Peachtree Creek WaterWatch - Atlanta, GA

Peachtree Creek is a major tributary to the Chattahoochee River in Atlanta, GA. The U.S. Geological Survey (USGS) has been monitoring stream stage and streamflow at or near the Northside Drive gage location since 1958. The Peachtree Creek watershed occupies a large portion of the northeast metro Atlanta area and collects precipitation runoff that drains into the Chattahoochee River. This website offers information about the streamflow and flooding characteristics of Peachtree Creek.

As the residents of Woodward Way, which runs parallel to Peachtree Creek near Northside Drive know too well, very heavy rainfall can cause Peachtree Creek to overflow its banks and send waters flooding into their houses. A number of houses on Woodward Way have actually been raised onto stilts. The massive flooding of Sept. 2009 once again caused major flooding damage to houses in this area.

Base flow and flood conditions comparison

The pictures below compares normal base-flow conditions (river stage at about 3 feet) on the left with the flooding in Sept. 2009 (river stage about 20 feet).

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The Peachtree Creek watershed

The Peachtree Creek watershed upstream of the U.S. Geological Survey (USGS) streamflow gaging station at Northside Drive (the blue circle on the map) is of vital importance to the metro Atlanta region. The watershed encompasses an area of 86.6 square miles in the northeast quadrant of the metro Atlanta area. Runoff in the watershed enters Peachtree Creek, which, in turn, flows into the Chattahoochee River a few miles to the west. The altitude at the gaging site is about 764 feet above sea level.

What is a watershed?

A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. The word watershed is sometimes used interchangeably with drainage basin or catchment. Ridges and hills that separate two watersheds are called the drainage divide. The water resources of a watershed include surface water–lakes, streams, reservoirs, and wetlands–and all the underlying groundwater.

A watershed is a precipitation collector

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Most of the precipitation that falls within the Peachtree Creek watershed upstream of Northside Drive collects in the creek and eventually flows by the Peachtree Creek gaging site. Many factors, some listed below, determine how much of the streamflow will flow by the monitoring site. Imagine that the whole basin is covered with a big (and strong) plastic sheet. Then if it rained one inch, all of that rain would fall on the plastic, run downslope into gullies and small creeks and then drain into Peachtree Creek. Ignoring evaporation and any other losses, then all of the approximately 1,512,000,000 gallons of water that fell (you can use our interactive rainfall calculator to find out how many gallons of water fall during a storm) as rainfall would eventually flow by the Peachtree Creek monitoring site.

Not all precipitation that falls in a watershed flows out

To picture a watershed as a plastic-covered area of land that collects precipitation is overly simplistic and not at all like a real-world watershed. A career could be built on trying to model a watershed water budget (correlating water coming into a watershed to water leaving a watershed). There are many factors that determine how much water flows in a stream (these factors are universal in nature and not particular to a single stream):

• Precipitation: The greatest factor controlling streamflow, by far, is the amount of precipitation that falls in the watershed as rain or snow. However, not all precipitation that falls in a watershed flows out, and a stream will often continue to flow where there is no direct runoff from recent precipitation.
• Infiltration: When rain falls on dry ground, some of the water soaks in, or infiltrates the soil. Some water that infiltrates will remain in the shallow soil layer, where it will gradually move downhill, through the soil, and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging ground-water aquifers. Water may travel long distances or remain in storage for long periods before returning to the surface. The amount of water that will soak in over time depends on meteorological conditions and several characteristics of the watershed
• Soil characteristics: Clayey and rocky soils of Georgia's Piedmont region (including the Atlanta area) absorb less water at a slower rate than sandy soils, such as in Georgia's Coastal Plain. Soils absorbing less water results in more runoff overland into streams.
• Soil saturation: Like a wet sponge, soil already saturated from previous rainfall can't absorb much more …thus more rainfall will become surface runoff.
• Land cover: Some land covers have a great impact on infiltration and rainfall runoff. Impervious surfaces, such as parking lots, roads, and developments, act as a “fast lane” for rainfall - right into storm drains that drain directly into streams. Flooding becomes more prevalent as the area of impervious surfaces increase.
• Slope of the land: Water falling on steeply-sloped land runs off more quickly than water falling on flat land.
• Evaporation: Water from rainfall returns to the atmosphere largely through evaporation. The amount of evaporation depends on temperature, solar radiation, wind, atmospheric pressure, and other factors.
• Transpiration: The root systems of plants absorb water from the surrounding soil in various amounts. Most of this water moves through the plant and escapes into the atmosphere through the leaves. Transpiration is controlled by the same factors as evaporation, and by the characteristics and density of the vegetation. Vegetation slows runoff and allows water to seep into the ground.
• Storage: Reservoirs store water and increase the amount of water that evaporates and infiltrates. The storage and release of water in reservoirs can have a significant effect on the streamflow patterns of the river below the dam.
• Water use by people: Uses of a stream might range from a few homeowners and businesses pumping small amounts of water to irrigate their lawns to large amounts of water withdrawals for irrigation, industries, mining, and to supply populations with drinking water.

