Inspire Biology

Headwater

River

Lake

Mouth

Estuary region

Earth’s water

97.5%

Saltwater

2.5%

Freshwater

68.9%

Glaciers

0.3%

Lakes and rivers

30.8%

Groundwater

Figure 18

The vast majority of Earth’s water is salt water. Most of the freshwater supply is locked in glaciers.

Figure 19

Mountain streams have clear, cold water that is highly oxygenated and supports the larvae of

many insects and the coldwater fish that feed on them. Rivers become increasingly wider, deeper, and

slower. At the mouth, many rivers divide into many channels where wetlands or estuaries form.

Rivers and streams

The water in rivers and streams flows in one

direction, beginning at a source called a head-

water and traveling to the mouth, where the

flowing water empties into a larger body of

water, as illustrated in

Figure 19.

Rivers and

streams also might start from underground

springs or from snowmelt. The slope of the

landscape determines the direction and speed of

the water flow. When the slope is steep, water

flows quickly, causing a lot of sediment to be

picked up and carried by the water.

Sediment

is material that is deposited by water,

wind, or glaciers. As the slope levels, the speed

of the water flow decreases and sediments are

deposited in the form of silt, mud, and sand.

The characteristics of rivers and streams change

during the journey from the source to the

mouth. Interactions between wind and the

water stir up the water’s surface, which adds a

significant amount of oxygen to the water.

Interactions between land and water result in

erosion, in nutrient availability, and in changing

the path of the river or stream.

Module 3 • Communities, Biomes, and Ecosystems