+ Erosion Surface Water moving, transporting, and depositing sediment.
+ Surface Water 2 Water from rainfall can hit Earth s surface and do a number of things: Slowly soak into the ground: Infiltration Change to a gas: Evaporation Flow across Earth s surface: Runoff Runoff Water that runs or flows across Earth s surface under the force of gravity Runoff causes erosion!!
+ RUNOFF 3 Water is a powerful agent of erosion that can start as a simple drop of rain. A tiny raindrop can loosen small particles such as clay sediment. Raindrops can combine with other drops to form a trickle of water. This trickle can merge into slightly larger streams that loosen and transport more sediment. As streams merge into one another, they form a river. June 27, 2013
+ RUNOFF 4 As a river or stream flows, it will create a path by weathering and eroding away soft layers of rock, like sandstone. Sometimes, rivers will encounter layers of rock that do not weather and erode as easily, such as granite. This can cause rivers to change course, or meander, as rushing water tends to flow in the direction of least resistance. However, even harder layers of rock, like granite, can be eroded by flowing water over time. June 27, 2013
+ RUNOFF 5 Surface water can be so powerful that it can even transport giant boulders downstream. Surface water can also spread sediment when rivers overflow their banks. Runoff typically occurs because of heavy rainfall. Sediment particles can travel great distances when moved by surface water.
+ RUNOFF 6 If water carrying sediment flows over a depression or basin, its sediment load can be deposited. If water meets an obstacle or has to spread out over a larger area, it might not have enough energy to carry its full load of sediment and will deposit it. Over time, sediment particles can gradually accumulate and can eventually form sedimentary rock.
+ What affects runoff and erosion? 7 The intensity and duration of rainfall The harder and longer it rains, the more water that will flow on the surface. Slope of the land Water flows fast down steep slopes which lead to more Erosional energy. The water content of the soil Is the soil saturated, or can rainfall soak into it? Vegetation Vegetation can slow runoff and allow the surface water to soak in. June 27, 2013
+ Running Water 8 If runoff does occur, it will occur in one or two ways: Sheet Flow Rill Erosion Sheet flow is when water flows as a sheet across Earth s surface. Rill erosion is when sheets of water collect and begin to flow and forms a channel or gully.
+ Running Water 9 Over time, continuous rill erosion leads to the development of a stream. A stream is any channelized flow of water. There are many different types of streams that differ in terms of their shape, speed, and volume of water.
+ River Systems 10 A river and all of its tributaries make up a river system. June 27, 2013
+ 11 June 27, 2013
+ WATERSHED 12 All the water that drains into a large stream or river is known as a watershed (a drainage basin). The land area that supplies water to a river system is also called a watershed.
+ PA River System Watersheds 13
+ STREAM DEVELOPMENT STAGES 14 Longitudinal Profile: The side-view of a stream from its head (start) to its mouth (end). Conditions upstream are much different from conditions downstream. We will follow a typical stream in its journey from upstream to downstream. - Mountain headwater streams flow swiftly down steep slopes. - Cut deep V-Shape Valleys. - Rapids and Waterfalls common. Headwaters Young River - Upstream Transition Zone - Middle Elevation streams merge and flow down gentler slopes (hills). - The valley broadens and the river begins to meander. - At low elevations the river wanders across a broad, nearly flat valley. - At its mouth it may divide into many separate channels as it flows across a delta. - Deltas are built from sediments deposited into the ocean Depositional Zone Mature River - Downstream
+ Stages of Development of a Stream 15 Gradient The slope of a stream channel or the vertical drop over a certain distance. Streams start with a steep gradient at their head and end with a gentle gradient at their mouth. The goal of a stream is to become flat. It will erode and erode until it reaches a gentle gradient.
+ Stages of Development of a Stream 16 Gravity makes it do this! The erosion water does under the force of gravity is called down-cutting. A stream will cut a V- shaped channel in the land to try to decrease its gradient.
