Landscape Development

Transcription

1 Landscape Development

2 Slopes Dominate Natural Landscapes Created by the interplay of tectonic and igneous activity and gradation Deformation and uplift Volcanic activity Agents of gradation Mass wasting Streams Wind Glaciers

3 Slope Formation Slopes are temporary, in the context of geologic time Change may result from renewal of the activity which created them, or by other actions Change may be rapid, but, overall, is typically quite slow Earthquakes, eruptions, floods, landslides, glaciation Creep: the very slow downward adjustment of hillsides Geologists attempt to understand landforms in light of their formation and continued adjustments

4 The hydrologic cycle Figure 16.3

5 Slope Formation Rainsplash, sheetflow, rills, runoff and streams Water falls on the land and quickly enters (forms) small channels These join together in a drainage network to remove the water Channels lengthen by headward erosion, intercept other channels and grow in size and flow volume An efficient drainage system is developed

6 Longitudinal profile of a stream Figure 16.7

7 Running water Changes from upstream to downstream Profile Profile is a smooth curve Gradient decreases downstream Factors that increase downstream Velocity Discharge Channel size

8 Running water Changes from upstream to downstream Factors that decrease downstream Gradient Channel roughness Base level and graded streams Base level is the lowest point to which a stream can erode

9 Running water Base level and graded streams Two general types of base level Ultimate (sea level) Local or temporary Changing conditions causes readjustment of stream activities Raising base level causes deposition Lowering base level causes erosion

10 A waterfall is an example of a local base level

11 Adjustment of base level to changing conditions Figure 16.9

12 Drainage basin Figure 16.30

13 The drainage basin of the Mississippi River

14 Watershed of Chesapeake Bay

15 Drainage patterns Figure 16.32

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17 Running water Stream valleys The most common landforms on Earth s surface Two general types of stream valleys Narrow valleys» V-shaped» Downcutting toward base level» Features often include rapids and waterfalls

18 Erosion in Fast-moving Streams Figure 7-7 A Steep-Gradient Stream Gradients are steep in headwaters, and streams flow fast, cut steep-sided channels, erode vigorously, and carry coarse sediment. Figure 7-8 Rivers Erode the Geosphere Arizona s Grand Canyon is an example of how effective river erosion can be. In this case the Colorado River downcut rapidly In response to uplift on the Colorado Plateau.

19 Sideways Erosion Cutbanks Figure 7-9 Erosion along River Cutbanks Can Be Hazardous Rivers obviously erode downward but they also erode sideways or laterally, especially if the gradient is lower. On the outside bend of a stream a channel may be eroded; this is called the cutbank (the stream cuts into the bank here). Opposite the cutbank is the pointbar a focus of deposition rather than erosion as streams migrate laterally.

20 Running water Stream valleys Two general types of stream valleys Wide valleys» Stream is near base level» Downward erosion is less dominant» Stream energy is directed from side to side forming a floodplain

21 Running water Stream valleys Features of wide valleys often include Floodplains» Erosional floodplains» Depositional floodplains Meanders» Cut bank and point bar» Cutoffs and oxbow lakes

22 Erosion and deposition along a meandering stream Figure Link: Map of Licking River

23 Erosion and Deposition Low Gradient Streams Near Base Level Broad loops and curves are common along the lower reaches of rivers, near their base levels. These bends, or meanders (Fig. 7-10a), change as a result of erosion and deposition working hand in hand. Figure 7-10 Meanders and Oxbow Lakes (a) Meanders erode banks where river s gradient is low. (b) If a river cuts across narrow neck of a meander loop, sedimentation may seal off the loop, transforming it into an oxbow lake.

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25 Sediment Transport Loads Bedload, is dragged, rolled, skipped, or bounced over the river bottom the latter is referred to as the process of saltation Finer particles carried along in the river water itself are the suspended load. Dissolved minerals and some organic materials leached from soil/rock make up the dissolved load. Figure 7-11 Mechanisms of Sediment Transport Gravel-rich bedload is dominant along steeper river gradients, whereas suspended and dissolved loads are common at lower gradients.

26 Stream Carrying Capacity At a specific velocity and discharge, a stream has a maximum amount of bed and suspended load it can carry = capacity. Capacity with Velocity and Discharge, therefore more erosion and sediment transport occurs during flooding. Material transported in large rivers consists mainly of suspended load, such as silt. Figure 7-12 The World s Muddiest River China s Huang Ho (Yellow River) carries a tremendous suspended load of silt up to 70% of the river s volume. Its watershed has much loess, a silt-rich windblown sedimentary deposit.

27 Running water Incised meanders and stream terraces Incised meanders Meanders in steep, narrow valleys Caused by a drop in base level or uplift of the region Terraces Remnants of a former floodplain River has adjusted to a relative drop in base level by downcutting

28 A meander loop on the Colorado River

29 Running water Headward erosion and stream piracy A stream can lengthen its course by either Building a delta Headward erosion Headward erosion may result in stream piracy the diversion of the drainage of one stream into another

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