Watershed Primer Part 1: Watershed Basics
Water covers nearly three quarters of the Earth’s surface. It is present in the atmosphere, and in the Earth’s crust and composes a large percentage of all living plants and animals (including humans). Of all of the hundreds of thousands of chemical substances that exist, none is more important to plant and animal life than water. Water is also the most widely used of all solvents, as it easily dissolves many gases and substances.
About one-half gallon of water is needed each day to satisfy the biological needs of a single person. However, in the U.S., each person also requires about 100 gallons per day to maintain cleanliness, to prepare food and to maintain a comfortable indoor environment (heating and cooling). About 3 gallons of water are required to produce 1 gallon of milk. About 10 gallons of water are required to produce 1 gallon of gasoline. About 80 gallons of water are required to produce 1 kilowatt of electricity. (van der Leeden, et. al. 1990)
Pure water is an odorless, tasteless and colorless liquid. The pleasant taste experienced when drinking "fresh water" is due to dissolved gases from the atmosphere and dissolved salts from the earth. All water in nature is impure and contains substances dissolved by the water as it falls through the atmosphere and flows over soil and rock. Rain collects dust and dissolved gases as it falls. Other impurities in the water depend upon the nature of the soil and rock that it passes over. Impurities can include suspended solids (sand, clay, mud, silt, leaves and organic matter, and microorganisms, etc.), dissolved chemicals (oxygen, nitrogen, carbon dioxide, ammonia, chloride, sulfates, calcium, metals, etc.), and dissolved organic substances (such as from the decay of vegetable and animal matter, microorganisms, bacteria, pathogens, etc.).
Impurities / Contaminants
In addition to natural impurities in water, many additional impurities (or "contaminants" or "pollutants") are introduced by activities and land uses of society. These occur through:
- Discharging or disposing of wastes and wastewater in the ground and streams.
- Spraying and spreading of chemicals (such as fertilizers and pesticides) or releasing gases and particles into the air (such as from automobiles and smoke stacks) that are carried off by the wind to eventually fall from the air onto the ground or into streams, lakes and ponds (a process referred to as "atmospheric deposition").
- Rain falling onto and flowing over buildings, pavement, cars and vehicles, stored chemicals and materials, and other lands where chemicals, dust, trash and microorganisms have accumulated from society’s activities (known as "polluted stormwater runoff").
Effects of Pollutants
The cumulative effect of these pollutants diminishes the ability of plants, animals, and humans to use the water to satisfy biological and other needs. The types of pollutants of greatest concern include:
- Nutrients (phosphorus, nitrates, nitrites, etc.) that accumulate in water and support the rapid growth of algae and other undesirable aquatic plant life. As these organisms die and decompose, they sink to the bottom of the water body and absorb the oxygen from the water in the decomposition process. The depletion of oxygen from the water (known as "eutrophication") renders the water unfit for fish that rely on the oxygen for life. Also, high concentrations of nitrogen in drinking water is a health risk to infants and pregnant cows.
- Microorganisms (including bacteria and pathogens such as cryptosporidia, and viruses, etc.) that are "shed" by human beings, pets, wildlife and waterfowl and are carried through the environment by rainfall runoff, streams and ground water.
- Metals (including copper, lead, zinc, iron, mercury, etc.) that are dissolved from cars, rain gutters, and many other objects in society composed of metal. Metals are also included in many other chemical substances that can be exposed to rainfall and washed into streams.
- Sediments (including clay, silt, sand, mud, etc.) that can be washed from unvegetated, exposed land surfaces or dislodged from the sides and bottom of stream channels. Sediments often have other pollutants attached to them, such as phosphorus or metals.
- Pesticides that are applied to landscaping and agricultural crops can be washed into the streams or carried into the ground with rain. When pesticides of sufficient concentration reach a stream or water body, their toxicity can harm or kill living organisms in the streams.
- Thermal pollution refers to increasing the temperature of a stream or water body either by bringing the water in contact with heat (such as for cooling of power plants) or by removing the shade canopy of tree cover from over a stream. Many aquatic living species require cool waters to survive.
