The Active River: Agents of Erosion and the Water Cycle

1. Rivers and Groundwater Chapter 11 Preview Section 1 The Active River Section 2Stream and River Deposits Section 3Using Water Wisely Concept Map

2. Section 1 The Active River Chapter 11 Bellringer Do you think a river can have a source? Describe where you would expect to find a river’s source. Write your answers in your science journal.

3. Section 1 The Active River Chapter 11 What You Will Learn • Moving water shapes the surface of Earth by the process of erosion. • The sun is the major source of energy that drives the water cycle. • Three factors that affect the rate of stream erosion are gradient, discharge, and load.

4. Section 1 The Active River Chapter 11 Rivers: Agents of Erosion • Six million years ago, the area now known as the Grand Canyon was as flat as a pancake. • Tectonic uplift raised the land, and water running downhill became the Colorado River. • The Colorado River cut down into the rock.

5. Section 1 The Active River Chapter 11 Rivers: Agents of Erosion, continued • The river formed the Grand Canyon over millions of years through the process of erosion. • Erosion is the process by which wind, water, ice, and gravity move soil and sediment from one place to another. • Rivers are agents of erosion that shape Earth’s landscape.

6. Section 1 The Active River Chapter 11 The Water Cycle • The water in rivers is part of the water cycle. • The water cycle is the continuous movement of water between the atmosphere, the land, and the oceans. • The major source of energy that drives the water cycle is the sun.

7. Section 1 The Active River Chapter 11 The Water Cycle, continued

8. Section 1 The Active River Chapter 11 River Systems • A river systemis a network of streams and rivers that drain an area of its runoff. • A stream that flows into a lake or into a larger stream is called a tributary. (the small stream is contributing to a larger one) • A tributary is part of a river system.

9. Section 1 The Active River Chapter 11 River Systems, continued • River systems are divided into regions called watersheds. • A watershed is the area of land that is drained by a river system. • Watersheds vary in size.

10. Section 1 The Active River Chapter 11 River Systems, continued • The largest watershed in the United States is the Mississippi River watershed.

11. Section 1 The Active River Chapter 11 River Systems, continued • The Mississippi River watershed has hundreds of tributaries. • These tributaries extend from the Rocky Mountains in the West to the Appalachian Mountains in the East. • A main stream that drains a large watershed and has many tributaries is called a river.

12. Section 1 The Active River Chapter 11 River Systems, continued • The Mississippi River watershed covers more than one-third of the United States. • Other major river systems are the Columbia River, the Rio Grande, and the Colorado River watersheds. • Watersheds are separated from each other by an area of higher ground called a divide.

13. Section 1 The Active River Chapter 11 Stream Erosion • A stream forms as water erodes soil and rock to make a channel. • A channel is a path that a stream follows. • As the stream continues to erode rock and soil, the channel gets wider and deeper.

14. Section 1 The Active River Chapter 11 Stream Erosion, continued • Over time, tributaries join the main channel. • The increased flow from the tributaries causes the stream to become longer and wider. • Increased flow can be caused by heavy rains and snow melting.

15. Section 1 The Active River Chapter 11 Stream Erosion, continued • Gradient is the measure of the change in elevation over a certain distance. (slope) • The water in a stream that has a high gradient moves very rapidly. (the steeper the slope the faster the water moves) • This rapid water gives the stream or river a lot of energy to erode rock and soil.

16. Section 1 The Active River Chapter 11 Stream Erosion, continued • A river or stream that has a low gradient has less energy for erosion. • The amount of water that a stream or river carries in a given amount of time is called discharge. • The discharge of a stream increases when a major storm occurs or when snow melts rapidly.

17. Section 1 The Active River Chapter 11 Stream Erosion, continued • As the stream’s discharge increases, the water’s speed and erosive energy increase. • The amount of solid material that the stream can carry also increases. • The faster the water is moving the more particles it can carry down stream.

