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Watershed Index Online

Introducing WSIO: A National Watershed Library and Tools

EPA developed WSIO so that resource managers, citizens and other users can learn about hundreds of watershed characteristics within user-selected geographic areas anywhere in the conterminous United States.  The content in this website is technical.

What is a watershed?

A watershed – the land area that drains to one stream, lake or river – affects the water quality in the water body that it surrounds.  Like water bodies (e.g., lakes, rivers, and streams), individual watersheds have similarities, but they also differ in many ways.  Healthy watersheds not only maintain good water quality but also provide benefits to the communities of people and wildlife that live there.  Every inch of the United States is part of a watershed – in other words, all land drains into a lake, river, stream, ocean or other water body and directly affect its quality.  We all live in a watershed; watershed conditions are therefore important to everyone.

There are literally hundreds of watershed characteristics (such as environmental traits, sources of degradation and community factors) that may influence water quality, environmental health and quality of life.  Comparing watershed characterisitics is one way in which citizens, communities and their government agencies:
  • learn about their environment,
  • identify water pollution control options, and
  • plan for effective lake, river or stream restoration and protection. 

A standardized, national source for mapping watersheds is the Watershed Boundary Dataset (WBD) ( The WBD includes maps of drainage areas identified by their Hydrologic Unit Codes, called HUCs.  An HUC is a sequence of numbers or letters that identify a hydrological feature like a river, river reach, lake, or area like a watershed, drainage basin or catchment.  Although all HUCs are delineated based on landscape surface runoff characteristics and boundaries, not all HUCs are true, complete watersheds; many are partial drainage areas within a larger, complete watershed.   In the WBD and in common practice, the terms “HUC” and “watershed” are often used interchangeably.  Except where identified explicitly as complete watershed data, the WSIO’s watershed data are measured characteristics of individual HUCs.

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What is the Watershed Index Online (WSIO)?

The Watershed Index Online (WSIO) is a free, public access comparative analysis tool and national watershed indicator data library. WSIO helps users compare watersheds in a user-defined geographic area, for a purpose of their choice, using factors most relevant to their comparison.  EPA developed this site to enable public or private resource managers, citizens, and other users to compare watersheds across any geographic area of interest for a wide variety of reasons, primarily related to learning how watersheds and the activities within them can influence the health of America’s waters.

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What are the intended uses and who are the intended users of the WSIO?

The WSIO is primarily a resource for technical users with a moderate level of understanding of environmental science and geospatial information. Federal, state, tribal and local water quality programs, non-government organizations, and anyone with environmental science and computer skills who are interested in water quality management and planning may use WSIO to:
  • Easily obtain information about specific characteristics of watersheds of interest.
  • Allocate limited resources more wisely for maintaining and restoring clean and healthy water.
  • Compare watersheds for restoration planning and prioritization.
  • Compare watersheds to target healthy watersheds for protection.
  • Understand and use geospatial watershed data to support resource and planning decisions.

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What are the main components of the WSIO?

The WSIO is a public, technical information resource that consists of:

  • Nationwide watershed indicator data library: A library of several hundred indicators measured on all conterminous U.S. watersheds at the HUC12 scale.  HUC12 average size is approximately 35 square miles, and there are about 83,000 HUC12s in the lower 48 states. Tables of watershed indicator data are downloadable, as are several of the most frequently used WSIO geospatial datasets.
  • WSIO Tool: A coded Windows application used to select a study area, download watershed indicator data, perform calculations, and compare and contrast watersheds. The WSIO utilizes an online, customizable tool creation process that draws from the WSIO watershed indicator data library.  Users may download a custom-designed watershed analysis tool copy for their area of interest, pre-loaded with the watershed data they have selected, for local use on their own computers.  Manuals and metadata files in this website describe how to use the tools and the WSIO indicator data. Other web links provide visitors with opportunities to visit project areas where WSIO data and tools have been used.
  • State-Specific Recovery Potential Screening (RPS) Tools: The RPS tools are pre-made, state-specific watershed comparison tools that use the same WSIO data. These tools perform most of the same functions as the WSIO in a modified Excel file, used in the local computer environment without the need for Internet access. These tools each contain full state coverage and an embedded data table with hundreds of WSIO indicators at HUC12 scale.  You can download any state or territory’s RPS tool as an alternative to creating your own custom WSIO tool.

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What are watershed indicators?

A WSIO watershed indicator is a measured attribute of a watershed that is useful for watershed comparison and can be consistently calculated for watersheds throughout the geographic area of interest.  Most indicators are derived from geospatial data analysis, but many also come from surface waters monitoring data or other ground-gathered information sources.

Indicator types in the WSIO database are categorized into four categories: Ecological, Stressor, Social and Base.  The major categories represent three very general types of attributes that determine watershed condition. These categories are based on an extensive, peer-reviewed literature search for factors affecting watershed condition.

