The term watershed describes an area of land that drains downslope to the lowest point. Water moves through a network of drainage pathways, both underground and on the surface. These pathways converge into streams and rivers, which become progressively larger as the water moves on downstream, eventually reaching the ocean. Other terms used interchangeably with watershed include drainage basin or catchment basin.

The Chartiers Watershed is the total geographic land area that drains water, sediment and dissolved materials by the network of tributaries that feed the main channel of Chartiers Creek.  The Chartiers Creek Watershed comprises 280 square miles. Chartiers Creek runs for 52 miles, beginning 6 miles south of Washington, PA, and flows into the Ohio River at McKees Rocks, 3 miles downstream of Pittsburgh. Chartiers Creek runs from an elevation of 1200 feet to 700 feet above sea level at the Ohio River.

It is relatively easy for you to delineate watersheds using a topographic map that shows stream channels. Watershed boundaries follow major ridgelines around channels and meet at the bottom, where water flows out of the watershed, into a stream or river.

Watersheds can be large or small. Every stream, tributary, or river has an associated watershed, and small watersheds join to become larger watersheds.  Two large sub-watersheds are delineated within the Chartiers Watershed - the Upper Chartiers Watershed and the Lower Chartiers Watershed.  The lower watershed, comprised of the sub-watersheds downstream of the confluence of Little Chartiers Creek and Chartiers Creek in Washington County, an area approximately 139 square miles in size, is shown in the map at right.

The hydrological cycle is a basic process of water cycling that is crucial to all life on earth. Precipitation waters crops for us to eat, replenishes streams, lakes, and wetland habitats, supports the growth of forests and recharges the groundwater that provides the water we drink. Evaporation withdraws water from earth and stores it in the atmosphere as clouds, until weather conditions stimulate a rain shower.

The drainage system includes the geographic area surrounding the stream system that captures precipitation, filters and stores water, and determines water release into stream systems. The stream system is the visible, aboveground portion of this larger drainage system.

The connectivity of the stream system refers to the physical connection between tributaries and the river, between surface water and groundwater, and between wetlands and water.  Connectivity is the primary reason for doing aquatic assessments at the watershed level.  Because water moves downstream, any activity that affects the water quality, quantity, or rate of movement at one location can affect locations downstream. For this reason, everyone living or working within a watershed needs to cooperate to ensure good watershed conditions. 

Floodplains are nature's method of disaster damage control. Not all ecosystems can tolerate inundation, or the damage and destruction left by raging floods.  Floodplains are special parts of the valley where rising waters can flood. Specially evolved plant communities occupy these areas. Grass-like sedges and rushes capture rushing sediments and help stabilize the stream banks. Trees like red maples and box elders like to grow with their roots in water, and can re-sprout from these roots if their trunks get broken by rushing debris during a flood. Other trees like sycamores are strong and solid and can withstand many floods.

Rivers and streams overflow predictably into the floodplain. This fact makes building in the floodplain a very dangerous option. Even building on a natural terrace slightly above the floodplain could spell disaster in the next 100 years.

The illustration below shows the probable extent of flooding over long time intervals on a hypothetical floodplain similar to the Chartiers Creek floodplain. 

The 2-3 year flood inundates the existing floodplain (Terrace 1). It is only this flooding which the Fulton Flood Control Project was designed to eliminate.  The 100 year flood inundates both the existing floodplain and a higher one (Terrace 2), which formed when the river stood at a higher level.   

The 500 year flood event inundates an even higher terrace and all lower terraces and floodplains.  This is the event presumed to have happened during Hurricane Ivan.  However, that assumption was based on traditional land use patterns where there was less runoff than with current land use and development.

After many flooding disasters, the government has realized the danger and expense of building on floodplains. Some regulations restrict the construction of new buildings within certain limits of the floodplain. 

In the illustration above, the regulatory floodway is kept open to carry flood water.  No building or fill is permitted.  Use in the regulatory floodway fringe is permitted if protected by fill, flood-proofed or otherwise protected.  The regulatory flood limit is based on technical study and is the outer limit of the floodway fringe.  The standard project flood (SPF) limit is the brown area subject to possible flooding by large floods.  This SPF area was the floodplain inundated by Hurricane Ivan.

However, in the past, planners did not recognize the power of water and the danger of the floodplain. Today, neighborhoods like Carnegie, Heidelberg and Bridgeville lie in the floodplain. The water may not be visible, but the land still remembers its floodplain identity.

Water is one of the most powerful forces known and has been harnessed by humans for thousands of years. Even today, the combined strength of water and gravity power mills and run hydroelectric power generating stations. All ecosystems on earth are shaped and influenced by water, or a lack of water. 

The eastern United States is a living, life-sized tribute to the action of water and rivers.   Fast moving water can move very large particles, as this diagram shows. As water slows down, it starts to deposit particles. This power of flowing water was evident in the wake of hurricane Ivan.

Diagram shows threshold stream velocities for erosion, transportation and deposition of varying particle sizes.  A higher water velocity is required to erode clay and silt than to move sand.

