Key Points
Key Points will give you the main information you should know to teach the activity.

• Watersheds, also known as drainage basins, are areas of land where all of the water that collects in the area from precipitation will eventually drain into the same river, lake, wetland or other body of water.
• Drainage divides are the boundaries between watersheds. They are areas of high elevation, such as ridges, hills or mountains, in which all of the precipitation on one side of the ridge will flow into one watershed and all of the precipitation on the other side of the ridge will flow into a different watershed.
• South Carolina has four major watershed systems:
  • The Santee River Watershed is the largest area in South Carolina and extends from the Blue Ridge Mountains of South and North Carolina to the ocean in South Carolina.
  • The Pee Dee River Watershed also extends to the Blue Ridge Mountains in North Carolina and covers the northeast corner of South Carolina.
  • The Savannah River Watershed extends to the Blue Ridge Mountains, but is found primarily in Georgia.
  • The Coastal River Watersheds are the only watersheds completely contained in South Carolina. This includes the watersheds to rivers such as the Ashley River, the Edisto River and the Combahee River.

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Detailed Information
Detailed Information gives more in-depth background to increase your own knowledge, in case you want to expand upon the activity or you are asked detailed questions by students.

People are so used to having water easily available to them that most probably do not give any thought to where the water comes from. Every glass of water has its origin in the natural world, and the water molecules in the glass have been traveling around the planet through most of Earth's history. At different times these molecules may have been rain, water vapor, water in rivers or oceans, frozen in glaciers or inside a living organism. To become a glass of drinking water, these water molecules came to a local area, were collected in local wells or reservoirs, and were pumped to a local faucet by water utility company. The way this water traveled from rainwater along the ground to a reservoir is through a watershed. This activity is designed to teach students the basic definition of a watershed and to locate the major watersheds of South Carolina.

A watershed is a delineated area of land where all of the precipitation water drains into a particular stream, river, lake or wetland. Watersheds can be as small as a few acres or as large as a subcontinent. For example, the watershed of the Ashley River is contained entirely in the Lowcountry of South Carolina, while the Santee River watershed extends all the way from the Atlantic Ocean to the Blue Ridge Mountains. Water flowing into the Atlantic Ocean from the Santee River may have come from Columbia, Rock Hill, Greenville, Laurens or even towns in western North Carolina, as the Santee River watershed extends to all of these areas. The Mississippi River watershed, the largest watershed in the United States, extends from the Gulf of Mexico to Southern Canada and from the Rocky Mountains to the Appalachian Mountains. 

The area a watershed drains can also be termed a drainage basin. To make it confusing, the tributary of a river will have its own watershed or drainage basin that is also part of the larger watershed of the river. Tributaries are the smaller streams and rivers that join a river and bring water into it. For example, the Saluda River has a drainage basin that is separate from the drainage basin of the Broad River. Both these rivers’ drainage basins, though, are part of the Congaree River drainage basin, as both of these rivers flow in to the Congaree. All of these river watersheds are part of the Santee River watershed, as all of them bring water into the Santee.

(Click on the picture to enlarge) The boundaries of a watershed are named drainage divides. Drainage divides are areas of relatively high topography in which water on one side of the divide will drain into one basin and the water on the other side will drain into a different basin. The Continental Divide in the Rocky Mountains is an example of a drainage divide, where water on the west side flows to the Pacific Ocean and water on the east side flows to the Gulf of Mexico.

Watersheds contain both surface runoff and groundwater. During rains, some water will flow across the surface, moving down the land as elevation drops until it joins local streams or rivers. This water is termed surface runoff. In South Carolina, the surface water continues moving southeastward in streams and rivers until it reaches the ocean.

Rainwater that infiltrates into the ground is known as groundwater. Water is pulled downwards by gravity until it reaches an impermeable surface. It then moves in a lateral downwards direction, following the topography of the watershed towards the ocean. Groundwater moves very slowly, sometimes being measured in centimeters per year. Though not visible to us, groundwater comprises over 95% of the liquid freshwater on earth.

Watersheds are determined by the shape of the land and are affected by elevation. As gravity pulls water downward, the water moves from higher to lower elevations. Areas of relatively high topography surround watersheds, and these areas become the drainage divides. In a flat area such as the Lowcountry, slight changes in elevation can define drainage divides. The water in the watershed will follow the sloping of the land downward to the sea.

Watersheds are also affected by the underlying soil and rocks. If the underlying soil and rock is resistant to erosion, then the water will flow along the top and will not be able to erode enough material to create much of a channel in the rock. If the underlying rock is highly susceptible to erosion, then deep streambeds and wide river valleys will be created. Each river will have many stream tributaries and the drainage pattern of the watershed will resemble a tree with many branches. This pattern of drainage is known as a dendritic pattern. The dendritic pattern can be found in all the watersheds of South Carolina.

South Carolina is drained by three major watersheds: the Pee Dee River drainage basin, the Santee River drainage basin and the Savannah River drainage basin, as well as a handful of smaller watersheds in the Coastal Plain. These follow the slope of the state from the high elevation of the northwest mountains to the low elevation of the southeast sea. The headwaters, streams at the beginning of watersheds, of these three major watersheds in South Carolina form on the slopes of the Blue Ridge Mountains in North Carolina, east of the Appalachian Divide. Smaller watersheds of rivers and streams, such as the Combahee, Edisto and Ashley River drainage basins, are situated entirely within the Coastal Plain of the state.

Of South Carolina’s three major river watersheds, the largest is the Santee. The Santee and its tributaries in the Blue Ridge Mountains and Piedmont drain nearly 40 percent of the state’s total area. The Pee Dee River winds 197 miles (317 km) from the North Carolina line to the Atlantic Ocean, and its watershed drains the northeastern corner of our state (about 25% of South Carolina). The Savannah River drains about 15 percent of South Carolina’s total area. Though the Savannah River extends 260 miles (418 km) along South Carolina’s border, its watershed only extends 15 to 60 miles (24 to 97 km) into the state. The majority of its watershed is in Georgia. The number of smaller watersheds, such as those of the Ashley, Edisto and Combahee Rivers, form a fourth drainage system south of the Santee watershed. Lying totally within South Carolina and originating in the Coastal Plain, these rivers together drain about 20 percent of the state’s total area. The major watersheds, because they drain a much larger area than the coastal river watersheds, carry a much larger volume of water and transport more sediment than these smaller watersheds.

Watersheds are increasingly a concern for many people because of issues of water quality and contamination. Contamination in rivers and streams not only affects human water supplies, but it has a major effect on the wildlife that depend on these watersheds for water and who do not have access to filters or water treatment plants. Because a river's watershed may extend across an entire state, or even more than one state, if there is a contamination problem in that watershed, it is necessary to look at the entire watershed to determine where the contamination might be coming from. For example, if the Pee Dee River near Georgetown were experiencing water quality problems, it would be necessary to look at the entire Pee Dee Watershed to determine what might be causing this, both in South and North Carolina, since this entire area could be picking up contaminants and bringing it to the Pee Dee River. Students need to be aware of watersheds to realize that people miles upstream from them can affect the local water quality, and also that the students themselves have the ability to affect the water quality of people and wildlife miles downstream from them.

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