Water: Wadeable Streams Assessment
Our WatersRivers & StreamsMonitoring & AssessmentWadeable Streams AssessmentWSA Frequently Asked QuestionsWSA Frequently Asked Questions
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- What are wadeable streams?
- Why did you survey only wadeable streams? What about other waters?
- What is new about this study? Why did we do it?
- What's the major news of this study?
- Who conducted this study?
- Why were 5% of the nation's stream miles not assessed?
- How does this data compare to what EPA has reported on water quality previously (e.g. in 305(b) reports?)
- How does this report relate to USGS water quality reports/programs?
- Who has reviewed the information in this report?
- When will the WSA report be available?
- How do standards we use in the WSA compare to state water quality standards?
- How do we know this doesn't over or under estimate pollution?
- Why are there so few sampling locations in certain areas of the country? (e.g., Florida)
- What is an ecoregion?
- Can WSA data be used to provide state-scale analysis?
- What does this report say about the impact of different sources of pollution, such as urban runoff or agricultural sources?
- Did this report look at the impact of invasive plant and animal species?
- How were the random sites selected?
- How were the thresholds for good, fair, and poor developed?
- How do the thresholds for nutrients compare to EPA criteria guidelines for nutrients?
- How will the information from the WSA be used to direct EPA's national water programs to improve water quality in the future?
- What is EPA's schedule for future surveys?
Wadeable streams are streams, creeks and small rivers that are shallow enough to be sampled using methods that involve wading into the water. They typically include waters classified as 1st through 4th order (and sometimes 5th) in the Strahler Stream Order classification system (based on the number of tributaries upstream).
Why did you survey only wadeable streams? What about other waters?
Wadeable streams were selected for study because they are a critical natural resource, because we have a well-established set of protocols for monitoring them, and because they are underrepresented in traditional sampling programs. Most of the perennial, flowing waters of the U.S. are small wadeable streams.
Wadeable streams are the second in a series of several major water resource categories that EPA, states and other partners are surveying. Coastal waters were the subject of the first national survey of this type, which resulted in the National Coastal Condition Report I and II. EPA and states are currently conducting a national survey of fish tissue in lakes (the National Lake Fish Tissue Study,) and are beginning discussions to survey aspects of lake condition like nutrients and biological communities. National surveys of rivers and wetlands will follow in future years.
What is new about this study? Why did we do it?
This report describes the condition of the nation's streams and establishes a national baseline we can use to compare to results from future studies. This information will help us evaluate the success and shortcomings of national efforts to protect and restore water quality. The Wadeable Streams Assessment, along with upcoming studies of other water resource types, responds to criticisms by the Government Accountability Office and others that the nation's monitoring programs are not providing answers to key questions about water quality.
This study is the first time EPA and states can report on the condition of all U.S. streams. It applies statistical survey techniques as a cost-effective alternative to sampling every mile of stream across the U.S. Results are presented both for the nation as a whole (the lower 48 states) and for major ecological regions of the country. Pilot projects are underway for Alaska and Hawaii.
What's the major news of this study?
The report finds that 28% of U.S. stream miles are in good condition comparable to the best available sites in their ecological regions. Another 25% are in fair or intermediate condition, and 42% are in poor condition. An additional 5% of stream miles were not assessed.
The most widespread stressors observed across the country are nutrients (phosphorus and nitrogen), riparian disturbance, and excess streambed sedimentation. These stressors can degrade stream conditions such that fish and other aquatic life may not be able to survive. Nutrients can increase the growth of algae, decrease levels of dissolved oxygen and water clarity, and degrade stream habitat. Riparian disturbance -- essentially, evidence of human activity alongside streams, such as trash, pipes, pastures and roadways -- is found in more than 70% of streams. Excess streambed sediments can cause streams to become unstable and smother habitat for aquatic organisms. The study notes that stream quality varies widely across the diverse ecological regions of the U.S.; a greater percentage of stream miles are in good biological condition in the West than in other regions of the country.
This study, like the National Coastal Condition Report and the National Lake Fish Tissue Study, was conducted by a partnership between EPA and Federal, state, tribal and agencies and other organizations.
Why were 5% of the nation's stream miles not assessed?
Streams in the New England area were sampled according to an earlier existing regional sampling design that did not include the smallest, first-order streams. The WSA chose to adopt this carefully-established sampling design rather than revise it to include smallest streams in this one area of the country.
How does this data compare to what EPA has reported on water quality previously (e.g. in 305(b) reports?)
