GEOL 101 Running Water
GEOL 101 Running Water GEOL 101
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This 6 page Class Notes was uploaded by Victoria Williams on Friday April 1, 2016. The Class Notes belongs to GEOL 101 at George Mason University taught by Mark Uhen in Fall 2016. Since its upload, it has received 17 views. For similar materials see Introductory Geology in Geology at George Mason University.
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Date Created: 04/01/16
Running Water Less than 1% of water in the world is drinkable fresh water. About 2% of the water on earth is in glaciers, which is fresh water. The Hydrologic Cycle, the Evaporation, Precipitation, Condensation cycle. Some precipitation infiltrates the earth and deposits into lakes and the ocean. Water is removed from this cycle with subducting plates, the amount that is lost is unknown Precipitation, or meteoric water, water that falls as precipitation (Snow, freezing rain, hail, rain, etc.) Precipitation soaking into the ground is called Infiltration. There are zones that the soil has that water passes through. The top is the soil zone, which can be completely dry, then the Water table, to the Capillary fringe, then the saturated zone. Runoff is water that flows across the surface of the ground. This happens when the soil is unable to absorb any more water. o It often begins as sheetflow. o Infiltration capacity is controlled by: Intensity and duration of rainfall, prior wetted condition of the soil, soil texture (packed clay won’t absorb as much, but sand and peat will absorb a lot), slope of the land, nature of the vegetative cover. o Sheetflow develops into tiny channels called rills. Streams and Rivers – rills join together into streams. The terms stream and river are often used interchangeably by geologists. Drainage Basins or watersheds are the land areas that contributes water to a river system or estuary (where it meets the ocean). Tributaries are rivers feeding into another river. An example of this is the Mississippi River Drainage Basin. o Drainage basins form in lower areas between local peaks in elevation because of gravity. Drainage divides separate these flows. o An example of this near us is the Chesapeake Bay. The bay used to be a valley in the ice age but then the ice melted and fills up the valley. Right now there is an effort to clean up this bay because of fertilizer from farms flowing into it and causing algae blooms. The algae dies and decomposes, which takes up the oxygen in the water so it’s taken away from creatures within it. River Systems – a river or stream is water flowing usually in a channel, but not always. o Water flows in two ways, Laminar (straight line) and Turbulent (erratic flow) Channel Shape and Flow Velocity – more water in contact with channel be means more drag and lower velocity. The water flows the fastest in the center because there it is not a lot of friction against the sides of the channel. The narrow, deep rivers are a lot faster than a wide, shallow river. Discharge: The volume of water flowing past a certain point in a given unit of time. The formula is: Discharge (m /s) = Cross Sectional Area (m) x Velocity (m/s) o Crosssectional area = Width x Height The river with the largest discharge is the Amazon River, this is mainly because the river is HUGE and there is a ton of precipitation in the area. Stream Gradient – Vertical drop over horizontal flowing distance. Average gradient = the head (highest point on a river) / The distance it runs = Sediment Transport o Dissolved Load: Minerals dissolved in water o Suspended Load: Tiny sedimentary particles that make no contact with the bottom (silt, clay) Most of the sediment transported is in this suspended load o Bed Load: Larger sedimentary particles that make some contact with the bottom (sand, gravel, boulders). Can move across the bed in several different ways by rolling, sliding, and saltation (or bouncing). Stream Erosion – Streams with sediment weather rocks a lot faster o Abrasion – The river with sediment wears rock away by banging it with sediment and the pieces are carried away by the water, like in a canyon o Potholes – Where the river is low and it bores holes downwards, there are usually a bunch together in one place. Capacity and Competence o Capacity is the maximum load of solid particles a stream can move per unit time o Competence is the measure of a stream’s ability to move sediment based on the maximum size particles. A small stream with a soft flow has a low capacity and low competence A small stream with a rushing flow has a low capacity but a higher competence Sediment Deposition – Streams often form in a ‘meandering’ way in a sinuous way (winding around). Around bends, the water is very fast and it erodes land at the corners which are called cut banks. On the other side of the bend, a deposition point bar is formed where there are sediment deposits. What fraction of the water in the Colorado River, flowing through the Grand Canyon reaches the Gulf of California? Less than 1% makes it to the gulf! There is no water so there are salt flats where water used to