+ Stages of Development of a Stream 17 As we move downstream, the gradient of a stream decreases. This is because the stream has down-cut to near base level. Base-Level: Lowest elevation a stream can erode its channel. At this point, gravity is no longer trying to bring the water down.
+ Stages of Development of a Stream 18 Upstream gravity was affecting the moving water and pulling on it so much that it cut and eroded through the land. Downstream, near baselevel, gravity has much less of an affect on the water and does not cause it to cut-down anymore.
+ 19 June 27, 2013
+ Stages of Development of a Stream 20 Now, instead of down-cutting, downstream erosion occurs from side-to-side. Moving water will erode side-to-side to form MEANDERS. MEANDERS are the curving, windy, path water takes downstream. A flood plain is created as meanders weave side-to-side over the land. A flood plain is the flat area land on either side of a meander. A flood plain is where a stream or river will flood during high waters.
+ STREAM CHARACTERISTICS 21 UPSTREAM: YOUNG STREAMS 1. Steep gradient 2. Steep gradients = fast-moving water. 3. Fast-moving water = down-cutting and erosion at a higher rate. 4. Down-cutting = deep but narrow V-shaped channelized water. Velocity is another word used to describe the speed of water in a stream. Velocity is highest in the CENTER of a downcutting stream = less friction. FEATURES OF A YOUNG RIVER - Waterfalls - Rapids - V-Shaped Valleys
+ Waterfalls form upstream 22
+ STREAM CHARACTERISTICS 23 DOWNSTREAM: MATURE STREAM 1. Gentle gradient. 2. Gentle gradient = slow-moving water and less down-cutting. 3. Less down-cutting = most erosion is side-to-side forming meanders. 4. Meanders indicate a stream is nearest base-level (Mature). Velocity is highest on the outside of a meander. FEATURES OF A MATURE RIVER - Meanders - Oxbow Lake - Flood Plains
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+ The work of a stream: Erosion 25 As a stream travels, it picks up and transports sediments. The more water (high discharge) = the more erosion and transport The faster water (high velocity) = the more erosion and transport. There are 3 major types of sediment transport: 1. Dissolved Load: Sediments dissolved in solution (salts) 2. Suspended Load: Silts and clays held or suspended in the water. 3. Bed Load: Sands and gravels rolling along the channel bottom. Sediments in bed load often move by saltation.
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+ The work of a stream: Erosion 27 The most stream erosion occurs: On a steep gradient. On the outside of a meander (called a cut-bank) The water is moving the fastest during each situation.
+ Cut Banks cause Oxbow Lakes As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders. 28 The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream. The force of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy due to decreased friction.
+ Cut Banks cause Oxbow Lakes 29 On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction. Over time the horseshoe become tighter, until the ends become very close together. As the river breaks through, e.g. during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cutoff from the main channel. The cut-off loop is called an oxbow lake.
+ 30 June 27, 2013
+ The work of a stream: Deposition 31 As a stream travels, it also deposits its load of sediment. The less water (low discharge) = the more deposition. The slower water (low velocity) = the more deposition. There are 3 major locations for deposition: 1. Inside of a meander: Water moves the slowest (Called point-bar) 2. Delta: Water slows to enter a river, lake, or ocean. The sediment is deposited in a fan-shape. 3. Levees: Deposited sediment on the banks of a stream or rivers. During a flood, waters rise and recede. In the process they drop out sediment along the banks.
+ The work of a stream: Deposition 32 A delta is a fan-shaped area of deposition at the mouth of a river or stream where water slows down to enter another body of water, depositing its sediment load in the process. June 27, 2013
+ The work of a stream: Deposition 33 Alluvial fan a fan-shaped alluvial deposit formed by a stream where its velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a mountain.
+ The work of a stream: Deposition 34 A natural levee is created during a flood when waters rise and recede dropping sediment load in the process.
+ 35 June 27, 2013