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Hydrology > Surface Water > Ground Water > Aquifers > Water Cycle

Hydrology is the study of water and its properties, scientific laws, and distribution. Hydrologists study surface water and groundwater and their interactions, which are part of the water cycle.

Surface water is freshwater we see above ground. It includes streams, lakes, ponds and wetlands. Groundwater is freshwater that lies beneath the earth's surface. Most groundwater is stored in aquifers. Aquifers are areas underground where rock and soil are very porous.

Many features of streams and rivers undergo constant change. Throughout a single day we may observe changes in water level, salinity, water flow speed, discharge (the amount of water passing a point on the river in a given amount of time), and sediment load (the amount of soil and gravel suspended in the water). These changes are caused by water input into the river. Rain and melting snow increase the amount of freshwater in rivers. Rain falls directly into rivers, and rainwater and melted snow on land surrounding the river runs downhill into the river. During flood tides, saltwater from the ocean is pushed into the mouths of rivers. The salinity in an estuary increases, and this mix of freshwater and saltwater reaches farther upstream.

Water Level and Salinity

Throughout the day, the water level in the lower part of the Altamaha River rises and falls in rhythm with the tides. Throughout the year, water levels in the Altamaha River watershed change in response to rainfall. In Georgia, winter and summer are the rainiest seasons. From year to year, water levels may change depending on annual rainfall. In the tidal portion of the river, salinity increases with flood tides and decreases with ebb tides. Overall, rainwater dilutes the river and decreases salinity.

Arrow shows a high water mark from a flood event. Click on picture for a larger view.

Water Flow Speed, Discharge, and Sediment Load

Geological changes in the river, such as erosion have several influences: the speed that water is flowing, the amount of water that is flowing, and the amount of sediment in the water. Water flow speed depends on the slope of the land. Flow speed increases as land becomes increasingly steep.

Discharge depends on the width and depth (water level) of the river and on water flow speed. As each of these factors increases, discharge increases. The discharge of the Altamaha River into Atlantic Ocean is 380,000 liters (100,000 gallons) of freshwater every second! If this sounds like a lot, consider the following. Although the Nile River in Africa is the longest river in the world (6,695 km or 4,160 miles), the Amazon River in South America (6,437 km or 4,000 miles) carries the greatest amount of water. The discharge of the Amazon is 199,880,000 liters (52,600,000 gallons) per second. One day's discharge from the Amazon is enough to supply New York City (population 13 million people) with freshwater for over 5 years! The Amazon maintains one-fifth (1/5) of the world's freshwater river flow.

The water in rivers is not pure. It contains suspended particles including soil, fine gravel, algae, and detritus. Many chemical compounds are dissolved in the water, such as salt and acidic gases (the result of acid rain). Particles and chemicals get into the river several ways. Most are produced within the river. Some are carried in by rainwater runoff from surrounding land. In the tidal portion of the river, some may be introduced with seawater.

The water in the picture of the Oconee River at left is reddish-brown because of the sediment in it after a heavy rain.

Click on the picture for a larger view.

As water flows through the river, particles and chemicals in the water erode the bottom and sides of the river. This action creates more sediment in the water. Some sediment settles to the bottom, at different points along the river. Some of the sediment remains suspended in the river. It gets carried all the way to the ocean, where it is deposited at the shore. Sediment load and its pattern of settlement depend on water flow speed and discharge. Faster streams can carry larger particles. Larger rivers can carry greater amounts of sediment. In fact, the third largest river in the world, the Yangtze River in China, carries 1,450,000,000 kg (3,200,000,000 lb) of sediment a year! That is enough to build a wall that would go all the way around the planet.

After a heavy rain you may notice the water in rivers looks very muddy. This may be due to several factors. Rain may have washed in loose soil and plant material. It is likely that the water level is higher, flow speed is faster, and discharge is greater. These changes in river flow result in greater erosion within the river. Together, these factors make a pretty muddy river. After a few days the river usually begins to look like itself again. Sediment settles, and the river becomes clear again.

To learn more about the effects of surrounding land areas on the Altamaha River, read about its Geology.