WHAT IS A WATERSHED?

Upper & Lower Chartiers Watershed in the Pittsburgh area

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.

Lower Chartiers watershed showing subwatersheds of tributaries

  
HYDROLOGICAL CYCLE

Hydrologic Cycle

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

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.

Floodplain dynamics

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. 

floodplain terraces

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.

regulatory floodway and building codes

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.

  
FLUVIAL
PROCESSES

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. 

fluvial processes determine creekbank morphology

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.

erosion of cut banks and point bar deposition is the result of flowing water and sediment transport

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. 

  

GROUNDWATER

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.  

gaining & losing streams

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.

  

RUNOFF

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.

The amount of storm water runoff is calculated by the equation at right.  The runoff coefficient C is calculated based on the permeability of the ground surface.  

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

Some types of surfaces encountered in a typical urban area are shown below:

table of runoff coefficients of some typical urban land surfaces

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.

graph showing increase in severe flood frequency with increase in impervious surface area

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.

  
URBAN IMPACTS

In urban and suburban areas, much of the land surface is covered by buildings and pavement, which do not allow rain and snowmelt to soak into the ground.  Instead, most developed areas rely on storm drains to carry large amounts of runoff from roofs and paved areas to nearby waterways.  The stormwater runoff carries pollutants such as oil, dirt, chemicals, and lawn fertilizers directly to streams and rivers, where they seriously harm water quality.  To protect surface water quality and groundwater resources, development should be designed and built to minimize increases in runoff.

  
HOW URBANIZED AREAS AFFECT WATER QUALITY

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.  

urban impacts on runoff, infiltration, and evapotranspiration

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.

 
MANAGING URBAN RUNOFF

what can homeowners do?

To decrease polluted runoff from paved surfaces, households can develop alternatives to areas traditionally covered by impervious surfaces.  Porous pavement materials are available for driveways and sidewalks, and native vegetation and mulch can replace high maintenance grass lawns.  Homeowners can use fertilizers sparingly and sweep driveways, sidewlks and roads instead of using a hose.  Instead ofdisposing of yard waste, you can use the materials to start a compost pile.  And homewoners can learn to use Integrated Pest Management (IPM) to reduce dependence on harmful pesticides.

In addition, households can prevent polluted runoff by picking up after perts and using, storin and disposing of chemical properly.  Drivers should check their cars for leaks and recycle their motor oil and iantifreeze when these fluids are changed.  Drivers can also avoid impacts from car was runoff (e.g. detergents, grime, etc.) by using car wassh facilities that do not generate runoff.  Households served by septic systems should have them professionally inspected and pumped every 3 to 5 years.  They should also practice water conservation meatsures to extend the life of their septic systems.

controlling impacts from new development

Paddlers on the canoe sojourns did not fail to notice the streamside development with houses sprouting like mushrooms after a spring rain along the banks of Chartiers Creek.  Low impact development includes measures that conserve these natural areas, particularly sensitive hydrologic areas like the streambank riparian buffers and infiltrable soils.  Developers and city planners must attempt to control the volume of runoff from new development by using these low impact development, structural controls, and pollution prevention strategies.  They can reduce development impacts and reduce site runoff rates by maximizing surface roughness, infiltration opportunities and flow paths.

controlling impacts from existing development

How about our current floodplain and watershed communities?  Controlling runoff from existing urban areas is often more costly than controlling runoff from new developments.  Economic efficiencies are often realized through approaches that target "hot spots" of runoff pollution or have multiple benefits, such as the high efficiency street sweeping which was instituted in Crafton, that addresses aesthetics, road safety and water quality.  

Urban planners, city managers and others responsible for managing urban and suburban areas can first identify and implement pollution prevention strategies and examine source control opportunities.  They should seek out priority pollutant reduction opportunities, then protect natural areas that help control runoff, and finally begin ecological restoration and retrofit activities to clean up degraded water bodies. In many communities in the Chartiers Watershed, these efforts are spearheaded by local conservation organizations, in cooperation with their local government and Environmental Advisory Councils.  

Local governments are encouraged to take lead roles in public education efforts through public signage and pollution prevention outreach campaigns. Crafton and Greentree  have marked storm drains, while Scott and South Fayette Townships have partnered with their local conservation groups in acquiring, preserving and remediating natural areas and promoting and enacting model ordinances. 

Citizens can help prioritize the clean-up strategies and  volunteer to become involved in restoration efforts, which often starts with their local conservation group..

read more

REDUCING STORMWATER RUNOFF The Penn State Cooperative Extension has urban and community forestry information for municipalities, community-based organizations and homeowners in the form of a central clearinghouse website for all things related to forests and watersheds. There have been many recent efforts towards managing urban forests for watershed health that have resulted in a variety of highly useful tools and training materials. This site compiles these resources into a format that can be easily accessed and downloaded. 

Visit ARTEMIS, the green building supply center in Lawrenceville, on our Links page, to build and remodel greener.  The US EPA, in cooperation with 3 Rivers Wet Weather have also included these related on-line publications on water quality and stormwater management:

TURN YOUR HOME INTO A STORMWATER POLLUTION SOLUTION  
This website links to an EPA homewoner's guide to healthy habits for clean water that provides tips for better vehicle and garage care, lawn & garden techniques, home improvement, pet care, and more...

NATIONAL MANAGEMENT MEASURES TO CONTROL NONPOINT SOURCE POLLUTION FROM URBAN AREAS    This technical guidance and reference document is useful to local, state and tribal managers in implementing management programs for polluted runoff.  Contains information on the best available economically achievable means of reducing pollution of surface waters and groundwater from urban areas.

ONSITE WASTEWATER TREATMENT RESOURCES  This site contains the latest brochures from EPA for managing onsite wastewater treatment systems such as conventional septic systems and alternative decentralized systems.  These resources provide basic information to help individual homeowners as well as detailed, up-to-date technical guidance of interest to local and state health departments.

LOW IMPACT DEVELOPMENT CENTER This center provides information on protecting the environment and water resources through integrated site design techniques that are intended to replicate pre-existing hydrologic site conditions.

STORMWATER MANAGERS RESOURCE CENTER Created and maintained by the Center for Watershed Protection, this resource center is designed specifically for stormwater practitioners, local government officials and others that need technical assistance on stormwater management issues.

COMMUNITY RESPONSES TO RUNOFF   The National Resources Defense Council developed this interactive web document to explore some of the most effective strategies that communities are using around the nation to control urban runoff pollution.  This document is also available in print form and as an interactive CD-ROM.

WATERSHED DEFINITION | WATER CYCLE | FLOODPLAIN | STREAM | GROUNDWATER | RUNOFF | URBANIZATION
ABANDONED MINE DRAINAGE
  |  FLOODS  |  HYDROLOGY  |  WATER QUALITY
  

WATER QUALITY & STORMWATER MANAGEMENT | LAND & CONSERVATION  | TRAILS & RECREATION 
SITE MAP
| PLAN | LINKS | HOME 


ChartiersGreenway.net
20 Mar 2009
Hit Counter
Google