Why doesn't a stream go dry after it stops raining?

Many streams in Georgia and throughout the world typically continue to flow even after weeks without rain. The amount of water flowing is often referred to as "base flow". Water in the streams during these times comes from groundwater coming from the soil along the stream bank. The groundwater often flows in the direction of the land slope, toward the stream, where it seeps out of the stream bed and banks. Base flow is critical to the stream ecosystem, as well as for human uses of the stream during drought periods. The amount of base flow in a stream depends on the amount of precipitation in the watershed in the previous months, the amount of infiltration in the watershed, and the characteristics of the groundwater aquifers that may supply water to the streams.

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Monitoring of Peachtree Creek

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There are many pieces of equipment, both mechanical and electronic, that are installed at the Peachtree Creek monitoring site to measure, record, and transmit both water-quantity and water-quality information. The U.S. Geological Survey (USGS) monitors "real-time" streamflow and water-quality conditions for thousands of streams nationwide and in Georgia. USGS has continously monitored streamflow at Peachtree Creek since 1958 and has recently begun monitoring water-quality parameters. A number of USGS National Water Information System (NWIS) Webpages displaying water data for Peachtree Creek are available (links below open a new window):

• Peachtree Creek NWIS home page
• Real time stage/streamflow
• Recent daily streamflow
• Historical daily streamflow
• Streamflow statistics: Daily | Monthly | Annual
• Peak streamflow

 Description of the USGS real-time streamflow system.

The job of the water scientist

Long-term monitoring of water resources requires knowledge, skills, and commitment developed over decades by the men and women of USGS. USGS streamgagers and hydrologists must develop and follow precise scientific procedures to assure the quality of water measurements and to properly interpret the meaning of water data. High-quality data is essential for the wise management and use of water resources. Technological advances have improved the amount and timing of data, and made stream monitoring more complex, but these have not reduced the need to physically measure the stream over the range of possible flows, particularly during floods, whenever they occur.

Stream-stage monitoring instrumentation

This picture shows one type of USGS gage structure which is attached to a stilling well (the vertical pipe), and the shelter contains measurement and data-transmission instruments. Equipment in the gage house continuously measure stream height (stage) every 15 minutes. Stilling wells are used at locations where they can built next to the stream, but other systems monitoring water pressure can be used in different situations.
Find out more.

The box on top of the pipe contains a measuring instrument (data logger) that has a pulley with a metal tape holding a float at one end. As the water in the stream rises and falls, the water in the vertical metal pipe also moves and the float on the wire goes up and down with the water level. As the wire moves, the pulley turns, which changes the stream-stage (gage height) reading. The stream-stage readings are recorded by the data logger. Peachtree Creek has a solar-powered GOES satellite system that transmits and uploads stream-stage and water-quality measurements directly to USGS databases to give virtually "real-time" readings of stream stage, streamflow (computed using the stream stage/streamflow relation), and water-quality measurements.

► Find out how USGS measures streamflow.

