An Introduction to Urban Watersheds

What is a watershed?

It doesn't matter how far you live from a stream, river, or lake, you live in a watershed. Everyone resides in a watershed. A watershed is an area of land from which all water drains, running downhill, to a shared destination - a river, pond, stream, lake, or estuary. A watershed is a catchment basin that is bound by topographic features, such as ridge tops.

A watershed has three primary functions. First, it captures water from the atmosphere. Ideally, all moisture received from the atmosphere, whether in liquid or solid form, has the maximum opportunity to enter the ground where it falls. The water infiltrates the soil and percolates downward. Several factors affect the infiltration rate, including soil type, topography, climate, and vegetative cover. Percolation is also aided by the activity of burrowing animals, insects, and earthworms.

Second, a watershed stores rainwater once it filters through the soil. Once the watershed's soils are saturated, water will either percolate deeper, or runoff the surface. This can result in freshwater aquifers and springs. The type and amount of vegetation, and the plant community structure, can greatly affect the storage capacity in any one watershed. The root mass associated with healthy vegetative cover keeps soil more permeable and allows the moisture to percolate deep into the soil for storage. Vegetation in the riparian zone affects both the quantity and quality of water moving through the soil.

Finally, water moves through the soil to seeps and springs, and is ultimately released into streams, rivers, and the ocean. Slow release rates are preferable to rapid release rates, which result in short and severe peaks instream flow. Storm events which generate large amounts of run-off can lead to flooding, soil erosion and siltation of streams.

Ultimately, the moisture will return to the atmosphere by way of evaporation. The hydrologic cycle (the capture, storage, release, and eventual evaporation of water) forms the basis of watershed function.

A watershed should be managed as a single unit. Each small piece of the landscape has an important role in the overall health of the watershed. Paying attention primarily to the riparian zone, an area critical to a watershed's release function, will not make up for lack of attention to the watershed's uplands. They play an equally important role in the watershed, the capture and storage of moisture. It is seamless management of the entire watershed, and an understanding of the hydrologic process, that ensures watershed health.

Protection of Urban Natural Areas

Development pressures found within urban areas place natural areas at great risk. Without specific laws mandating their protection, natural areas make way for transit corridors, residential development, commercial and industrial projects, and associated parking/storage areas. Public parks and greenspaces provide a refuge from the built environment, but fall very short of retaining the habitat components necessary to sustain native fish and wildlife populations, clean air and water, aesthetics, and adequate recreational opportunities. As a result, the livability of a city is compromised when natural areas are converted to more intense land uses.

Local governments have a large role in protecting urban natural areas. This role is reinforced by legislation and laws which requires them to develop and implement plans which conform with planning standards - conserving open space and protecting natural and historic resources for future generations; protecting urban natural areas at the local level; and other issues.

A Watershed-Based Approach to Resource Management

Each river system - from its headwaters to its mouth - is an integrated system and must be treated as such. The focus of water resource management is on wise and efficient use of water resources for such purposes as energy production, navigation, flood control, irrigation, and drinking water. It also places emphasis on improving ambient water quality. The Federal Water Pollution Control Act of 1956 provided large scale funding of publicly owned treatment works. The Water Quality Act of 1965 required States to develop water quality standards for interstate waters. River Basin compacts were formed to protect major systems, like the Delaware and Colorado Rivers. Some state sanitation commissions adopted a river basin approach to their work. Some developed basin plans that classified individual bodies of water according to their best uses. These early water quality managers walked, boated, and drove throughout entire basins, documenting outfall pipes and collecting ambient samples.

In 1972, the Federal Water Pollution Control Act Amendments established the national goal of restoration and maintenance of the physical, chemical, and biological integrity of the nation's waters. The dominant features of this Clean Water Act were a Federal permitting program (the National Pollutant Discharge Elimination System or NPDES) and massive funding for waste water treatment and state water quality programs.