The heat from the sun evaporates the drop into water vapor. It rises into the air and continues rising until winds carry it over land. There, warm updrafts coming from the heated land surface take the drop (now water vapor) up even higher, where the air is quite cold. When the vapor gets cold, it changes back into a liquid (the process is "condensation"). The vapor condenses on tiny particles of dust, smoke, and salt crystals to become part of a cloud. The drop combines with other drops and falls to the Earth as "precipitation".
Once it starts falling there are many places for water drops to go. It can land on a leaf in a tree, in which case it would probably evaporate and begin its process of heading for the clouds again. The drop could land on a patch of dry dirt in a flat field. In this case it might sink into the ground to begin its journey down into an underground aquifer as "ground water". The drop will continue moving (mainly downhill) as ground water, but the journey might end up taking tens or hundreds of years until it finds its way back out of the ground at a spring, or seep or flow into the sides or bottom of a stream channel.
Then again, the drop could be pumped out of the ground via a water well and be sprayed on crops (where it may be taken up by the plant through its roots or leaves and "transpired" back to the atmosphere, evaporate from the ground or leaves of plants, flow over the ground into a stream, or percolate back down into the ground). Or the well water containing the drop could end up in a baby's drinking bottle or be sent to wash a car. From these places, it goes back again into either the air, down sewers into rivers and eventually into the ocean, or back into the ground.
But plenty of precipitation ends up staying on the Earth's surface to become a component of surface water. If the drop lands in an urban area it might hit your house's roof, go down the gutter and your driveway to the curb. It could run down the curb into a storm sewer and end up in a small creek. It is likely the creek will flow into a larger river and the drop will begin its journey back towards the ocean.
If no one interferes, the trip will be fast back to the ocean, or at least to a lake where evaporation could again take over. But there is a good chance that the drop will get picked up and used before it gets back to the sea. A lot of surface water is used for irrigation. Even more is used by power-production facilities to cool their electrical equipment. From there it might go into the cooling tower to be evaporated. But maybe a town pumped the drop out of the river and into a water tank. From here the drop could go on to help wash your dishes, fight a fire, water the tomatoes, or flush your toilet. Maybe the local steel mill will use the drop, or it might end up at a fancy restaurant mopping the floor. The possibilities are endless -- but it doesn't matter to the drop, because eventually it will get back into the environment. (adapted from USGS website, 2000)
What is a Watershed?
No matter where you are, you're in a watershed! The simple definition of a watershed is that it is the area of land that catches rain and snow that drains or seeps into a common marsh, stream, river, or lake. You're sitting in a watershed now. Homes, farms, ranches, forests, small towns, big cities and virtually all lands make up watersheds.
A watershed is the area of land where all of the water that is under it or drains off of it goes into the same water body. In addition to illustrating the hydrologic cycle, the hydrologic cycle figure also depicts how land is part of a watershed. John Wesley Powell, scientist geographer, put it best when he said that a watershed is: "that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community". Watersheds come in all shapes and sizes. Some cross county, state, and even international borders. Some are millions of square miles, others are just a few acres. Just as creeks drain into rivers, watersheds are nearly always part of a larger watershed. The ridge lines along hilltops separate one watershed from another.
Watersheds can be subdivided into smaller watersheds or "subbasins", which collectively flow together to form larger watersheds and "basins".
Each watershed includes the ridge line that forms it border or outer limit, the land area across its surface, the network of streams that drain the watershed, and the underlying soils, rock and ground water. But, most streams flow continuously, regardless of whether or not it is raining. And over time, the stream channels seem to shift and bend. These things happen because a watershed is also a series of continuous natural processes that involve the movement of water and soil usually acting under the influence of gravity. Some of the processes that make a watershed "work" include:
Evapo-transpiration" – is the combined processes of evaporation of water from any surface, and the transpiration of water taken up by plants back into the atmosphere. This process accounts for about 55% of the total annual rainfall that falls in the study area.
Infiltration" – rainfall that is not returned to the atmosphere through "evapo-transpiration" can soak into the soils of the ground as it falls to the surface of the Earth. Some water that is infiltrated to soils is then taken up by roots of plants and trees and transpired. But some will continue to move downward (or "percolate") deeper into the ground, and eventually reach (or "recharge") the ground water. Infiltration accounts for about 28% of the total annual rainfall that falls in the study area.