18. Section 1 The Active River Chapter 11 Stream Erosion, continued • The materials carried by a stream are called the stream’s load. • The size of particles in a stream’s load is affected by the stream’s speed. • Fast-moving streams can carry large particles.

19. Section 1 The Active River Chapter 11 Stream Erosion, continued • Rocks and pebbles scrape along the bottom and sides of the stream bed. • A stream that has a load of large particles has a high rate of erosion. • Streams that move more slowly carry smaller particles and have less erosive energy.

20. Rivers and Groundwater Chapter 11 Types of Load

21. Section 1 The Active River Chapter 11 Describing Rivers • William Morris Davis was a geomorphologist who studied how rivers shape the landscape. • He developed a model to describe the stages of river development. • Davis described rivers as evolving from a youthful stage to an old-age stage.

22. Section 1 The Active River Chapter 11 Describing Rivers, continued • Today, scientists support a different model of river development. • Scientists now take into account climate, local geology, gradient, and load to model the development of a river. • However, scientists still use some of Davis’s terms to describe a river.

23. Section 1 The Active River Chapter 11 Describing Rivers, continued • A youthful river erodes its channel deeper rather than wider. • Its channel is narrow and straight. • The river tumbles over rocks in rapids and waterfalls.

24. Section 1 The Active River Chapter 11 Describing Rivers, continued • Youthful rivers have very few tributaries. • These rivers flow quickly because they have steep gradients. • Many youthful rivers have steep gradients because the areas that these rivers drain have been tectonically lifted.

25. Section 1 The Active River Chapter 11 Describing Rivers, continued • A mature river erodes its channel wider rather than deeper. • The gradient of a mature river is not as steep as that of a youthful river. • Therefore, a mature river has fewer falls and rapids.

26. Section 1 The Active River Chapter 11 Describing Rivers, continued • A mature river is fed by many tributaries. • Because of its large watershed, a mature river has more discharge than a youthful river does. • A mature river also tends to curve back and forth. These curves or bends in the river channel are called meanders.

27. Section 1 The Active River Chapter 11 Describing Rivers, continued • A rejuvenated river forms when land has been raised by tectonic activity. • When land rises, the river gradient becomes steeper and the river flows more quickly. • Rapid flow allows the river to cut more deeply into the valley floor, causing step-like formations called terraces.

28. Section 1 The Active River Chapter 11 Describing Rivers, continued • An old river has a low gradient, so it has little erosive energy. • Instead of widening or deepening its banks, the river deposits rock and soil in and along its channel. • An old river has wide, flat floodplains, or river valleys, and many bends.

29. Section 1 The Active River Chapter 11 Describing Rivers, continued • An old river has fewer tributaries than a mature river does. • The old river has fewer tributaries because its smaller tributaries have joined together. • An old river commonly forms an oxbow lake.

30. Section 1 The Active River Chapter 11 Describing Rivers, continued • An oxbow lake forms when the strip of land that separates parts of a meander is eroded. • An oxbow lake looks like a horseshoe. • The process of forming an oxbow lake shortens the river’s length.

31. Rivers and Groundwater Chapter 11 Old and Rejuvenated Rivers

32. Section 2 Stream and River Deposits Chapter 11 Bellringer Even though flooding along rivers is potentially harmful, many farms are located near rivers. Why do people grow food along rivers? Write your answers in your science journal.

33. Section 2 Stream and River Deposits Chapter 11 What You Will Learn • Three types of stream deposits are deltas, alluvial fans, and floodplains. • Rivers and streams flood their banks in natural and recurring patterns, and these floods affect humans and wildlife habitats.

34. Section 2 Stream and River Deposits Chapter 11 Stream Deposits • Rivers and streams carry large amounts of material, such as soil and rock. • Rivers may carry fertile soil to farmland and wetlands. • Although erosion is a big problem, rivers also renew soils and help form new land.