Under Section 303(e) of the Clean Water Act, states prepare basin plans for controlling their point source difficulties. These plans are collections of known information about dischargers and water quality and help form the basis for grant decisions on wastewater treatment proposals. However, after the initial plans were complete, most states have retained only a limited basin planning function and focused on individual point source problems.

In the 1987 amendments to the Clean Water Act, Congress required States to expand their programs for dealing with toxicants, nonpoint sources, wetlands, and water quality standards. The states' progress in eliminating point source pollution has made it apparent that nonpoint source pollution and habitat degradation are the nation's biggest water quality challenges.

Theoretically, a watershed approach can provide benefits to individual citizens, the public sector, and the private sector. Individual citizens benefit when watershed protection improves the environment and the livability of an area.

The public sector benefits because agencies can accomplish more through cooperation with other stakeholders than they can working solo with limited resources. The watershed- wide participation of local citizens and organizations ensures that those who are most familiar with a watershed, its problems and possible solutions, play a major role in watershed stewardship. The privatesector can benefit because the burden of water resource protection is distributed more equitably among pollution sources.

A comprehensive approach to water resource management is needed to address the myriad water quality problems that exist today from nonpoint and point sources as well as from habitat degradation. Watershed based planning and resource management is a strategy for more effective protection and restoration of aquatic ecosystems and for protection of human health. The watershed approach emphasizes all aspects of water quality, including chemical water quality (e.g., toxins and conventional pollutants), physical water quality(e.g., temperature, flow, and circulation), habitat quality (e.g., stream channel morphology, substrate composition, and riparian zone characteristics),and biological health and biodiversity (e.g., species abundance, diversity, and range).

To deal with nonpoint source pollution in an effective manner, a smaller and more comprehensive scale of analysis and management is required. While point source pollution control programs encourage identifying isolated polluters, nonpoint source strategies recognize that small sources of pollution are widely dispersed on the landscape and that the cumulative impacts of these pollutants on water quality and habitat are great. A whole basin approach to protecting water quality has proved most effective because it recognizes connected subbasins.

Effects of Urbanization

Almost eighty percent of our country's population resides in metropolitan areas. Many city residents think of natural areas in a wilderness context. However, as cities spread outward, more and more streams, especially small creeks feeding directly into lowland rivers, are transformed into urban streams. These natural areas are perhaps the most neglected watercourses

An urban creek may experience many things on it's journey from its headwaters in the upper watershed to it's mouth at the confluence of a larger creek, stream, or river. It may pass through a park or residential area in one reach, then flow past a shopping mall or industrial park the next. In a small, steep, wooded canyon behind a residential area, it may become a convenient place to dump grass clippings or garbage. It may pass through vacant lots, becoming lost among the discarded appliances, shopping carts, and tires. It may follow-through an over-fertilized golf course. It may flow for long distances inside a culvert under ground. It may receive stormwater run-off from oily roads, parking lots, and factory drainage ditches. Uninformed neighbors may even dump used motor oil or antifreeze into storm drains that empty directly into the stream.

There are countless threats to water quality and habitat due to urbanization. The urban environment contributes 11.8% of nonpoint source pollution into the nation's waters. Impacts result from residential, transportation, commercial, and industrial land uses. Specific impacts include toxic substances that enter the food chain, petroleum products that are harmful to plants, fish, and wildlife; excessive nutrients that increase algal blooms; and a reduction of water quality.

New development Run-off from developing areas creates problems regarding both water quality and quantity. Earth moving during construction contributes sediment to the run-off. Construction alone contributes 3.3% of the nation's nonpoint source water pollution.

Once buildings and pavement are introduced, less water is able to penetrate the soil to be filtered of contaminants such as automobile by-products, pesticides, fertilizers, and excess sediment. Not only is the land less able to filter such contaminants, but increasing numbers of people produce a greater pollutant load.