While to some extent the findings of the 305(b) report appear somewhat similar to the findings of the WSA, there are many differences between the scope and findings of these two reports and they should not be compared.
The Wadeable Streams Assessment (WSA) represents the condition of virtually the entire wadeable stream resource in the 48 contiguous states. The 305(b) report, on the other hand, summarizes information reported by the states for only a small portion (approximately 20%) of U.S. large rivers and smaller streams. Wadeable streams are generally under-represented in state monitoring programs.
Samples were gathered for the WSA using consistent methods for a specific set of indicators -- two macroinvertebrate indicators, four chemical stressors (nitrogen, phosphorus, salinity, and acidification) and four physical habitat stressors (excess fine sediments, in-stream fish habitat, riparian disturbance, and riparian vegetative cover). The 305(b) report is based on data collected using a variety of state methods, standards, and many parameters that change over time and between states.
For the WSA, sites were selected using a probability-based sample design that ensures that the survey results represent the character of streams across the U.S. For the 305(b) report, sites are selected to meet a variety of state needs (such as identifying and tracking impaired waters), and may not fully represent all conditions within a state.
Because of its consistent methods and sampling framework, the WSA will be useful as a baseline for trend analysis; the 305(b) report, because of variations in state approaches, is not useful for trend purposes.
How does this report relate to USGS water quality reports/programs?
The WSA and the USGS NAWQA program differ in scope and design. The USGS NAWQA studies are based on a sampling design that targets specific watersheds that are influenced primarily by a single dominant land use (e.g., agricultural or urban). The NAWQA sampling design was selected to support analysis of relationships between water quality and land use, and is used by analysts to develop models that predict water quality. USGS also summarizes the monitoring results by describing the number of sites where chemicals are detected and at what level.
The WSA, on the other hand, does not consider land use in its site selection, has a random sample design (meaning that every site in the target population of all perennial 1st through 5th order streams had an equal chance of being selected for sampling) so results are not limited to the sampled sites, but rather, represent the entire population of perennial, wadeable streams.
Both of these designs provide important data and tools to support water quality management. The random design describes the extent of streams in good to poor condition. Water quality models help identify where specific water quality problems are likely to be found.
Who has reviewed the information in this report?
EPA has worked closely with state water quality agencies and Federal and other partners in developing this report. State and other partners collected and analyzed most of the samples in the study. Frequent meetings and conference calls were held with workgroup members involved in data collection, laboratory processing and data analysis, and quality assurance/quality control at each step of the survey process. WSA data were included as candidate indicators for the Report on the Environment, which underwent external peer review. The WSA report itself underwent external peer review in March 2006.
When will the WSA report be available?
We plan to post the draft report for public comment on OWOW's WSA website in early May. After a 60 day comment period, we will make any necessary revisions and release the final report. We plan to issue the report in final form in the summer.
How do standards we use in the WSA compare to state water quality standards?
The WSA does not compare water quality conditions at its sample sites to state water quality standards, but rather to biological reference conditions (the best available sites) determined based on a defined set of scientifically-based decisions.
The data collected are available for states to analyze, along with other available data, using their methodologies for assessing attainment of state water quality standards.
How do we know this doesn't over or under estimate pollution?
Statistical survey designs, like that used for the WSA, have been used for many years to characterize the condition of large populations based on a representative sample of a relatively few members or sites within the population. The ability of these designs to provide accurate estimates, with documented confidence levels, of the condition of populations of interest is well documented. These surveys are used in a variety of fields including election polls, monthly labor estimates, forest inventory analysis, and the national wetlands inventory.
Why are there so few sampling locations in certain areas of the country? (e.g., Florida)
Qualifying stream sites (e.g. perennial, wadeable streams and rivers) were picked according to their density within certain strata, including EPA region and ecological region. Thus if an area such as the Southern Appalachians has a high density of streams, more sites are likely to be selected in that area. Coastal plains such as those in Florida have fewer streams, and of those, many are tidally influenced and did not fit the WSA selection criteria.
Qualifying stream sites (e.g. perennial, wadeable streams and rivers) were picked according to their density within certain strata, including EPA region and ecological region. Thus if an area such as the Southern Appalachians has a high density of streams, more sites are likely to be selected in that area. Coastal plains such as those in Florida have fewer streams, and of those, many are tidally influenced and did not fit the WSA selection criteria.
Can WSA data be used to provide state-scale analysis?