Water-quality monitoring instrumentation

Like other streams in urban settings, Peachtree Creek is affected by the pressures of urban development, and thus, the potential for water-quality problems are high. Possible sources of problems are:

• Sediment runoff from construction sites
• Potential pollution from runoff from roads and parking lots
• Inflow of warmer water from impervious surfaces (roads, parking lots)
• Fertilizer (nitrogen and phosphorus) runoff from yards and gardens
• Bacteria and pathogens from animal and human wastes
• Runoff containing pesticides and pharmaceutical residue
• Industrial wastes
• Trash

It is important to monitor water quality not only to establish baseline water-quality information about Peachtree Creek, but also to allow for timely notification when water quality changes. USGS recently installed an automated water-quality monitoring site at Peachtree Creek. In the picture, the white tube on the right contains probes and instruments submerged in the stream. The structure on the left houses the data-transmission and automated stream-sampling and storage equipment.

The tube shown in this picture contains probes that continually measure temperature, specific conductance, dissolved oxygen and pH. It also contains tubes to collect water samples during storm events and store the samples in the refrigerated housing structure. It is important to sample the water during storm events, because it is during high water that some water-quality problems, such as high levels of bacteria, sediment, and nutrients (nitrogen and phosphorus) occur.

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Flooding at Peachtree Creek

Stream stage at Peachtree Creek

The U.S. Geological Survey (USGS) uses the term "stream stage", or stage or gage height, measured in feet above an arbitrary altitude, to refer to how high streams are. During dry periods (baseflow conditions), Peachtree Creek typically is at a stage of about 3 feet. During a typical flood, Peachtree Creek might reach a stage of 17 feet. At 17 feet, streamflow is almost 150 times greater (about 5,750 cubic feet per second (ft³/s)) than at 3 feet. On March 8, 1998 stream stage reached 20.16 feet, which means that about 8,800 ft³/s of water was flowing. It is easy to imagine the magnitude of this flood when, at a stage of 3 feet, only 67 ft³/s of water is flowing.

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Comparison of low flow and high flow

The pictures below help show the difference in streamflow during baseflow and a flood. The picture on the left shows Peachtree Creek at a stage of about 3 feet. The picture on the right was taken during the September 2009 flooding, when stage peaked at 23 feet.

► Find out how impervious surfaces in urban areas can cause flooding
► Find out about high-water marks and stream stage

Effects of floods

When Peachtree Creek overflows its banks, water floods nearby streets and can inundate yards and basements of nearby houses, as well as clog streets with mud and debris. High water can also cause problems at bridges. This picture shows a huge pile of debris on the upstream section of the bridge at Northside Drive.

The debris contains whole tree trunks that must weigh tons. If enough debris piled up above the bridge then detrimental effects, such as scouring out of the bridge piers, could eventually occur, or the pipes holding back the debris could break. Local governments have to bring in cranes and heavy equipment to remove this debris.

Largest floods at Peachtree Creek

Peachtree Creek reacts very quickly when heavy rains occur. As is typical with smaller streams in urban areas, a heavy rain can cause the stream to rise in a matter of hours or even minutes. Also, as is typical with smaller urban streams, high water peaks quickly and then falls quickly; thus, streamflow at Peachtree Creek can go from base flow to flooding and back to near base flow in a single day.

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The chart below shows the highest peak streamlows at Peachtree Creek, using historical USGS records. Peak streamflow is the maximum instantaneous measure of the flow of water. Some of the values in the chart are estimates. This chart goes only to about 2005; note that the flood of September 2009 reached a gage height of 22.91 feet (the hightest stage on record) with a streamflow peak of about 9,050 ft³/s. This storm was unique in that the excessive flooding was not caused by rainfall that fell in the watershed of Peachtree Creek, but rather was caused by rainfall that fell west of Atlanta. This rainfall caused historic flooding and high river levels on the Chattahoochee River in the Vinings area. The Chattahoochee River got so high that the water from Peachtree Creek, which flows into the Chattahoochee River essentially "backed up" and could not drain into the Chattahoochee River. So, much of the flooding caused by Peachtree Creek was from water that could not flow where it normally did, so it flowed outward, until the Chattahoochee River dropped enough to let Peachtree Creek flow out.