Erosion and Deposition Water Sculpting the Earth’s Surface

Water Water covers over 70 percent of the surface of the planet..

Hydrologic Cycle A continuous renewal of the Earth’s supply of fresh water Sources: The Ocean – Largest source All other bodies of water

Processes of the Water Cycle The water cycle operates through a series of processes. These processes are: Evaporation – from ocean, lakes, rivers Condensation – water vapor cools and forms clouds Precipitation – falls to the Earth’s surface as rain, snow, etc.

After falling to the ground… 1 of 3 process can occur: 1. Infiltration - absorbed into the ground 2. Runoff – moves across the Earth’s surface into lakes, rivers, oceans 3. Transpiration – absorbed by plants and then released back into the atmosphere

The Water Cycle Water’s movement between the atmosphere, earth and biosphere makes up the water cycle.

Stream flow Influenced by gravity. The time that it takes water to flow from the land to the sea depends on the water’s velocity.

Where rivers run straight, their velocity is fastest at midstream Where rivers run straight, their velocity is fastest at midstream. In bends, water runs fastest along the outer banks.

Stream flow The ability of a stream to erode and transport sediment depends on velocity Three factors that determine velocity of a stream Gradient Channel Characteristics Discharge

Gradient - The steepness of a stream channel. - The steeper the gradient, the more energy the water has therefore more erosion will take place Mississippi River

Channel Characteristics A stream channel is the course the water follows. As the water flows, it encounters friction from the stream’s sides and bottom. Large smooth streams flow the most. Small rough streams flow the least.

Discharge Discharge is the amount of water passing a point in a given time. Discharge is usually measured in meters3/second.

Factors affecting discharge 1. Precipitation: As additional water reaches the stream due to precipitation, discharge increases. 2. Stream Channel Width: The wider the stream channel, the more water it discharges 3. Stream Contour: The smoother the stream channel, the more the discharge.

The world’s largest river, the Amazon, has a discharge rate of 212,000 meters3/second.

Changes Between Upstream and Downstream While gradient decreases between the head waters and the tail waters of a river, discharge increases. This is because all along its course, rivers are fed by tributaries. Mississippi River Basin

Base Level At Base level (or sea level) rivers enter wide, flat valleys. Here rivers and streams begin to bend back and forth and form meanders. Thames River, England

Meanders

Erosion Streams and rivers eroded their channels by : 1. abrasion 2. grinding 3. dissolving soluble materials.

Sediment Transport Streams transport sediment in three ways: In solution In suspension Rolling material along the bottom.

In solution Material carried in solution by a river is called its dissolved load.

In suspension Material in suspension is called its suspended load. The majority of sediments carried by a river is its suspended load.

Rolling material along the bottom. These materials are called the bed load.

Competence versus Capacity A streams competence measures the largest particle it can carry. Competence increases with velocity. Capacity is the maximum load that a stream can carry. Capacity is directly related to its discharge.

Deposition Deposition occurs when stream flow drops below a critical velocity thus allowing a particle to settle out. Alluvial Iron deposits in a Steam in New Zealand

Deposition During deposition, like-sized particles tend to fall out. This causes deposits to be constructed of similar-sized particles. This process is called sorting. These deposits are called alluvium. Alluvial deposits following the eruption of Mt. St. Helen

Depositional Environments When a river enters the relatively still waters of a lake or ocean, its velocity decreases and forms deltas, or broad, fan-shaped alluvial deposits. Niger River delta

Depositional Environments When streams overflow their banks, they create mounds of sediment parallel to the stream called natural levees.

Stream Valleys Narrow Valleys V-shaped valleys with rapids and waterfalls The primary work of the stream is downcutting toward base level

Stream Valleys - continued Wide Valleys Flat valley floors or floodplains, streams with meanders Downcutting becomes less and as a result the stream’s energy is directed against the sides widening the valley

Floods and Flood Control Most floods are caused by rapid snowmelts in the Spring and heavy rains over a large region.. Upper Mississippi River Valley Flood-1993

Measures to Control Flooding Artificial Levees Limiting Development Flood-control Dams