The WSA was designed to provide regional and national assessments of water quality. In most states, there are not enough sites to make statistically-valid assessments.
However, 15 of our state partners supplemented the WSA by sampling at additional sites to obtain state-scale results. We should be seeing the results of their surveys in future state 305(b)/IR reports (see Virginia example). It is one of the goals of the WSA to encourage and support states in conducting these kind of state-scale, statistically-valid monitoring and assessment programs. Our aim is to produce future national reports based on aggregation of comparable and consistent state scale surveys.
What does this report say about the impact of different sources of pollution, such as urban runoff or agricultural sources?
This report addresses key chemical and physical stressors on stream condition, but it does not examine their sources. The exception is that, in the case of acidification, the chemistry data was used to discern among major types of sources, e.g, acidic precipitation, acid mine drainage, and naturally occurring organic acids.
Did this report look at the impact of invasive plant and animal species?
This report did not look at the impact of invasive species on stream quality. However, future Wadeable Stream Assessments may do so.
How were the random sites selected?
The 1,392 sites sampled for the WSA were identified using a particular type of random sampling technique called a probability-based sample design, in which every element in the population has a known probability of being selected for sampling. Rules for site selection included weighting to provide balance in the number of stream sites from each of the 1st through 5th order size classes and controlled spatial distribution to ensure that sample sites were distributed across the United States. The WSA random sites were allocated by EPA region and by ecological region, based on the distribution of 1st through 5th order streams within those regions. Within each EPA region, the random sites are more densely distributed where the perennial 1st through 5th order streams are more densely located. Sites are more sparsely distributed where streams are sparse.
To pick a random sample, one must first know the location of members of the population of interest. The target population for the WSA was the perennial wadeable streams in the 1st through 5th Strahler stream order size classes. The WSA design team used the U.S. Geological Survey (USGS) National Hydrography Dataset (NHD)-a comprehensive set of digital spatial data on surface waters at the 1:100K scale- to identify the location of perennial streams. They also obtained information about stream order from the EPA's River Reach File, a related series of hydrologic databases that provide additional attributes about stream reaches. Using these resources, researchers determined the length of wadeable streams in each of the ecological regions.
How were the thresholds for good, fair, and poor developed?
Consistent with EPA guidance for development of biological and nutrient criteria, WSA used a reference condition approach to set expectations and benchmarks for interpreting the data on stream condition. The benchmarks used to define distinct condition classes (e.g., good, fair, poor) are drawn from this reference distribution.
The reference condition approach involves identifying a set of least-disturbed or best available reference sites in each region. The WSA uses physical and chemical data collected at each site (e.g., riparian condition, nutrients, chloride, turbidity, excess fine sediments) to determine whether any given site is in least-disturbed condition for its ecoregion. The range of conditions found in the reference sites for an ecoregion describes a distribution of those biological or stressor values expected for the least-disturbed condition. The WSA examined the range of values for a biological or stressor indicator in all of the reference sites in a region and used the 5th percentile of the reference distribution for that indicator to separate the poor sites from fair sites in that region. Using the 5th percentile means that stream sites and associated miles in the poor category were worse than the best 95% of the least-disturbed sites used to define reference condition. Similarly, the 25th percentile of the reference distribution was used to distinguish between fair sites and those in good condition. This means that stream miles reported as being in good condition were as good as or better than the best 75% of the least-disturbed sites used to define reference condition.
How do the thresholds for nutrients compare to EPA criteria guidelines for nutrients?
The WSA did not use the EPA criteria guidelines for this report. Thresholds based on the least-disturbed reference sites in each of the 9 WSA ecological regions were developed following the approach recommended in EPA´s guidance for developing nutrient criteria. The differences between the WSA thresholds and the criteria guidelines varied from region to region.
How will the information from the WSA be used to direct EPA's national water programs to improve water quality in the future?
The WSA provides statistically-valid information that allows us to track water quality changes over time and provides insights on the importance of key stressors in different parts of the country.
What is EPA´s schedule for future surveys?
We plan to repeat the WSA in five years. This is part of a series of surveys to evaluate all U.S. waters. Coastal condition has already been evaluated. EPA will sample the condition of lakes, large rivers, and wetlands, then repeat the process to provide ongoing comparisons and point to possible future action.
The next coastal condition report will be released in 2007. A report on the condition of the nation´s lakes is scheduled for 2009. It will be followed in 2011 by a report on rivers and stream, combined. A 4th coastal condition report will be published in 2012, and a report on wetlands is scheduled for 2013.