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Stream stage and streamflow at Peachtree Creek
Peachtree Creek WaterWatch
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 ground water.
A watershed is a precipitation collector
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 ground water coming from the soil along the stream bank. The ground water 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 ground-water aquifers that may supply water to the streams.