The clearing of vegetation decreases a watershed's capacity to capture moisture, increasing the amount run-off. The loss of vegetation also destabilizes stream banks vegetation and reduces the shade produced by the canopy. Increased solar pollution raises stream water temperatures during the summer months, destroying habitat for fish and disrupting the ecosystem

Impervious Surfaces As a watershed area becomes more populated, natural surfaces, that absorb water and recharge ground water supplies, are covered with hard, impervious surfaces (streets, sidewalks, rooftops, driveways, and parking lots). Conversion from predominantly vegetated land use to urban uses may result in tremendous reductions in watershed's absorption capacity.

The traditional approach to run-off has been to remove it as quickly as possible from developed areas. The cumulative result of such changes throughout the watershed is an increase in the volume of run-off to streams, wetlands, and rivers. The increased volumes of run-off also travel more quickly to surface waters, which in turn produces higher peak flows and velocities. Flooding may occur as flows exceed natural, designed, or available system capacities, threatening homes and businesses located along the stream.

Streams also suffer consequences from changes in peak water volumes and flows. Streams are always in a process of change, but human actions can accelerate the rate of change. As development in the watershed occurs, the flows associated with storm events increase. The stream low flow channel will erode to accommodate the increased flows becoming wider or deeper.

Storm induced urban run-off carries pollutants from roadways, yards, parking lots, storage areas, and flows directly into streams via storm drains and ditches. Up to ninety percent of the atmospheric pollutants, deposited on impervious surfaces, are delivered to receiving streams.

Erosion Run-off can erode land and carry sediment into streams. Erosion reduces the value of property, and the resultant sediment loads streams, degrading the stream's water quality, fish habitat, and diversity of macro-invertibrate. Larger sediment particles may clog spawning gravels. Sediments also reduce the hydraulic effectiveness of the surface water system as they settle into ditches, creek beds, and culverts.

Pollution Pollutants such as oil, gas, fertilizers, and pesticides carried with run-off can adversely affect fish, wildlife, plants, and may impact drinking water supply. Pollutants that originate from one source, like a sewage treatment plant, are known as point source pollutants. Point source pollution is regulated at the local, state, and federal level. Some examples of point source pollution include municipal, industrial, and commercial wastewater discharge; combined sanitary sewage and stormwater overflows; discharges from confined animal feedlot operations; and urban storm sewer discharges.

Pollutants that originate from diffuse sources are known as nonpoint source pollutants. Nonpoint sources are the most significant source of water pollution. Some examples of nonpoint source pollution include sedimentation from logging operations; chemicals and fertilizers from agricultural operations; oil, toxic chemicals, and heavy metals from commercial and industrial operations. Cumulative effects from residential activities are also significant nonpoint source pollutants, including household chemicals, paints and solvents; fertilizers, pesticides and herbicides used on gardens and lawns; nutrients and fecal matter from septic systems and domestic animals; and metals and toxins from wash water, oil, antifreeze, transmission and brake fluids; and fuel from automobile maintenance.

Watershed Management Practices

Nonpoint source pollution poses a serious threat to the health of urban watersheds. It results from an accumulation of many small actions, and, although the individual impacts may seem minor, the combined cumulative effects are significant. Control measures and best management practices (BMPs) exist that can be utilized for improved watershed health. The effectiveness of the measures varies, depending on the specific pollutants addressed; the watershed hydrology and characteristics, such as soils, slopes, type of vegetative cover, and the nature and extent of area development; the waterbodies in the watershed; and the sources of the pollution. Effectiveness also depends on correct application of the control measure or practice.

All types of land uses have the potential to create nonpoint source pollution. Most of this pollution results from changes to and disturbances of the land. Some key sources include residential areas, agricultural activities and forest practices. Residential

Residential problems stem from neighborhoods containing typical single- or multi-family dwelling units. The problems arise from impervious surfaces that increase the flow and volume of runoff causing stream channel erosion and flooding, and from sedimentation from eroded lawns and gardens. Runoff can become contaminated by household chemicals such as fertilizers, pesticides and herbicides, paints, solvents, and street/auto contaminates like oil.

