GG101 Lab 8 Background Reading F23
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Dec 6, 2023
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Page 1 Wilfrid Laurier University, Department of Geography and Environmental Studies GG101 Introduction to Physical Geography
Lab 8: Fluvial Features
References: Chapter 15 in Geosystems Introduction This lab exercise involves map and image interpretation with a focus on fluvial landforms and features. We will be contrasting meandering streams and braided streams, two common forms of streams in Canada. The main features of the stream channels will be illustrated using portions of a topographic map and satellite imagery that can be accessed using Google Earth. We will be examining Big East River in central Ontario and White River in southwestern Yukon. The portions of the two streams that we will examine can be seen in the 1:50,000 topographic map sheets: NTS 31E/6 Huntsville and NTS 115K/8, Snag Creek. The Snag Creek map sheet covers a portion of White River in southwestern Yukon that is northwest of Whitehorse, it has a topography that is typical of the northern part of the Western Cordillera, with mountains, plateaus and depressions. The Huntsville map sheet covers an area to the west of Algonquin Park in central Ontario, with a topography that varies from a steep, rocky areas, rolling hills, deep valleys, and wide floodplains. White River (red line in Figure 1) is a major tributary of the Yukon River. White River has a watershed area of 46,900 km
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(medium green area in Figure 2), with its headwaters in the Wrangell - St. Elias Mountains. Fed by mountain glacier meltwaters, White River flows northward to join the Yukon River. The discharge pattern of White River reflects the importance of glacial meltwater to the volume of water in the stream, with high flows sustained through much of the summer.
Figure 1.
The watershed of Yukon River in Yukon Territory and Alaska shown by the dashed black line. White River is highlighted by a red line (Bradford, Duncan and Jang, 2008).
Page 2 Figure 2.
Major watersheds (drainage basins) of the Yukon River (Brabets, Wang and Meade, 2000 USGS, Water-Resources Investigations Report 99-4204). Big East River is a tributary of the Muskoka River. Big East River, shown by the red line in Figure 3, flows southwestward from its headwaters where it is fed by cold water streams, lakes and ponds in Algonquin Park to where it discharges into Lake Vernon, near Huntsvile. The watershed of Big East River is approximately 740 km
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(Figure 4) and its discharge pattern reflects the input of water from surface waters, with peak flows during snowmelt, low flows in the summer and increasing flows in late August and September when precipitation levels are higher. Figure 3. A map of the Muskoka River Watershed (blue line). Big East River, shown by the red line, is located in the North Branch Muskoka River sub-watershed (MNRF, 2020).
Page 3 Figure 4. The watersheds of the Muskoka River. The Big East River watershed is shown by the orange polygon (Muskoka Watershed Council, 2023). Google Earth Pro You may also use Google Earth Pro to view the area to help you visualize the topography, additional detail about the surface features, and the streams. The age and resolution (detail) of the imagery varies when viewing different areas and the historical imagery of southwestern Yukon is limited. The file called Locations Lab 8 F23.kmz shows a series of placemarkers that will used in the questions for this lab. Open this file in Google Earth and examine the region, follow the streams to their headwaters and examine the terrain and surface features. Adjust your elevation exaggeration, angle of view, and direction of view to help you visualize the topography. To determine the elevations for a location, move your curser to the location and elevation is reported in the bottom right of the screen, shown to the right of the Latitude and Longitude data. Zoom in tightly on the location to get a more accurate elevation estimate. Stream Channels and Fluvial Features (Landforms, Sediments, Processes) In this lab exercise we will illustrate a variety of fluvial features including fluvial landforms, sediments, and processes. These topics are covered in lecture and students should review thereadings that accompany those materials.