Ground water aquifer"– underlying every watershed is an "aquifer" that includes the bedrock lying deep below the ground surface whose cracks and openings are fully saturated with water. Because this water exists well below the ground surface, it is referred to as "ground water" In the study area, the upper surface (or "water table") of the ground water may be situated several tens to several hundreds of feet below the ground surface. Because the underlying bedrock of the study area is dense rock, the water can exist only in cracks or fractures in the rock or cavities created by the bedrock being dissolved by the water. Aquifers can be several hundreds of feet thick, and contain very large volumes of ground water.
Ground water flow" – while the ground water exists in very small spaces in the rock, it continues to move and flow, generally in a downhill direction under the influence of gravity. Ground water moves very, very slowly through these tiny openings, sometimes moving only a few inches to a few feet per day.
Ground water discharge" – as ground water continues to move downhill (or "down gradient"), it eventually seeps to a point where it again intersects the ground surface and emerges (or "discharges") from the aquifer to the surface. This "discharge" can occur at a spring or seep where it will form the beginning of a new stream, or it can "discharge" or seep into the bottom or side of a flowing stream. In the study area, ground water discharge to springs and streams contributes approximately 60% of all of the water flowing in the streams. In most of the streams of the study area, ground water is seeping or discharging into the stream channel throughout the entire length of the stream.
Stream" - a stream is a flowing water body, generally in a natural open earthen channel. Streams are sometimes called runs, creeks or rivers, generally dependent upon their size and local nomenclature. When sufficient volume of ground water is discharged to the channel, a continuous flow of water occurs. A stream that flows continuously and year-round is called a "perennial stream". A stream that only flows during wet seasons or after prolonged rain events is called an "intermittent stream".
Stream baseflow" – when the water flowing in a stream is solely comprised of water that was discharged from ground water, it is referred to as "baseflow". Under "baseflow conditions", the stream is solely supported by ground water discharge and is receiving no water from surface runoff or rainfall. Baseflow conditions occur well after the last rainfall event. They also occur during drought or extended dry weather periods. The fact that the streams of the study area generally continue to flow without interruption even during prolonged droughts (such as were experienced in 1997, 1998, and 1999), is an indication of the immense volume of water stored in the ground water aquifers beneath the study watersheds.
Q7-10" baseflow – scientists use many different statistics to describe streams and their flow conditions. The most common statistic used to describe baseflow conditions is "Q7-10". This is determined by statistical analysis of stream flow records, and represents the lowest stream flow experienced for seven consecutive days with a recurrence interval of ten years. This flow rate is generally expressed in cubic feet per second (cfs).
1 in 25 year average annual stream baseflow" – this is another statistic used by scientists to describe certain stream baseflow conditions. This statistic refers to an "average annual" rate of flow, compared to the Q7-10 baseflow, which refers to a "7-day" flow rate. The "1 in 25 year average annual stream baseflow" is a statistic developed by USGS that represents the lowest average annual stream baseflow (generally given in cubic feet per second, or cfs) that is expected to occur on average once in every 25 years. Generally, this flow rate is larger than the Q7-10 flow rate for the same point in a stream. The 1 in 25 year average annual baseflow rate is the baseflow statistic used in this Technical Report.
Headwater stream"– a stream begins where ground water discharges sufficient volume of water via a spring or seep (or series of springs or seeps) to create a flow of water. This beginning stream is called a "headwater stream" or "headwater tributary" because it is at the "head" of the watershed and stream network.
First order streams" – a perennial (or continuously flowing) headwater stream can also be called a "first order stream" because it is the first stream segment to drain that land area and has not yet intersected another stream. When two "first order streams" intersect, they form a second order stream. When two "second order streams" intersect, they form a "third order stream", and so on. The larger the stream, the larger the stream "order" that is used to classify it.
Surface runoff" – rainfall that is not evaporated, or transpired, or infiltrated, is referred to as "surface runoff" because it flows over the land surface, in a downhill direction, until it finds its way into a drainage channel. In the study area, "surface runoff" accounts for approximately 40% of the water that flows in a stream, and accounts for about 17% of the total annual rainfall volume that falls in the study area.