35. Section 2 Stream and River Deposits Chapter 11 Stream Deposits, continued • After rivers erode rock and soil, they drop, or deposit, their load downstream. • Deposition is the process in which material is laid down or dropped. • Rock and soil that are deposited by streams are called sediment.

36. Section 2 Stream and River Deposits Chapter 11 Stream Deposits, continued • Rivers deposit sediment where the speed of the water decreases– often along the inside bank of a bend. • A river’s current slows when a river empties into a large body of water, such as a lake or the ocean. • As the current slows, a river may deposit its load in a fan-shaped pattern called a delta.

37. Section 2 Stream and River Deposits Chapter 11 Stream Deposits, continued • A delta forms on a flat surface and is made mostly of mud. • These mud deposits form new land and cause the coastline to grow. • The world’s deltas are home to a rich diversity of plant and animal life.

38. Section 2 Stream and River Deposits Chapter 11 Stream Deposits, continued • In the United States, the Mississippi Delta has formed where the Mississippi River flows into the Gulf of Mexico. • Each of the fine mud particles in the delta began its journey far upstream. • Parts of Louisiana are made of particles from as far away as Montana and Minnesota.

39. Section 2 Stream and River Deposits Chapter 11 Stream Deposits, continued • When a fast-moving mountain stream flows onto a flat plain, the stream slows down quickly. • As the stream slows down, it deposits sediment. • The sediment forms an alluvial fan.

40. Section 2 Stream and River Deposits Chapter 11 Stream Deposits, continued • Alluvial fans are fan-shaped deposits of material that, unlike deltas, form on dry land. • Alluvial fans can be found in some desert regions of California.

41. Section 2 Stream and River Deposits Chapter 11 Floods • The amount of water in a stream usually varies seasonally. • During periods of high rain or rapid snowmelt, the amount of water in a stream increases. • As a result, the stream may overflow its banks in a flood.

42. Section 2 Stream and River Deposits Chapter 11 Floods, continued • Flooding can cause a stream to change its path. • During a flood, a stream’s banks may collapse and may change the water’s course. • This process commonly forms oxbow lakes.

43. Section 2 Stream and River Deposits Chapter 11 Floods, continued • When a stream floods, a layer of sediment is deposited on land. • The area along a river that forms from sediment deposited when a river overflows its banks is called a floodplain. • Floodplains are rich farming areas because periodic flooding brings new soil to the land.

44. Section 2 Stream and River Deposits Chapter 11 Floods, continued • Floods are natural events that happen in recurring patterns. • These natural events can cause a great deal of damage. • Wildlife habitats can be buried or washed away.

45. Section 2 Stream and River Deposits Chapter 11 Floods, continued • Human property may be damaged. • The flooding of the Mississippi River in 1993 caused damage in nine states. • Farms were destroyed, and whole towns were evacuated.

46. Section 2 Stream and River Deposits Chapter 11 Floods, continued • Many people have lost their lives in floods. • Barriers can help to control floods. • One type of barrier is called a dam.

47. Section 2 Stream and River Deposits Chapter 11 Floods, continued • A dam is a barrier that can redirect and hold a portion of the floodwater in a reservoir. • Another type of barrier is a levee. • A levee is the buildup of sediment deposited along the channel of a river.

48. Section 2 Stream and River Deposits Chapter 11 Floods, continued • This buildup helps keep the river inside its banks. • Sandbags can be used to build artificial levees to control water during flooding.

49. Section 3 Using Water Wisely Chapter 11 Bellringer Identify each of the following resources as either renewable or nonrenewable: air soil rocks minerals petroleum fresh water wildlife forests What makes a resource renewable? Write your answers in your science journal.

50. Section 3 Using Water Wisely Chapter 11 What You Will Learn • California’s water needs are supplied by using groundwater and surface water, by transporting water from one region of California to another, and by importing water from other areas. • Conserving water and protecting water sources are important in California because the water supply is limited.