The most effective control measures to address residential nonpoint source pollution include:

  • public education
  • use of vegetated swales and wetlands for contaminate filtration before runoff enters receiving streams
  • sediment traps in urban stormwater systems
  • stormwater retention (e.g. detached downspouts)
  • landscape design for erosion control
  • reduction in use of household chemicals
  • recycling and proper disposal of household chemicals
  • proper maintenance of on site septic systems to reduce nutrient loading
  • combined sewer overflow management
  • vegetative planting and riparian enhancement of neighborhood streams
  • frequent street sweeping and use of vacuum sweepers to reduce suspended solid loading and decrease heavy metals and phosphorus contamination to receiving streams
  • limited development on steep slopes
  • limited amount of impervious surface
  • increased use of cluster developments
  • utilization of erosion control ordinances, especially on construction sites
Agricultural Activities Agricultural activities include land uses such as orchards, nurseries, crop production, feedlots, and grazing. Most nonpoint source pollution from agricultural practices comes from erosion or chemical contamination of receiving waters.

The most effective control measures to address agriculture-related non-point source pollution include:

  • limit intensive livestock uses
  • riparian area protection and enhancement
  • revised management practices for livestock grazing and manure handling
Forestry Activities Forest practices generally lead to nonpoint source pollution problems of soil erosion and chemical contamination. The most effective control measures to address these problems include:
  • technical assistance to landowners
  • limits on road building and management
  • use of erosion control standards
  • chemical application controls (pesticides and herbicides)
  • riparian area protection and enhancement

Getting Involved and Taking Action

Watershed residents can be the "watchdogs" of the watershed, reporting occurrences of flooding, contamination, and illegal dumping. Residents are also in the best position to act as stewards of their watershed; they can help monitor water quality and enhance wildlife habitat. At the national level, citizens can establishing groups to protect and restore watersheds.

So where do you start?
Find out what information is available There is a lot of information available from a myriad of sources. However, much of the existing information and data about natural resources is organized based on political boundaries, such as counties or neighborhoods. An important first step is to get this data organized along hydrologic boundaries. The challenge is working together to get specific data on a watershed basis.

Define your watershed
When beginning to define your watershed, it is helpful to:

  • identify watershed boundaries;
  • identify water bodies showing visible signs of point or nonpoint source pollution (erosion, siltation, etc.);
  • identify water bodies and aquifers or recharge zones
  • identify current land uses in the watershed
Identify critical areas.
Two types of critical areas occur in every watershed: areas adjacent to or near the waterbody and areas that may contribute large amounts or high concentrations of pollutants to the watershed irrespective of their location. The following critical areas should be considered:
  • areas adjacent to a waterbody
  • areas within 1/4 mile of a waterbody
  • areas that contain direct discharges to a waterbody (pipes, ditches, tanks)
  • areas that have intense land use patterns
  • areas that are used for higher-risk purposes
  • geologically vulnerable areas (unstable soils, steep slopes, etc...)
It is critical to develop relationships with agencies, additional scientific experts, and other active watershed groups to identify what information already exists or is being collected, to get assistance in determining what information gaps exist and might be addressed, and to find out about the successes and lessons learned in other watersheds. Ask specific questions, such as:
  • What discharges are located on the stream and what are their permit limits?
  • How many households in this watershed use septic tanks five years or older and which area of the watershed has the most concentrated use?
  • How many cattle are being grazed in this watershed? Based on the average manure production of a cow, what are the nitrogen and phosphorus impacts to the stream?
Information such as this can be used to make a list of pollution sources and to roughly calculate the pollution loads coming from each source. Determining actual pollutant load is a very complex task, best accomplished by an technical expert.
  Source: Adopted using information from several sources, and based on guides prepared by the EU and US-EPA
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