Streams play an important role in erosion Streams play an important role in erosion. The stream banks collapse in mass movement. The soil is then carried away by the stream as sediment.

The primary force of erosion is gravity, which pulls sediment and water downhill. Deposition is the process in which sediment is laid down in new locations. Most sediment is moved and deposited by flowing water.

Running Water Erodes the Land What is the most important factor influencing the ability of a stream to cause erosion?

Running Water Erodes the Land Erosion begins when runoff carries small particles of soil downhill. Runoff forms small channels in the soil. Channels join together to form gullies. Gullies flow together to form streams.

Running Water Erodes the Land Most of the sediment in streams is carried in suspension, in which tiny sediment grains move along with the water. Larger particles of sediment slide, roll, or bounce along the bottom of a stream. The process of particles bouncing along a stream bottom is called saltation. A large amount of sediment is also carried in solution.

Running Water Erodes the Land The water in a mountain stream moves quickly because it is flowing down a steep slope. Fast streams carry more sediment, which causes more abrasion. Fast streams can also carry large sediment grains. As a stream flows toward the sea, its slope decreases, and larger sediments settle on the stream bottom.

Running Water Erodes the Land Streams continue the process of erosion by transporting sediment. Direction of flow

Features Formed by Water Erosion What features are formed by surface water erosion?

Features Formed by Water Erosion V-Shaped Valleys Near a stream’s source, the stream flows fast down steep slopes. Mass movement on the stream slopes causes a V-shaped valley with sharply angled sides to form. A waterfall may develop where a stream crosses rock layers that differ in hardness.

Features Formed by Water Erosion As a river winds its way from the mountains to the ocean, it changes the surrounding landscape through erosion and deposition. Waterfall V-shaped valley Meander Oxbow lake Sediment River mouth

Features Formed by Water Erosion Flood Plains Where a river or stream crosses gently sloping land, it forms a flood plain, a flat area along a stream that is entirely covered only during times of flood. Sediment builds up into long, low ridges called natural levees, which help prevent a river from spilling over its banks.

Features Formed by Water Erosion At a curve in a river, the water on the outside of the curve moves more rapidly than the water on the inside and causes more erosion. Sediment is deposited on the inside of the curve, where water moves more slowly. This process forms a looplike bend in the river called a meander.

Features Formed by Water Erosion Sometimes during a flood, the river erodes through a narrow neck of land at the base of a meander and forms a new path. Sediments build up and cut the old meander off from the rest of the river. The result is a separate, curved lake, called an oxbow lake.

Features Formed by Water Erosion Rivers often form winding meanders and oxbow lakes across their flood plain.

Features Formed by Water Deposition What features are deposited by running water?

Features Formed by Water Deposition Deposition is the process in which sediment is laid down in new locations. As a stream flows out of the mountains and onto the plains, it slows down and sediment settles out. An alluvial fan is a fan-shaped deposit of sediment on land. Alluvial fans often grow into thick deposits of sediment. A delta is a mass of sediment deposited where a river enters a large body of water.

Features Formed by Water Deposition This alluvial fan in Death Valley, California, formed from sediment deposited when a mountain stream reached flat land. When a large river empties into the ocean, the deposited sediment often forms a delta.

Groundwater Erosion What causes groundwater erosion?

Groundwater Erosion Erosion and deposition occur below ground as well as at the surface. Carbon dioxide in the air combines with rainwater to form carbonic acid, which reacts with some rocks.

Groundwater Erosion Limestone easily erodes away through this process, forming caves, or caverns. Caves usually form in the saturated zone, below the water table. When the water table drops, it leaves dry caves.

Groundwater Erosion Sometimes water drips into the cavern from the rock layers above, carrying dissolved minerals that are left behind when carbon dioxide escapes from the water. Water dripping from the ceiling forms a stalactite. Water dripping to the floor forms a stalagmite.

Groundwater Erosion If erosion weakens a layer of limestone, entire portions of the ground can suddenly collapse. The resulting hole is called a sinkhole. Sinkholes can appear suddenly, swallowing buildings and roads.

Groundwater Erosion Carlsbad Caverns in New Mexico contain spectacular formations, such as the stalactites and stalagmites shown here.

Assessment Questions What is the most important factor in determining a stream’s ability to cause erosion? width of the stream depth of the stream composition of the water speed of the stream

Which feature is most likely to form near the source of a river? Assessment Questions Which feature is most likely to form near the source of a river? meander flood plain V-shaped valley alluvial fan

Assessment Questions Caves and sinkholes are geological features that are generally found where the bedrock consists of granite. quartzite. sandstone. limestone.