Watersheds and Drainage Basins
Surface Water Data for the Nation
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When looking at the location of rivers and the amount of streamflow in rivers, the key concept is the river's "watershed". What is a watershed? Easy, if you are standing on ground right now, just look down. You're standing, and everyone is standing, in a watershed.
• Water Science School HOME • Surface Water topics •
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. Watersheds can be as small as a footprint or large enough to encompass all the land that drains water into rivers that drain into Chesapeake Bay, where it enters the Atlantic Ocean. This map shows one set of watershed boundaries in the continental United States; these are known as National hydrologic units (watersheds).
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 watershed consists of surface water--lakes, streams, reservoirs, and wetlands--and all the underlying groundwater. Larger watersheds contain many smaller watersheds. It all depends on the outflow point; all of the land that drains water to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area "above" the river-outflow point.
A watershed is a precipitation collector
Most of the precipitation that falls within the drainage area of a stream's USGS monitoring site collects in the stream and eventually flows by the monitoring 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 that rain would fall on the plastic, run downslope into gulleys and small creeks and then drain into main stream. Ignoring evaporation and any other losses, and using a 1-square mile example watershed, all of the approximately 17,378,560 gallons of water that fell as rainfall would eventually flow past the watershed-outflow point.
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. A water budget is an analysis of the relationship between the amount of water coming into the watershed (from precipitation) and the amount leaving the watershed (in the forms of evaporation, recharge to aquifers, and streamflow coming out at the bottom of the watershed). There are many factors that determine how much water flows in a 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 down, driven by gravity, through the soil. Some of that water never leaves the shallow part of the soil and it eventually is taken up by plants and returns to the atmosphere. This is discussed below in the section on evapotranspiration. Some of that water will move towards a stream and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging groundwater 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 several characteristics of the watershed:
- Soil characteristics: some soils can absorb large amounts of water very rapidly. These are soils containing large amounts of sand or gravel. Other soils absorb water very slowly. These include soils containing large amounts of clay (very fine particles). In some places what lies at the surface is solid rock which may absorb very little water, although some types of rock have fractures that do allow water to enter them. 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, sidewalks, and rooftops, act as a "fast lane" for rainfall - right into storm drains that drain directly into streams. Floods become larger in watersheds where the amount of impervious surfaces is increasing.
- 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/Evapotranspiration: 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. These uses decrease the flow of water in streams, but in many situations a significant part of the water that people use eventually returns to the streams. This is true of irrigation (the water flowing back to the stream is called irrigation return flow). Much of the water taken from the river to supply water to homes and businesses eventually returns to the river because it moves through appliances such as toilets, bathtubs, showers, sinks, dishwashers, and washing machines, and then enters the sanitary sewer system which takes it to a wastewater treatment plant (or “sewage treatment plant”) where some pollutants are removed from the water and the remaining water is returned to a river.
Watershed Example: A Swimming Pool
You're expecting to see a picture of a watershed — so why am I showing you a picture of a common swimming pool in winter with a pool cover? This covered pool is really a good example of a watershed.
The blue cover represents the watershed, the area in which precipitation that falls flows "down-gradient" towards the lowest part of the basin, the center of the pool cover in this case. Likewise, in nature, water flows towards a valley, river, or lake. The lake in the center of the pool forms for the same reason that a lake will form on the landscape — it is the lowest area around and the water comes in, through a river, seepage into the ground, or by evaporation, faster than it can get out. In most valleys, the land slopes downhill somewhere, in which case, a river will form.
Do you see why this pool cover is not exactly the same as a real drainage basin? There is no outlet for water to exit the "watershed", as almost all watersheds have, such as a stream flowing out of the watershed. Thus, in our pool watershed, you can see how a large "lake" is forming. In a dry climate, the water ponded on this pool cover may all evaporate, which can also happen in watersheds situated in arid environments.
Below are other science projects associated with watersheds and drainage basins.
How Does the USGS Collect Streamflow Data?
