Surface Water

Surface Water
Have you ever walked outside during a rainstorm and noticed sheets of water flowing down the sidewalk? Water that flows over Earth’s surface is called runoff. It comes from rain, melting snow or ice, or any water that does not soak into the soil or evaporate.

When rain falls on a pointed roof, it runs down the roof in opposite directions on either side due to gravity. When rain falls to Earth, water flows downhill. A watershed is an area of land that drains runoff into a particular stream, lake, ocean, or other body of water. The boundaries of a watershed are the highest points of land that surround it. These high points are called divides.

Streams
A body of water that flows within a channel is a stream. Scientists use the term stream to refer to any naturally flowing channel of water. For example, a river is a large stream.  Small streams that form near divides are called headwaters. Streams begin at the headwaters and may end at the mouth of a river or where runoff drains into a lake, an ocean, or another large body of water. They can also end when they merge with other streams, disappear underground to become groundwater, or evaporate. 

Lakes
When runoff flows into a basin, or a depression in the landscape, a lake can form. A lake is a large body of water that forms in a basin surrounded by land. Most of Earth’s lakes are in the Northern Hemisphere. Lakes are reservoirs that store water. Most lakes contain
Freshwater.

Wetlands
Water often collects in flat areas or depressions that are too shallow to form lakes.  Conditions like these can create a wetland—an area of land that is saturated with water. for part or all of the year. Wetlands also form in areas kept moist by springs and in areas
along the shores of streams, lakes, and oceans. The water in a wetland can remain still or flow very slowly. 

Glaciers, Sea Ice, and Ice Sheets
Glaciers are large masses of moving ice that form on land. They cover about 10 percent of Earth's surface and are located near the North Pole and the South Pole and on mountaintops. A glacier that spreads over land in all directions is called an ice sheet. Ice sheets cover large areas of land and store enormous amounts of freshwater. The only two ice sheets currently on Earth are in Antarctica and Greenland. Snow and sea ice are also frozen forms of freshwater. Sea ice is ice that forms when seawater freezes. As seawater freezes, salt is left behind in the ocean.

Human Influences on Surface Water
Surface water in a watershed includes streams, lakes, and wetlands. Some natural systems are pushed beyond their limits by humans. There are many ways that surface water in a watershed can be influenced by human activity, including pollution, eutrophication, stream modification, and conservation.
 

Pollution
Runoff can carry fertilizers, pesticides, sewage, and other pollutants that are harmful to organisms living in or using that water. Pollution is the contamination of the environment with substances that are harmful to life. Runoff from construction sites might contain excess sediment, which makes water in streams and lakes cloudy.

This reduces the amount of sunlight available for photosynthetic organisms. Runoff from mines can be acidic and contain toxic metals. It can contaminate surface water, affecting the growth and reproduction of aquatic organisms and water supplies for people.

Eutrophication
The process of a body of water becoming nutrient-rich is called eutrophication. Excess nutrients from fertilizers or sewage can enter a stream and result in an increase in the population of algae. When the algae die, bacteria break down the algae and use oxygen as they decay. If decay rates are too high, oxygen levels in the water can be so low that fish and other animals cannot survive.

Stream Modification
People worldwide depend on sources of freshwater for their water supplies. One method that people use to obtain water is through dams. Streams are often dammed to create reservoirs that store water. But dams can also have negative consequences on the environment and the biodiversity of ecosystems around the river. Dams can increase the rate of erosion along the banks of the streams. They also act as a geographic barrier for migratory fish. Because of dams, some rivers, such as the Colorado river, are nearly dry before they reach the ocean

Urban Development
Wetlands maintain water quality, support ecosystems, and reduce the severity of flooding. Many wetlands throughout the world have been drained and filled with soil for roads, buildings, airports, and housing developments. The disappearance of wetlands has also been associated with rising sea level, coastal erosion, and the introduction of species that are not naturally found in wetlands. Scientists estimate that more than half of all wetlands in the United States have been destroyed over the past 300 years.