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 today’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.