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An Introduction to Fluvial Geomorphology Streams and Rivers Lab: An Introduction to Fluvial Geomorphology Geomorphology is the study of the Earth's landscapes and landforms, the processes by which the landforms originated, their age, and the nature of the materials underlying them. Fluvial geomorphology is the study of landforms and processes associated with rivers. The Fluvial System Model A model is a simplified idealized representation of reality. The basic fluvial system can be represented as a model in order to help us understand its structure and processes. The basic fluvial system is often considered to be a drainage basin. Drainage basins differ in size and complexity, and are often subject to scale considerations. Often, a small watershed is part of a tributary to a much larger drainage basin. The following diagram of a fluvial system is an idealized representation to help you conceptualize its structure. The diagram is presented as three nested basins of small, medium, and large scale. Courtesy of Dr. Zbigniew Zwolinski Institute of Quaternary Research and Geoecology Adam Mickiewicz University Fredry 10, 61-701 Poznan, Poland voice: +48-61 8294676 or 8294569, fax: +48-61 8530234 Original Image online at = http://main.amu.edu.pl/~sgp/gw/sf/sf.html Zone 1: Drainage Basin or Watershed principal area from which runoff flows principal area from which dissolved solids and sediments are derived Zone 2: Area of Transfer inflow of water roughly equals outflow assuming stability, influxes of dissolved solids roughly equals fluxes out Zone 3: Area of Deposition discharge of water into larger streams or bodies influxes of dissolved solids roughly equals fluxes out alluvial-fan and alluvial-plain environments estuarine and deltaic environments costal environments Exercise I: Stream Drainage Patterns PRE-LAB ACTIVITY: Below are descriptions and illustrations of a number of classic drainage patterns. In lab you will be asked to identify some of these features (and others) on topographic maps, aerial photographs and remotely sensed imagery. You should review your knowledge about drainage systems by studying these illustrations as well as those shown in your textbook. You will be asked to answer some questions in writing on a lab sheet that will be provided. Try also to answer the questions included here - how many of these places can you identify? Dendritic -- a tree-like arrangement of small streams that join to form a larger river. It is the type of stream one expects to find in a region that has adequate rainfall and no unusual geologic features. The tiny creeks form a dendritic pattern and the modifying effects of any underlying geologic structure are not apparent until the stream gets quite large. Where do you think this might be? Trellis -- a squared off drainage pattern in which streams often flow directly toward each other from opposite directions and then make right angle turns when they meet. Trellis patterns are common in places where layered sedimentary rocks are tilted up from the horizontal. The rivers usually follow the layers of less-resistant rock until they get big enough to break through the resistant layers. Those gaps have enormous strategic value for transportation. What is the name of one of the US's most famous "gaps"? Where in the US would expect to find this drainage pattern? Parallel -- an elongated variant of the dendritic pattern, in which the tributary streams flow in the same general direction and usually join at small angles. Parallel drainage occurs in areas with a regional slope, prevailing wind, or some other factor that causes streams to flow unusually far in one direction before merging with another. What type of structural geological processes do you think produced this landscape and the pattern of rock structure and drainage systems observed? Where do you think this place is located? Parallel -- Another view of drainage patterns produced by structural geological processes. Here one can see the short parallel drainage patterns down the "back-tilted" side of a cuesta landform in the "Green River Formation" area of Wyoming. Radial -- a circular arrangement of streams that flow outward in all directions. , away from a central high area. Radial drainage patterns are common in the vicinity of volcanic cones, salt domes, granite intrusions, and other localized uplifts. A type of feature that usually has a radial drainage pattern - where do you think this place is located? One of the most famous stratovolcanoes in North America - what is its name and where is it located? It also would have a radial drainage. Centripetal -- a circular arrangement of streams, where water flows inward from all directions toward the center of the area. Centripetal drainage is likely in karst topography and in deserts where intermittent streams flow toward a temporary salt lake or basin. The map also show clearly some large alluvial fans (agriculture is shown in green) another typical fluvial landform feature of regions such as this. Where do you think this place is located? Hint - it is one of the largest basins of interior drainage in the world? Meanders -- a pattern of sweeping curves and loops. Meanders are common where terrain is flat enough to allow a river to move sideways, undercutting its bank on the inside of the curve. This lateral stream movement is responsible for a distinctive cycle of loop formation and enlargement, followed by abandonment of old channels when the river takes a shortcut across the neck between two large meanders. In what type of climatic region do you think this fluvial system is found? Would this be in the "up-stream" watershed area or in the lower reaches-the floodplain--where deposition is the more dominant process? Braided -- a rope-like pattern of twisting channels that separate and then join again all along the stream. Stream braiding is common in semi-arid regions, where floods bring more sediment into the channel than the normal flow of the stream is capable of carrying. A maze of sandbars and low islands may form during periods of low water and then be destroyed when floodwaters carry the material farther downstream. This stream is chocked with alluvium it can't transport downstream. What type of climatic or geologic situation might produce this situation? Where do you think this river is located? Another braided stream: the river flows from a series of glaciers in a very humid "marine west coast climate" region. Where do you think this river is located? What causes the heavy milky-white sediment in much of the stream--what is it called?