flow. People take all the water for irrigation and drinking water. Stream Channels: Bedrock vs. Alluvial Channels. Streams that cut through bedrock incise the rock like in the Grand Canyon. Streams that go through Alluvium (softer sediment) are alluvial, they are more free flowing. Alluvial Channels o Braided – more often found in high gradient, upstream areas. o o Meandering – More often found in low gradient, downstream areas. Meander Cutoff – When a meandering river/stream erodes and starts to get closer together until they touch. Where the loop used to be, sediment now deposits there and cuts it off into an oxbow lake. These lakes then just collect sediment and in time become swampy areas. Base Level – The lowest level to which a stream can erode its channel. Local base level is controlled by local geology and topography. The Sea level is the ultimate base level for ALL streams. Except in very rare cases, like in the Dead Sea. When there was a stream going downhill to the ocean, a fault comes up and creates a resistant bed that the stream cannot erode. The stream will still flow down into a waterfall and still continue to the sea. But over a long time, the water erodes the resistant bed and will bring it back down again. Valley Deepening – ‘V’ shaped valleys erode channels through rock rather quickly because the water is moving very quickly in these areas most of the time. The valley will keep getting deeper until it hits sea level. Valley Widening – As the river erodes the valley, the river will begin to meander a bit and make the valley wider. Eventually it will start to flatten the landscape into a Floodplain that was once an elevated area. Incised Meanders are meanders in steep, narrow bedrock valleys formed when the meandering channel cut down from soft sediment into hard bedrock. Mass wasting is also a factor in creating incised meanders in canyons. Stream Terraces are flat areas around a river channel that represent former levels of the floodplain. Around the edges of these floodplains are terraces, which are raised areas. These normally form in tectonically active areas where the land is raised into terraces and the floodplain is ‘dropped.’ Many terraces can form in one area. Stream Deposition: Deltas – form where sedimentladen streams enter larger, relatively still bodies of water. The stream loses its competence and deposits sediment in a delta (a triangleshape area of deposited sediment, the Nile River Delta is a prime example). o Sometimes when enough sediment is dumped, the stream channel can’t flow there as much. So it moves to an area with less resistance, which is called evulsion. There is normally one large channel and then a few smaller ones branching out. Natural Levees – Form with flood stages. When an alluvial stream channel in a valley floods, sediment is dropped right where the water escapes the channel because the water’s competence drops. The sediments then creates a natural levee, and it keeps layering up with each flood. Humans create these to protect towns from floods. But this can cause issues with rainwater trying to flow into the river. The excess water will sometimes become a Yazoo tributary, a small stream that appears alongside the river. Alluvial Fans occur when a stream flows through a channel and thin into a wider, open space. These form on dry land and not in the water because the area is so dry. When the water hits the open area, it loses its capacity and drops the sediment in a fan. Drainage Patterns – Geomorphology (the change in earth) o Dendritic – Like branches of a tree, one large channel that goes into smaller streams and then into tiny creaks and then to nothing. o Radial – Formed around places like a cone volcano, all the streams flow in a radial pattern away from the peak. o Rectangular – Happens when there are sets of joints in a rock at sharp angles to one another. o Trellis – Forms in places with bands of more and less resistant rocks. Most form along the bands but a few will form right through several bands. Water Gap Formation – In a place with synclines and anticlines, when a river is flowing through the area and the soil erodes and shows the layers, creating water gaps. Stream Piracy – When two rivers flow near each other and when one of the rivers erodes towards the river in the water gaps. The river will then ‘latch’ onto the water gap river and steal the water away. When no more water flows through the water gaps, they’re just called wind gaps. Floods occurs when a stream overflows a channel. o Regional floods occur when large areas are inundated with a great deal of rain or snowmelt over a span of time. You tend to know these are coming, predictable. o Flash floods happen when a small area receives a great deal of rain or snowmelt in a short period of time, they can be extremely sudden especially in places near mountains and valleys. These happen fast and cause more devastation because of the ‘surprise.’ o Dam Failure Floods are caused by human dams that, of course, fail and break apart and let the massive amount of water go and cause massive amounts of damage.
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