HSES_1eTE_C06.qxd 9/23/04 9:49 PM Page 164 Section 6.2 6.2 The Work of Streams 1 FOCUS Section Objectives 6.6 Explain how streams erode Key Concepts Vocabulary Reading Strategy their channels and transport How do streams erode ◆ bed load Monitoring Your Understanding Preview sediments. their channels and ◆ capacity the Key Concepts, topic headings, vocabulary, transport sediment? ◆ alluvium and figures in this section. List two things you 6.7 Explain how stream deposition expect to learn about each. After reading, state How does stream ◆ delta occurs. what you learned about each item you listed. deposition occur? ◆ natural levee 6.8 Identify the two general types What are the two types ◆ floodplain What I Expect to Learn What I Learned of stream valleys. of stream valleys? ◆ flood 6.9 Predict the causes of floods What causes floods, and ◆ drainage basin what are the major flood and describe major flood ◆ divide control measures? control measures. What is the relationship 6.10 Explain the relationship between a stream and between streams and drainage a drainage basin? basins. Reading Focus Streams are Earth’s most important agents of erosion. They can downcut or erode their channels. They can also transport enormous L2 Build Vocabulary amounts of sediment. Most of the sediment a stream carries comes Paraphrase Ask students to write the from weathering. Weathering produces huge amounts of material that vocabulary terms on a sheet of paper. are delivered to the stream by sheet flow, mass movements, and Instruct students to write a definition, in groundwater. Eventually, streams drop much of this material to create their own words, for each term as they many different depositional features. encounter the term while going through the chapter. After writing their own Erosion definition, encourage students to write Streams generally erode their channels lifting loose particles by the term in a complete sentence. abrasion, grinding, and by dissolving soluble material. When the L2 flow of water is turbulent enough, it can dislodge loose particles from Reading Strategy the channel and lift them into the moving water. In this manner, the Answers will vary. Sample answer: force of running water rapidly erodes some streambeds and banks. What I Expect to Learn—how erosion The stronger the current is, the more erosional power it has and the happens and what features it can form more effectively the water will pick up particles. What I Learned—streams erode by Sand and gravel carried in a stream can erode solid rock channels lifting loose particles and by abrasion like sandpaper grinds down wood. Moreover, pebbles caught in meanders swirling stream currents can act like cutting tools and bore circular “potholes” into the channel floor. 2 INSTRUCT What are three ways that streams erode their channels? Erosion Build Reading Literacy L1 Refer to p. 334D in Chapter 12, which 164 Chapter 6 provides guidelines for outlining. Outline Have students outline the section, leaving room for notes. Then have students scan through each heading and find the main idea. Allow students to refer to their outlines when answering the questions in Section 6.2 Assessment. Logical, Verbal 164 Chapter 6 HSES_1eTE_C06.qxd 5/16/04 9:09 AM Page 165 Sediment Transport Sediment Transport Integrate Chemistry L2 Streams transport sediment in three ways. Solutions and Suspensions Streams 1. in solution (dissolved load) transport sediment in solution and in 2. in suspension (suspended load) suspension. In chemistry, a solution is 3. scooting or rolling along the bottom (bed load) a homogeneous mixture of dissolved substances. A suspension is a Dissolved Load Most of the dissolved load enters streams heterogeneous mixture that separates through groundwater. Some of this load also enters by dissolving rock into layers over time. Ask students to along the stream’s course. The amount of material the stream carries write a paragraph explaining how in solution changes depending on climate and the geologic setting. streams are both solutions and Usually the dissolved load is expressed as parts of dissolved material suspensions. (Streams are composed, per million parts of water (parts per million, or ppm). Some rivers may in part, of groundwater that contains have a dissolved load of 1000 ppm or more. However, the average dissolved substances, and in this way figure for the world’s rivers is estimated at 115 to 120 ppm. Streams streams are solutions. Streams also supply almost 4 billion metric tons of dissolved substances to the transport fine sand, silt, and clay that are oceans each year. not dissolved, but rather suspended in moving water. In this way, streams are Suspended Load Most streams carry suspensions.) the largest part of their load in suspension. Verbal The