Some rain water soaks through the ground while some evaporates into gas. The rain water that doesn’t do wither one of these, runs over the ground and ends up in lakes, streams and oceans and is called run off.
The amount of rain and how long it rains for are two factors that affects run off.
Light rain over a long period will have time to soak into the ground, where as heavy rain over a short period of time will not.
Another factor for the amount of run off is steepness or slope of the land, because gravity causes water to move down slopes more rapidly.
When water moves down a slope quickly, it does not have a chance to soak into the ground and vice versa.
Vegetation increases the amount of water soaked into the ground. The presence of plants, slows down water and allows it to be soaked into the ground better.
When water travels down the same slope time after time, it erodes a path into the slope.
Rill erosion begins when a small stream forms during heavy rain. As the stream flows along, it erodes and carries soil away.
As water continues to move down this path, it creates a groove on the slope called a channel.
If this continues, rill erosion turns into gully erosion, where the channel becomes broader and deeper.
Sheet and Stream Erosion
Water erodes soil without having to be in a channel. Water often runs off in a thin broad sheet before forming rills and streams like water flowing over a car.
Sheet erosion occurs when water that is flowing as sheets, picks up and carries away sediments.
As water in a stream moves along a channel, it picks up sediments from the bottom and sides of it.
The sediment that it being carried here is called the streams load. The lighter part of this load is called suspended load. The heavier part of the load is called the bed load.
The bed load rolls along the bottom of the stream channel eroding other segments of the channel.
River System Development
Streams are part of river systems. The water comes from rills, gullies and smaller streams that are upstream.
Run off enters streams, and these streams flow into rivers.
A drainage basin is the area of land from which a stream or river collects run off ( like a bathtub). Like water in a bathtub goes towards the drain, water in a river system flows to the main river. The largest one being the Miss. River drainage basin.
Stages if Stream Development
Some streams are narrow and swift, others are wide and slow. This is because they are in different stages of development.
Streams are classified as young, mature, or old.
Young streams flow swiftly through steep valleys. They may have white water rapids and waterfalls.
Mature streams is the next stage, and it flows more smoothly through its valley. A mature stream erodes along its sides and causes curves to form. Water in a shallow area will move slower, and water in a wider part of the channel will flow faster.
As the curve of a mature stream becomes a broad arc, it is known as a meander
This is the last stage. Ole streams flow smoothly through a broad, flat flood plain, the flat valley floor formed by a meandering stream.
Old streams meet at drainage basins and form a major river.
Too Much Water
Heavy rains or melting of snow and ice can cause large amounts of water to enter a river system.
This could cause rivers to overflow, and can bring disaster by flooding homes or destroying bridges and crops.
Dams and levees are built to prevent rivers from overflowing their banks.
Delta’s and Fans
A delta is a triangular or fanned shaped deposit of sediment that is located where water empties into an ocean or lake.
Where water enters the Gulf of Mexico from the Miss. Is called the Mississippi delta.
H.W. pg 248, 1-4. Page 266, 1-3,5,8,9,11.
Water that soaks into the ground and collects in the pores of the soil is called ground water
Scientists estimate that 14% of fresh water on Earth exists as ground water.
Soil and rock are permeable because the pore spaces are connected and water can pass through them. Rock and soil that do not have large connected pores are considered impermeable and water cannot pass through them.
Ground Water Movement
Ground water continues to go deeper and deeper until it reaches impermeable rock. The layers of permeable rock that let the water through are called aquifers.
When all of the pores on top of permeable rock are filled with water, it is called a zone of saturation.
The upper surface of this zone is called the water table.
The water table is where many people get their water from by using wells drilled into the water table.
Wells and Springs
Wells extend into the zone of saturation past the top of the water table. Ground water flows into the well and a pump brings the water to the surface.
Wells go dry when the water table drops below the well during dry season.
If the water table is too close to the surface, the water table flows out and forms a spring. Water from springs is a constant cool temperature, however in some places magma heats up rock surrounding the spring and causes a hot spring.
A geyser is a hot spring that erupts periodically shooting water and steam into the air.
They occur when ground water is heated to high temperatures causing the water to expand underground and causing it to shoot out the opening at the surface like a teakettle.
Water dissolves limestone to create caves.
Sometimes water dripping from cracks in the cave can contain calcium ions causing stalactites to form. These hang off the ceiling. Where drops of water fall to the floor of the cave, a stalagmite forms.
Sinkholes are depressions on the surface of the ground that form when the roof of a cave collapses.
H.W. pg 254, 1-3. Page 264, 1-10.
Waves constantly pound against the shore and break rocks into small pieces. Currents then move many metric tons of sediment along the shoreline.
The sediment grains are carried out to deep water and when the tide returns, the tide brings new sediments with it.
These forces are always at work and slowly change the shape of a shoreline. Three major forces are waves, currents and tides.
Wind blowing across water makes waves.
Rocky Shore Lines
Rocks and cliffs are common along rocky shorelines. Waves crash against these rocks and cause them to weather and erode.
These sediments are transported by longshore currents, which are currents that run parallel to the shore line.
Smooth gently sloping shorelines are where beaches form. Beaches are made of rock fragments, grains of quartz, and seashell fragments.
Waves break rocks and seashells into sand sized particles and deposit them parallel to the shore. Longshore currents also carry sand along beaches to form barrier islands which are sand deposits that are parallel to the shore, but separated from the mainland.