Ch 06.1: Running Water

6.1: Explain how the water cycle circulates Earth’s water supply in an unending cycle.

6.2: Explain how the water cycle is kept in balance.

6.3: Describe the ability of a stream to erode and transport material.

6.4: Compare and contrast the changes in gradient and discharge between a stream’s headwaters and mouth.

6.5: Define base level.

​Water is found everywhere on EArth.

Water constantly moves among the oceans, the atmosphere, the geosphere, and the hydrosphere.

water cycle - unending circulation of Earth’s water supply

• possible because water readily changes from one state of

  matter to another at temperatures and pressures common on Earth’s surface

• worldwide system powered by energy from the sun and by gravity

Water evaporates into the atmosphere from the ocean.

Condensation causes moisture to form clouds.

Precipitation causes water to fall back to the surface of the Earth.

When precipitation falls on land:

• infiltration - the movement of surface water into rock or soil through cracks and pore spaces

• runoff - the process by which excess water flows over the surface of Earth into lakes and streams when the rate of rainfall exceeds Earth’s absorption ability

• transpiration - the process through which plants absorb water and release it into the atmosphere

• in cold areas, precipitation may become part of a glacier

Balance in the water cycle means the average annual precipitation over Earth equals the amount of water that evaporates and transpires.

Gravity influences the way water makes its way to the oceans:

• Streams and rivers carry water downhill from the land to the sea.

• The time this takes depends on the velocity of the steam.

• Velocity is the distance that water travels in a period of time.

straight stretches - higher steam velocity is near the center of the channel just below the surfacee

• center of the channel is where friction is the lowest

curved stretches - zone of maximum velocity shifts toward its outer bank

The ability of a steam to erode and transport materials depends largely on its velocity.

• factors determining stream velocity include: gradient; shape, size, and roughness of the channel; and discharge

stream channel - the course the water in a stream follows

gradient - the slope or steepness of a stream channel

• usually expressed as the horizontal drop of a steam over a certain distance

• varies over a stream’s length and between streams

• the steeper the gradient, the more energy the stream has as it flows downhill

Friction slows the forward movement of water.

• shape, size, and roughness of the channel affect the amount of friction

• water in a smoother channel flows more easily than water in a rougher channel

• water in a larger channel flows more easily than water in a smaller channel

Discharge

discharge - the volume of water flowing past a certain point in a given unit of time

• usually measured in cubic meters per second

• increases with rainfall and snowmelt

• as discharge increases, channel widens and deepens to handle additional water

• as size of channel incrases, there is less friction and water

  flows more swiftly

• construction can cover soil that once absorbed water; decreased water absorption can increase runoff, leading to an increase in magnitude and frequency of floods

stream profile - a cross-sectional view of a stream or river from its headwaters to its mouth

headwaters - the source or starting point of a stream or river

mouth - the point downstream where a stream or river empties into another body of water

While gradient decreases between a stream's headwaters and mouth, discharge increases.

• the amount of discharge increases because more and more tributaries enter the main channel as it moves downstream

tributary - a stream that empties into another stream

Base level is the lowest point to which a stream can erode its channel.

• the base level at which the mouth of a stream enters the ocean, a lake, or another stream

ultimate base level - sea level - the lowest level that stream erosion can lower the land

temporary base level - lakes, resistant layers of rock, and main streams that act as base level for their tributaries

meander - the many bends in a stream in a broad, flat-bottomed valley that is near its base level

downcutting - occurs when a stream erodes the bottom of a channel, resulting in a deepening of the channel