Flooding" – when the volume of surface runoff of rainfall entering the streams is greater than the volume of water that the stream channel can physically contain, water begins to flow "out of bank" of the streams and into the adjacent "floodplain". When a stream is flowing "out of bank" it is considered to be a "flooding" stream. The "floodplain" is generally a broad, relatively flat land area adjacent to the stream that has been made over many thousands of years as flooding streams eroded through the underlying soils and bedrock. Larger streams have larger floodplains. Small streams may have no distinguishable floodplain. Severe flooding creates obvious hazards to people, roads, homes and other structures in its path. However, flooding is an important ecological process that provides water and flushing to the vegetation of the riparian area and floodplain.
Erosion" - as water flows over or against unprotected soil, its energy and movement dislodge soil particles and carry them away. This is called "erosion" and is an undesired process as it strips farm and agricultural lands of valuable soil resources and because the soil particles often have other chemicals or pollutants attached to them that impair water quality of streams (such as phosphorus or metals). Greater volumes and velocities of water will remove greater volumes of sediment and greater sized particles, sometimes as large as rocks and boulders. Substantial erosion occurs within stream channels as part of the natural stream evolution. However, as high stream flow rates and volumes occur more frequent from developed areas, the instream erosion becomes excessive, which is undesirable.
Sedimentation" - when the streamflow slows down, the sediment particles fall out of suspension and are deposited in the stream bed. This process of flowing water depositing sediments is known as "sedimentation". This is often an undesirable process as the deposited sediments often fill in and destroy the underwater spaces around rocks and fallen tree trunks and branches where fish and other aquatic living resources hover for food or shelter.
Meandering" – stream channels move and shift over many hundreds and thousands of years. As they mature over time, they generally form larger meanders to accommodate the changes in the slope of the channel bottom as the water wears or erodes further down into the Earth. The meanders help control the rate of flow of the water and balance its energies across the cross-section of the stream. They work in balance with the range of flows that the stream experiences to efficiently convey low streamflows as well as flood flows. This meandering is most evident in very large, old streams, such as the Mississippi River. The streams of the study area are relatively small and their meanders are not as evident. Still, riparian land owners should expect the adjacent stream channels to move and meanders to expand over time. It is important to allow room along the stream corridor for the stream to move without interference from permanent structures.
Dynamic equilibrium" - each of the above processes continues perpetually. Over hundreds and thousands of years, they reach a balance (or "dynamic equilibrium") with each other. A stream that has reached a "dynamic equilibrium" is considered to have "stable stream channel conditions". As society expands its land development patterns and covers the natural land surfaces with buildings and pavement, the rate and volume of surface runoff become too great to maintain that balance or equilibrium. This causes the streams to become unstable as they attempt to adjust to the larger flows too quickly. The result is unstable conditions within the stream channel, generally including excessive instream erosion, sedimentation, and frequent flooding.
Stream downcutting" – when excessive instream erosion and sedimentation develop within a stream, the forces of the flowing water act against the channel banks and bottom, eroding them away at rapid rates. The process of the channel bottom being eroded quickly, creating a deeply incised stream, is called "downcutting". This is undesirable because it causes the stream to also become straightened, losing its meanders. Once straightened, the stream flows and erosion rates are accelerated. When the sediment-laden water reaches a flatter segment of stream or some other feature that slows the rate of flow, the sediment load is dropped, creating an "imbedded" stream bottom.
Stream geomorphology" – the above processes of change, equilibrium, and evolution between the water, the sediments, and the stream channel are collectively referred to as the "stream geomorphology" (or "fluvial geomorphology").
Riparian area" – this is the corridor of land immediately adjacent to a stream or water body and serves as a transition between aquatic and terrestrial environments. This area directly affects and/or is affected by the adjacent water body (PADEP 1998).
Forested riparian buffer" – is an area of trees, usually accompanied by shrubs and other vegetation, adjacent to the water and managed to maintain the integrity of the stream channels and shorelines, to reduce the impact of upland sources of pollution by trapping, filtering and converting sediments, nutrients and other chemicals, and to supply food, cover and thermal protection to fish and other wildlife (PADEP 1998).