent. Dry or mudflow alluvial fans are typically created under dry conditions and their streams are ephemeral. Mud flows comprise the large part of deposition from rainfall or failures. Dry fans tend to be relatively small compared to wet fluvial fans. The wet fluvial fan is different because it has boulders and other coarse material near Zone 1 and is a gradual shift towards small sized material and minerals near the zone of deposition.
The modern dry alluvial fan occurs in two situations, the first being when deposition is near the mountain front and the fan surface is undissected. The second situation is when deposition is at the toe of the fan and water and sediment move to this location through a fan-head trench. See figure 2-1 for the difference between the two situations. The main explanation for these two scenarios is because of an external variable such as tectonics or climate change.
Wet fluvial fans have a progressively shifting channel and do not have random distribution of erosion and deposition. The most widely known wet fluvial fan is the Kosi River in India. The Kosi River drains from the high Himalayas and delivers a tremendous amount of sediment into the Ganges piedmont zone, where it has constructed the large â€œmega-coneâ€ of the Kosi fan. Because of the progressively shifting channel the Kosi River has migrated 70 miles from 1736 to 1964. See figure 3-1 for a detailed view of the migration process. This process caused 3500 square miles of reworked land and laid waste because of sand deposition and bank erosion.
At the head of the fan the channel consist of coarse material for nearly 10 miles downstream at which point it is rarely transported further. Then the river widens and begins to have braiding characteristics and interlacing channels. From the head of the fan the gradient
changes from 5 to 1 foot per mile. Near the lower gradients the river is divided into several channels which then spread into an area in the Ganges piedmont for 10 miles. The cause of the shift of the river was determined because it is a large wet fan with enormous amounts of deposition. This causes an almost chaotic system around the piedmont. Also the river began to move westward by avulsion, or tearing away of the landscape, which was then built up by deposition. This continual tearing and redeposition always created a lowland to the west of the river and so it continued to migrate that way. The steepness of slopes and tilting of the Himalayas to the east helped aid the river to a westward direction. See figure 3-2 for contour lines and river diagram.
Schummâ€™s ideas and interpretations of fluvial systems to me seemed to be very accurate and well supported. He uses numerous references in each section of the book as well as field examples, laboratory studies, and past historical geology to build his explanations for fluvial geology. With the amount of credibility and research he has put forward for geomorphology it is evident that he understands these systems and processes to an unbelievable extent.
Reading this book and understanding the concepts, variables, and processes involved in fluvial geology helped me to appreciate the geomorphology a lot more. Because this is such an influential and important process in geomorphology learning about it with specific field examples made things much clearer. Since we learned about these systems in class in a very broad way it was nice to read things from another personâ€™s perspective and see the attention to
detail from as aspects. Since one of the three concepts is uniformity I believe this book will help me to understand fluvial systems all around the world for a long time.
Schumm, Stanley, 1977, The Fluvial System: Wiley-Interscience pp 1-14; 17-20; 245-258