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Page 4 Channel Shape, Pattern Channel shape refers to the width to depth ratio in a stream channel. Channel pattern refers to the form of a stream channel. There are several possible channel patterns, including meandering and braided. In a meandering stream, the stream flow is contained within a single sinuous channel. A braided stream has its flow divided between several branching and relatively straight channels. Typically, meandering channels have a low width to depth ratio (they are relatively narrow and deep) and braided channels have a high width to depth ratio (they are wide and shallow). The channel pattern is controlled by an equilibrium that is established between valley gradient (slope), discharge and flow characteristics, and sediment supply. Altering these parameters can cause the channel pattern to change. A suite of landforms are associated with meandering and braided streams, in this lab our focus is on meandering streams. Meandering Stream Features Meandering streams typically occupy valleys that are bounded by moderately sloping ridges. Within the valley, the stream cuts down and produces a broad meandering river floodplain by migrating laterally across the valley floor. A variety of illustrations in Geosystems show the range of landforms that are associated with meandering streams (see Chapter 15). The illustrations and the text descriptions will be of assistance when answering the lab questions. Review those sections in the text. There are three animations that depict aspects of meandering streams available from Chapter 15 of Geosystems. In MyLS go to Academic Resources (eText), Geosystems, Launch Courseware, then Open MyLab and Mastering, Study Area, and then select Animations. View the animations listed on the right. Big East River in central Ontario is a meandering stream and it has very well developed fluvial landforms such as a floodplain, oxbow lakes, meander scars, scroll bars, point and lateral bars, cut banks, and backswamps. The process of meander formation and migration that are illustrated in the animations, operate in the valley of Big East River and we can see evidence of those processes in the imagery that we will view through Google Earth Pro. Braided Stream Features Braided streams are characterized by multiple wide and shallow channels that transport considerable amounts of bedload (coarse sand, gravel, pebble and cobble sized sediments). They are typically found in mountainous terrain where runoff brings coarse sediment to the channel. Gradients along the valley of a braided stream tend to be relatively steep. In braided streams the positions of the channels can change frequently as the discharge rises and falls. There are some photographs and a description of braided streams in the text. Gradients In the lab exercise there will be a series of questions that will highlight some of the features of meandering and braided streams. We will use the measuring tools and the digital elevation data that are built into Google Earth to determine the elevations of stream channels at a series of locations. We will also measure the distances between locations along the channels. This will allow us to calculate gradients (changes in elevation over a distance) and compare the gradients of meandering and braided channels. Some of these
Page 5 types of measurements can also be done in Toporama. Valley and Channel Gradients To calculate the gradient of a valley along the axis of the valley (Valley Gradient), we establish a point in the centre of the valley in an up slope position and determine its elevation, we then move downslope to a second point at a lower elevation in the valley centre. The distance between the two points is the Valley Length and it is directly measured. The elevation change between the two points is also determined. The Valley Gradient is the Change in Elevation divided by the Valley Length. The gradient can be expressed in a variety of units, such as (i) percentage slope, (ii) as an angle, (iii) as a elevation change over a distance (e.g. m/km), or (iv) as a dimensionless value. To measure a Channel Gradient, we establish two points in the channel that are widely separated. We determine the elevation at each location and then digitize the pathway from the upstream to the downstream location by following along the channel itself. That distance is the Channel Length. The Channel Gradient is the Change in Elevation between the two points divided by the Channel Length. The difference between a valley length and a channel length is captured in the photograph to the right. If a channel is perfectly straight, there would be no difference between the valley length and the channel length, and the gradients would also be the same. If a channel is highly sinuous (meandering or winding) then the channel length will be longer than the valley length. The Sinuosity of a channel is the ratio of the Channel Length to the Valley Length. We can calculate the Sinuosity from the formula: Sinuosity = Channel Length/Valley Length Meandering streams have a high Sinuosity value while braided streams have a low Sinuosity value. In the images shown below, the upper image shows a section of the Donjek River in southwestern Yukon Territory, which is a tributary of the White River. For scale the red line in the image is 100 m in length. The water is divided into a series of wide shallow channels on a steep gradient, and the sediments are dominated by gravel and sand. The coordinate of the location is 61.259337° -139.626357°. The lower image shows an example of meandering stream, in this case it is the Pembina River in Alberta at 54.552514° -114.140252°, again the scale bar in red is 100 m. We see the Pembina River flow is in a sinuous single channel that is deep, it is developed on a gentle or moderate gradient and is dominated by finer sediments. The brown colour of the water is due to a high concentration of suspended fine sediments
Page 6 (silt and clay). The gradient is an important control on the characteristics of a stream channel, when gradients are relatively large (steeper) there is more energy in the system and the stream may be capable of mobilizing and transporting coarser sediments along its bed. Measuring Distances and Elevations in Google Earth We will use Google Earth Pro for length measurements and for elevations. There is a digital elevation model built into Google Earth and it will yield an approximate estimate of the elevation at a given point. To determine the elevation of a position, zoom into the image and position the cursor above the point desired, the elevation can be read from the lower right of the screen. Using the Ruler tool in Google Earth it is possible to measure the ground distance between two points on a straight line. If we need to determine the distance along a path that is comprised of many points we use the Path option. To digitize a path on Google Earth, select Ruler from the ‘Tools’ menu or select the Ruler icon, this will open a pop up box, then click on the Path tab, adjust the distance to kilometres or metres. To begin digitizing on the screen, move the cursor to the first point of interest and click the left mouse button, this will add a point to the image, then move the cursor to the second point and click the left mouse button, a line (yellow) will join the two red points that have been added, continue to add points in this manner, as
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Page 7 you add points the total distance of the path will be shown. If you add a point that is poorly situated and needs to be removed, click the right mouse button. The image below shows a path that was digitized along a section of the Pembina River. The total length of the path digitized along the channel is 3948 metres. As you digitize a long path it may be necessary to drag the image to a new position, because you have digitized to the edge of the screen. To do so, depress the left mouse button and hold it down, then drag the cursor, it will change to a closed hand shape, release the mouse button when you have finished repositioning the image and then continue digitizing. To accurately measure the length of a long path it is best to zoom in closely to the image and place many points along the path that you digitize. Once a path is digitized it can be saved by clicking on the Save button in the ruler dialogue box, when you save the path a window will pop up (Google Earth - New Path) that will allow you to name a path, enter a description of it, and change its attributes (colour, width, opacity). To prepare for your lab, download the files from MyLS and examine the area in Google Earth.