Chapter 15 – River Systems Section -The Water Cycle Section 2- Stream Erosion Section 3 – Stream Deposition

Section 1 – The Water Cycle The origin of the Earth’s water is a mystery. The Earth’s surface receives 5 times as much water as rivers carry to the ocean. Water is essential to all humans and other organisms and it needs to be available in all three phases for life to continue. 2/3 of the Earth’s surface is covered in water: it is held in oceans, water flows in rivers and streams, it is held in lakes, it is frozen in icecaps, and it flows through rock underground as ground water. Water is also found in the tissues of all organisms. Water is also an invisible gas found in the atmosphere.

Earth’s water is constantly changing from one form to another. Water vapor falls from the sky as rain Glaciers melt to form streams Rivers flow to the ocean where liquid water escapes into the atmosphere as water vapor This continuous movement of water on the Earth’s surface from the atmosphere to the land and oceans and back into the atmosphere is called the Water Cycle.

The Water Cycle

Evaporation – The process by which liquid water changes into water vapor. Transpiration – the process by which plants release water into the air-a byproduct of photosynthesis. Evapotranspiration – The total loss of water from an area that includes the sum of the evaporation and transpiration amounts. Condensation – the change of state from a gas to a liquid usually due to cooling temperature. Precipitation – Any form of water that falls to the Earth’s surface from clouds: rain, snow, sleet, and hail.

The Water Budget is the continuous cycle of evapotranspiration, condensation, and precipitation. Precipitation is the income, and evapotranspiration and runoff are the expenses. The water budget for the whole Earth is balanced because the amount of precipitation is equal to the amount of evapotranspiration and runoff. The water budget in a local area is usually not balanced due to temperature, vegetation, wind, and the amount and duration of rainfall. The local water budget is also affected by seasons of the year. (More evaporation in summer.)

The average person uses 20,890. 5 gallon of water each year The average person uses 20,890.5 gallon of water each year. A person uses water for bathing, washing clothes and dishes, watering lawns, flushing the toilet, and drinking. Agriculture and industry use large amounts water. As the population increases, the demand for water increases. 90% of the water used by cities and industry is returned to rivers and the oceans as wastewater with pollutants that can harm plant and animal life.

There is only a small amount of water that is fresh and available for human use. There are two ways to ensure fresh water for today and the future: Conservation – Individuals must limit water use as much as possible. Governments can help conserve by enforcing conservation laws and antipollution laws to prevent dumping into bodies of water Desalination – a process for removing salt from ocean water. Now it is expensive and impractical.

Section 2 – Stream Erosion Rivers form from runoff when the ground has soaked up as much water as it can. A river system is made up of a main stream and tributaries, which are the feeder streams that flow into the main stream. The land from which all this water flows is called the watershed. The ridges or elevated regions that separate watersheds are called divides. The narrow depression that a stream follows as it flows downhill is called its channel. The edges of a stream channel above the water level is the stream’s banks. The part of the channel below the stream’s water level is the bed.

A stream transports soil, loose rock fragments, and dissolved minerals as it flows downhill. The materials carried by the stream are called the stream load. The suspended load – all particles of silt and fine sand that float The bed load - larger, coarser materials such as coarse sand, gravel and pebbles that slide or jump down the stream bed The dissolved load – mineral matter that is transported in liquid solution

A stream’s discharge is the volume of water moved by a stream in a given time period. The faster a stream flows, the higher its discharge and the greater the load it can carry. Swift streams erode their channels more quickly than slow-moving streams do. The change of elevation over a given horizontal distance is its gradient. In the beginning of the stream, the gradient is steep. At the mouth of the river, or the end, the gradient becomes flatter.

When a stream becomes wider and deeper due to erosion, it is called a river. As a river changes, it may develop curves and bends. A river with a low gradient has more bends than a river with a steep gradient. The winding pattern of wide curves in a river is called a meander. The faster moving water on the outside of a river bend erodes outer bank of that bend. On the inside of the same bend the water is moving more slowly and dropping its load. Meanders can become so curved that they almost form a loop. The meander can become isolated from the river and become an oxbow lake.

The Making of an Oxbow Lake

Section 3 – Stream Deposition A stream carries the greatest load when a large volume of water is flowing swiftly. As the velocity of the water slows, the ability of the stream to carry its load decreases. Delta – a triangular-shaped deposit that forms where the mouth of a stream enters a larger body of water. The exact shape of the delta depends on the waves, tides, offshore depth and sediment load of the stream. Alluvial fan - a fan-shaped deposit formed on land where a stream deposits some of its load on a level plain at the base of the slope.

Nile Delta from space

Close up of Nile Delta

Delta

Alluvial Fan in Iran

Alluvial Fan in Death Valley, CA

The floodplain is the part of a valley floor that may be covered with water when there is a stream with an extremely large volume of water due to heavy rains or large snowmelt. Frequent flooding can occur when vegetation, trees and grass are removed for agriculture, logging, or housing development. The speed and volume of runoff increases when there is no vegetation. The most common method of direct flood control is the building of dams. The artificial lakes behind the dams are used to generate electricity, supply fresh water, and irrigate farmland.

Sometimes water from streams collects in depressions in the land forming lakes. Most water in lakes comes from precipitation and the melting of ice and snow. The life span of most lakes is short-lived. Many lakes disappear because too much of their water drains away or is evaporated. A lake basin can also disappear when the lake fills with sediment. Organic deposits from vegetation can accumulate on the lake floor. As the deposits grow denser, a bog or swamp may form. Eventually, the lake basin can become dry land.