Where would be the most strategic point on a levee for a landowner to detonate some dynamite? (see pp.41– 42) 5. List and explain two factors that contributed to the current mess of a situation in the lower Mississippi region. Topic Overview Unlike most planets, earth has an abundance of liquid water, and this water does a lot of work moving sediment around and sculpting the landforms we study in physical geology. Water has the power to weather, transport, and deposit sediment creating both erosional and depositional landforms. This week, we will examine the two major processes where water sculpts the surface of earth: running water in stream channels and coastlines of oceans. Water drains off continents to the oceans in streams organized into drainage basins or watersheds. Drainage basins scale up from small streams to large river networks. An area of high ground that separates one drainage basin from another is called a divide. All drainage networks have a zone of erosion near the headwaters where the smallest streams originate. Further downstream, drainage networks have a zone of transport with welldeveloped floodplains. At the mouth of a river, a zone of deposition creates a delta where the stream reaches base-level (ultimately the ocean). On floodplains, streams meander from side to side and the channel changes location often. High velocity water on the outside curves of meanders (cutbanks) erode sediments and widen the floodplain. Sediment is deposited by slow velocity water on point bars—the inside curves of meanders. When discharge is increased and the amount of water exceeds the capacity of the stream channel, a flood occurs and sediment is deposited on floodplains. The largest and heaviest sediments are deposited close to the stream channel and build up natural levees. Streams are equilibrium systems. For example, if the gradient of a stream is reduced by building a dam, the velocity of the stream will decrease, and sediment will be deposited. If the gradient is steepened by uplift of the land relative to base level, velocity will increase, and the stream will cut down into its channel until a new equilibrium gradient is reached. The landforms along coastlines are shaped primarily by the action of waves breaking along the shore. Emergent coastlines are areas where there has been uplift of the land relative to the sea. Many erosional features such as sea stacks, sea arches, sea cliffs, and wave-cut platforms can be observed along emergent coastlines. Submergent coastlines are areas where sea level has increased relative to the land. In areas with submergent coasts, the shoreline is often irregularly shaped and includes estuaries. As sediment is deposited along coastlines by rivers, that sediment is moved and shaped by wave action. Longshore currents can move sand parallel to the coastline in a process called longshore drift. Drifting sand can create depositional features such as spits, baymouth bars, and barrier islands. People often build structures such as jetties, groins, and seawalls to stabilize beaches or protect human structures in the face of erosion and deposition. However, these structures are only temporary because the forces of erosion and deposition are always at work. Mini-Lecture Videos • • • Running Water (14:09 min. in length, CC available) Coastlines (17:35 min. in length, CC available) Atchafalaya (17:17 min. in length, CC available) Textbook Readings • • An Introduction to Geology: chapters 11 & 12 The Control of Nature: Atchafalaya Discussion Question: In Atchafalaya, John McPhee discusses some of the hazards that come about from the dynamics of the Mississippi River and the efforts of humans to control that system. Which of the following would be most effective strategy for reducing the negative consequences (money spent, lives and property at risk) of the current situation on the lower Mississippi? Briefly justify your choice based on what you know about river systems. A. Dismantle the levees and flood control projects B. Build higher levees and larger flood control projects C. Increase zoning restrictions on land use in floodplains D. Restrict land uses that lead to greater runoff upstream E. Stop government subsidies of flood insurance and flood control F. Another solution? In addition to your post, please reply to and constructively critique a classmate’s post. Classmate’s post: Isabella Holman In my opinion, the most effective strategy for reducing the negative consequences of the current situation on the lower Mississippi would be option D, restrict land uses that lead to greater runoff upstream. I think this is a good option because the system is already surrounded by levees and other flood control projects that are not only directing the water from the Mississippi River through the Basin to the Gulf of Mexico but have also been successful with measures that are already in place like the use of levees to prevent the overflow of water. Since those are in place I believe measures that prevent more water runoff will allow the system to work effectively with knowing the levels won’t rise as much. With that, nature isn’t exactly predictable but controlling the human causes of runoff may help as the risk of runoff poses a threat to increased flooding and polluted water.
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