As a river flows, it deposits sediments at intervals. These deposition spots change the velocity of the river and encourage the deposition of more sediments. The river soon takes on a winding, meandering shape as continued erosion and deposition occur.   

Erosion and deposition patterns on a meandering stream are shown at right.  Erosion of the cutbank and deposition of a point bar on the slip-off slope is illustrated.  Arrow length is proportional to stream velocity.

These forces of erosion and deposition happen continually.

Sometimes the effect of these processes is obvious overnight after a torrential rain. Other times, the subtle movement of the streamcourse may not be visible at all in the course of a human lifetime. However, rivers always move. Human attempts to confine and alter the flow of river are usually not successful in the long run. It is wiser to stay out of the floodplain and build in upland locations. 

In most areas of the Eastern United States, groundwater occupies the pore spaces between soil particles at some depth below the surface. These deposits can be fairly small, or enormous underground reservoirs, or aquifers. Near streams and wetlands, the watertable is usually very close to the surface. Streams and the watertable have a cooperative relationship. After a rain when the rivers are full, they recharge the underground water supply. When the streams run dry in the summer, the groundwater feeds the stream to support the aquatic environment that lives there.  

When groundwater is extracted for residential, commercial, and industrial uses the watertable can be altered. This means the water resides further from the surface than it once did. If more and more water is taken out, eventually the groundwater supply may be exhausted.  Under natural conditions, precipitation would percolate through the soil and continually recharge the groundwater. However, in cities, the precipitation is usually piped away as runoff and released in a different area, or so quickly that it cannot seep back into the ground.

Under almost all situations, rainfall results in some amount of runoff. Any water that cannot immediately seep into the ground flows downslope as runoff. Ground permeability affects runoff significantly. Hard packed clay soils, such as those prevalent in the Chartiers watershed, absorb very little water while a loose sand might absorb almost all the precipitation that falls onto it.  The amount of runoff is related to the amount of rain a region experiences. 

However, urban and rural areas experience the effects of runoff very differently. The presence of vegetative cover slows the journey of raindrops from sky to soil and reduces the amount of runoff. Impermeable surfaces, such as concrete, absorb almost no water at all. The management of storm runoff is a significant issue in cities, especially when considering the destructive power of raging water.

Q=CIA 
Q = peak runoff rate 
C = runoff coefficient 
I = rainfall intensity 
A = drainage area (acres)2

Note that the C value for unimproved areas (forests, native meadows) is very low, almost all the water is absorbed. The C value for downtown areas containing a lot of asphalt, concrete, and roof surfaces is very close to 1.0, which means almost all of the water runs off these surfaces. Impermeable urban areas can create huge volumes of stormwater  runoff. 

Cities have a large proportion of paved areas and few natural areas with trees and shrubs. Because so much of the city surface is impervious to water, most of the precipitation that falls flows away as runoff. Urban storm runoff is usually directed through storm sewers, eventually emptying into nearby rivers. Under pre-urban conditions, much of this volume of water would have absorbed into the ground. Riverbeds often cannot accommodate this increased volume of water and massive flooding results downstream from urban areas.

The graph above illustrates that the risks of severe flooding and flood frequency increase with the percentage of area impervious to water as a result of paving and urbanization.

increased runoff

The porous and varied terrain of natural landscapes like forests, wetlands, and grasslands traps rainwater and snowmelt and allows them to filter slowly into the ground.  In contrast, impervious (nonporous) surfaces like roads, parking lots and rooftops prevent rain and snowmelt from infiltrating, or soaking, into the ground.  Most of the rainfall and snowmelt remains above the surface, where it runs off rapidly in unnaturally large amounts.  

Storm sewer systems concentrate runoff into smooth, straight conduits.  This runoff gathers speed and erosional power as it travels underground.  When this runoff leaves the storm drains and empties into a stream, its excessive volume and power blast out streambanks, damaging streamside vegetation and wiping out aquatic habitat.  These increased storm flows carry sediment loads from construction sites and other denuded surfaces and eroded streambanks.  They often carry higher water temperatures from streets, roof tops, and parking lots, which are harmful to the health and reproduction of aquatic life.  The loss of infiltration from urbanization may also cause profound groundwater changes.  Although urbanization leads to great increases in flooding during and immediately after wet weather, in many instances it results in lower stream flows during dry weather.  Many native fish and other aquatic life cannot survive under these conditions.

increased pollutant loads

Urbanization increases variety & amount of pollutants carried into streams, rivers, and lakes.  The pollutants include:

1. Sediment
2. Oil, grease and toxic chemicals from motor vehicles
3. Pesticides and nutrients from lawns and gardens
4. Viruses, bacteria and nutrients from pet waste and failing septic systems
5. Road salts
6. Heavy metals from roof hingles, motor vehicles and other sources
7. Thermal pollution from dark impervious surfaces such as streets and rooftops

These pollutants can harm fish and wildlife populations, kill native vegetation, foul drinking water supplies,and make recreational areas unsafe and unpleasant.  Chartiers Creek and many of its tributaries in the lower watershed are adversely impacted by these urban pollutants.