Sediment and Suspended Sediment
Rain and Precipitation
Impervious Surfaces and Flooding
When looking at the location of rivers and the amount of streamflow in rivers, the key concept is the river's "watershed". What is a watershed? Easy, if you are standing on ground right now, just look down. You're standing, and everyone is standing, in a watershed.
• Water Science School HOME • Surface Water topics •
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. Watersheds can be as small as a footprint or large enough to encompass all the land that drains water into rivers that drain into Chesapeake Bay, where it enters the Atlantic Ocean. This map shows one set of watershed boundaries in the continental United States; these are known as National hydrologic units (watersheds).
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 watershed consists of surface water--lakes, streams, reservoirs, and wetlands--and all the underlying groundwater. Larger watersheds contain many smaller watersheds. It all depends on the outflow point; all of the land that drains water to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area "above" the river-outflow point.
A watershed is a precipitation collector
Most of the precipitation that falls within the drainage area of a stream's USGS monitoring site collects in the stream and eventually flows by the monitoring 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 that rain would fall on the plastic, run downslope into gulleys and small creeks and then drain into main stream. Ignoring evaporation and any other losses, and using a 1-square mile example watershed, all of the approximately 17,378,560 gallons of water that fell as rainfall would eventually flow past the watershed-outflow point.
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. A water budget is an analysis of the relationship between the amount of water coming into the watershed (from precipitation) and the amount leaving the watershed (in the forms of evaporation, recharge to aquifers, and streamflow coming out at the bottom of the watershed). There are many factors that determine how much water flows in a 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 down, driven by gravity, through the soil. Some of that water never leaves the shallow part of the soil and it eventually is taken up by plants and returns to the atmosphere. This is discussed below in the section on evapotranspiration. Some of that water will move towards a stream and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging groundwater 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 several characteristics of the watershed:
- Soil characteristics: some soils can absorb large amounts of water very rapidly. These are soils containing large amounts of sand or gravel. Other soils absorb water very slowly. These include soils containing large amounts of clay (very fine particles). In some places what lies at the surface is solid rock which may absorb very little water, although some types of rock have fractures that do allow water to enter them. 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, sidewalks, and rooftops, act as a "fast lane" for rainfall - right into storm drains that drain directly into streams. Floods become larger in watersheds where the amount of impervious surfaces is increasing.
- 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/Evapotranspiration: 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. These uses decrease the flow of water in streams, but in many situations a significant part of the water that people use eventually returns to the streams. This is true of irrigation (the water flowing back to the stream is called irrigation return flow). Much of the water taken from the river to supply water to homes and businesses eventually returns to the river because it moves through appliances such as toilets, bathtubs, showers, sinks, dishwashers, and washing machines, and then enters the sanitary sewer system which takes it to a wastewater treatment plant (or “sewage treatment plant”) where some pollutants are removed from the water and the remaining water is returned to a river.
Watershed Example: A Swimming Pool
You're expecting to see a picture of a watershed — so why am I showing you a picture of a common swimming pool in winter with a pool cover? This covered pool is really a good example of a watershed.
The blue cover represents the watershed, the area in which precipitation that falls flows "down-gradient" towards the lowest part of the basin, the center of the pool cover in this case. Likewise, in nature, water flows towards a valley, river, or lake. The lake in the center of the pool forms for the same reason that a lake will form on the landscape — it is the lowest area around and the water comes in, through a river, seepage into the ground, or by evaporation, faster than it can get out. In most valleys, the land slopes downhill somewhere, in which case, a river will form.
Do you see why this pool cover is not exactly the same as a real drainage basin? There is no outlet for water to exit the "watershed", as almost all watersheds have, such as a stream flowing out of the watershed. Thus, in our pool watershed, you can see how a large "lake" is forming. In a dry climate, the water ponded on this pool cover may all evaporate, which can also happen in watersheds situated in arid environments.
Below are other science projects associated with watersheds and drainage basins.
How Does the USGS Collect Streamflow Data?
Sediment and Suspended Sediment
Rain and Precipitation