visible cloud of sediment suspended in the water is the most obvious portion of a Build Science Skills L2 stream’s load. Streams usually carry only sand, silt, and clay this way. However, streams Using Models Some students will have also transport larger particles during a flood difficulty comprehending the tiny scale because water velocity increases. The total of one part per million. To help build amount of material a stream carries in sus- number sense and awareness of how pension increases dramatically during floods, small a part per million is, ask students as shown in Figure 8. to imagine they have a budget of one million dollars, and have them think of Bed Load Bed load is that part of a the cost of their lunch in terms of parts stream’s load of solid material that is made up per million. A four-dollar lunch would be of sediment too large to be carried in suspen- equal to 4 ppm. This represents only a sion. These larger, coarser particles move along the bottom, or bed, of Figure 8 During this 1997 flood, tiny fraction of their total budget. the suspended load in the muddy Logical the stream channel. The suspended and dissolved loads are always Ohio River is clearly visible. The moving. But the bed load moves only when the force of the water is greatest erosion and sediment great enough to move the larger particles. The grinding action of the transport occur during floods. Applying Concepts What other bed load is very important in eroding the stream channel. types of load might account for the muddiness of the river? Competence and Capacity The ability of streams to carry a load is determined by two factors: the stream’s competence and its capacity. Competence of a stream measures the largest particles it can transport. A stream’s competence increases with its velocity. In fact, the competence of a stream increases four times when the velocity doubles. The capacity of a stream is the maximum load it can carry. Capacity is directly related to a stream’s discharge. The greater the volume of water in a stream is, the greater its capacity is for carrying sediment. Running Water and Groundwater 165 Customize for English Language Learners Students who are learning English can benefit news. Have them discuss the type of damage from real-life examples that relate to science done by the flood and some of the amazing content. Encourage students to think of actual pictures of rescues and houses floating flooding events that may have occurred in downstream. Encourage students to share Answer to . your area or that they have heard about on the their knowledge and examples with the class. Figure 8 Dissolved load might account for the muddiness of the river. Streams erode their channels lifting loose particles by abrasion, grinding, and by dissolving soluble material. Running Water and Groundwater 165 HSES_1eTE_C06.qxd 9/23/04 11:04 PM Page 166 Section 6.2 (continued) Mississippi Delta Region A t Deposition c Figure 9 h a Baton Rouge fa la M Movement This map y a is shows the growth of the R s i is v s Mississippi River delta over e ippi Riv r er the past 5,000 to 6,000 years. New Orleans Alluvium L2 As you can see, the river has 2 built a series of sub-deltas, Purpose Students learn how one after the other. The deposition occurs in a stream. numbers indicate the order in 4 Materials mixed sediment (pebbles, which they were deposited. 6 Locating In which overall sand, silt, and clay-sized particles), clear direction has the Mississippi 1 3 glass jar with lid, water River built its delta over the 5 7 past few thousand years? Procedure Fill the jar about one-third Gulf of Mexico full with the mixed sediment. Pour water Locating How has the growth of the delta changed into the jar and tightly close the lid. the location of the mouth of Shake the jar and allow sediment to the Mississippi River in settle out. relation to New Orleans? Expected Outcome Sediment should settle out in layers, with pebbles on the bottom and clay-sized particles at the Deposition top. The water sorts the material as the energy from the shaking dissipates, the Whenever a stream slows down, the situation reverses. As a stream’s heavier material drops out sequentially. velocity decreases, its competence decreases and sediment begins to drop If this is not illustrated, try using a larger out, largest particles first. Each particle size has a critical settling veloc- jar, which will allow more room for the ity. Deposition occurs as streamflow drops below the critical materials to settle out. settling velocity of a certain particle size. The sediment in that cate- gory begins to settle out. Stream transport separates solid particles of various sizes, large to small. This process is called sorting. It explains why particles of similar size are deposited together.