  • The ecological indicators reflect overall current condition and resilience (i.e., the capacity of the watershed to maintain or regain functionality) based on metrics related to natural watershed processes and structure.
  • The stressor indicators reflect the pressures on watershed condition from several primary sources of pollutants and water quality impairments.
  • The social indicators can include many factors, such as community involvement, incentives, economics, governance, regulation, and planning status, that do not constitute watershed condition or stressors but often strongly influence the level of effort and complexity of achieving improvements.

The Base category contains only reference attributes, such as watershed name, ID number and size, that are unrelated to watershed condition.

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How does the WSIO Tool work?

The WSIO Tool is designed to help users:
  • identify a geographic area containing watersheds of interest,
  • select WSIO indicators relevant to their objective, and
  • run comparative screenings among selected watersheds.

Some uses of the WSIO Tool are performed online, while other uses occur after download in the user’s local computer environment. Online, the user delineates their geographic area of interest and the WSIO Tool identifies the HUC12s that comprise the area. The user then selects the indicators of interest for those HUC12s and saves the data pertaining to them through the WSIO Tool. This creates a customized WSIO tool copy that the user can operate in their local environment. To screen and compare watersheds, the Tool has a series of macros built into it that are used to calculate four indices using the selected indicators.  The Tool’s macros also store selected indicator data and indices, and display screening results in map and graph formats.

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How should a user choose which watershed indicators to download?

Before choosing indicators, the user should have a clear idea of the kind of comparison among watersheds they plan to make, and what kind of factors may be relevant to that comparison. When initially selecting indicators from the online WSIO data, users should be inclusive and select a variety of indicators that may become useful to their intended analytical objectives at any point. Selecting a wide variety of indicators when the Tool is first created allows the user to perform multiple screenings for multiple project objectives later on their local computer. It is also good to select indicators from as many components as practical for the user’s objectives. This ensures that as many as possible of the EEAs have been addressed.

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How do users select indicators for screening?

Unlike the inclusive approach to initially selecting indicators when creating their custom version of the Tool, users should choose a smaller number of only the most relevant indicators for a specific screening run.  This may be just a small subset of the indicators in their tool. It is recommended that users select between three and 12 indicators of each category (ecological, stressor, social) for a screening run. At least one indicator in each of the three categories is required for the customized tool to run properly.

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What are Ecological, Stressor, Social and Watershed Indices?

The indicators in each category (Ecological, Stressor, Social) that a user selects for a WSIO Tool analysis together become an index of that category that the tool generates during a screening analysis. These indices offer alternative ways to compare watersheds.  The indices are:

  • Ecological: Reflects the overall condition and the capacity of the watershed to maintain or regain functionality, based on metrics related to natural watershed processes and structure. Higher scores imply better watershed condition.
  • Stressor: Reflects pressures (drivers of change) on watershed condition from several primary sources of pollutants and water quality impairments. Higher scores imply worse watershed condition (i.e., higher stressors).
  • Social: Includes many factors that strongly influence the level of effort and complexity in making or maintaining improvements in the watershed. Higher scores imply more favorable context for restoring or maintaining watershed condition.
  • Watershed Index (WSI): An integrated index of watershed condition calculated from the combined effects of the Ecological, Stressor, and Social Index values. Higher scores imply generally better watershed conditions from an overall perspective.

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How does the WSIO Tool display the results from a watershed screening?

Results can be displayed and analyzed using provided tables, bubble plots and maps.

  • The Tool displays a table of Ecological, Stressor, Social and combined WSI Scores for each screened watershed, and a rank order of each watershed.
  • The Tool also displays results as a bubble plot, with each watershed plotted based on its stressor score (x axis), ecological score (y axis), and dot size reflecting its social score.
  • The Tool contains a mapping function allowing users to display indicator data and/or index scores on a map. Access to online maps enables the user to overlay their watersheds and their color-coded score ranges onto other base maps.

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Where can I find more detailed WSIO Tool instructions?

Detailed, step-by-step instructions for using the WSIO Tool are available in the WSIO User Manual and the Quick Start Users Guide.

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I am a less technical user and am not sure that I can create a WSIO custom tool.  Is there any other way I can use the data available on this website?

Although the WSIO User Manual provides detailed directions on all steps, the tool may seem complex to less technically trained users. 
  • If you would like to simply examine watershed indicator scores, you might wish to download a copy of the Regional WSIO Indicator data table for your region. You can find the name of your HUC12(s) of interest online and look them up in this table. The table also contains definitions and source information about every indicator.
  • Also, you may be unsure about defining a watershed comparison purpose and selecting the most relevant indicators, but would still like to explore different indicators and indices just out of curiosity. In this case, consider downloading a pre-made tool for your whole state at the State-Specific RPS Tools page.  These tools perform nearly all of the same functions as the online WSIO tool, and they have over 200 indicators for every HUC12 watershed in the state already embedded in the tool. 

After becoming familiar with the tabular watershed data or with a state-specific RPS tool, you may find that you can revisit the WSIO with more understanding and create exactly the custom tool you want.

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