Report on the Environment
Streambed Stability in Wadeable Streams
What are the trends in extent and condition of fresh surface waters and their effects on human health and the environment?
The above question pertains to all 'Fresh Surface Waters ' Indicators, however, the information on these pages (overview, graphics, references and metadata) relates specifically to "Streambed Stability in Wadeable Streams". Use the right side drop list to view the other related indicators on this question.
- Benthic Macroinvertebrates in Wadeable Streams
- High and Low Stream Flows
- Lake and Stream Acidity
- Nitrogen and Phosphorus Loads in Large Rivers
- Nitrogen and Phosphorus in Streams in Agricultural Watersheds
- Nitrogen and Phosphorus in Wadeable Streams
- Pesticides in Streams in Agricultural Watersheds
- Streambed Stability in Wadeable Streams
Click to enlarge exhibit
Streams and rivers adjust their channel shapes and particle sizes in response to the supply of water and sediments from their drainage areas, and this in turn can affect streambed stability. Lower-than-expected streambed stability is associated with excess sedimentation, which may result from inputs of fine sediments from erosion—including erosion caused by human activities such as agriculture, road building, construction, and grazing. Unstable streambeds may also be caused by increases in flood magnitude or frequency resulting from hydrologic alterations. Lower-than-expected streambed stability may cause stressful ecological conditions when, for example, excessive amounts of fine, mobile sediments fill in the habitat spaces between stream cobbles and boulders. When coupled with increased stormflows, unstable streambeds may also lead to channel incision and arroyo formation, and can negatively affect benthic invertebrate communities and fish spawning (Kaufmann et al., 1999). The opposite condition—an overly stable streambed—is less common, and generally reflects a lack of small sediment particles. Overly stable streambeds can result from reduced sediment supplies or stream flows, or from prolonged conditions of high sediment transport without an increase in sediment supply.
This indicator is based on the Relative Bed Stability (RBS), which is one measure of the interplay between sediment supply and transport. RBS is the ratio of the observed mean streambed particle diameter to the “critical diameter,” the largest particle size the stream can move as bedload during storm flows. The critical diameter is calculated from field measurements of the size, slope, and other physical characteristics of the stream channel (Kaufmann et al., 1999). A high RBS score indicates a coarser, more stable bed—i.e., streambed particles are generally much larger than the biggest particle the stream could carry during a storm flow. A low RBS score indicates a relatively unstable streambed, consisting of many fine particles that could be carried away by a storm flow. Expected values of RBS are based on the statistical distribution of values observed at reference sites that are known to be relatively undisturbed. RBS values that are substantially lower than the expected range are considered to be indicators of ecological stress.
This indicator is based on data collected for EPA’s Wadeable Streams Assessment (WSA). 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 in the Strahler Stream Order classification system (Strahler, 1952). The WSA is based on a probabilistic design, so the results from representative sample sites can be used to make a statistically valid statement about streambed stability in wadeable streams nationwide.
Crews sampled 1,392 randomized sites throughout the U.S. using standardized methods (U.S. EPA, 2004). Western sites were sampled between 2000 and 2004; eastern and central sites were all sampled in 2004. Sites were sampled between mid-April and mid-November. At each site, crews measured substrate particle size, streambed dimensions, gradient, and stream energy dissipators (e.g., pools and woody debris), then used these factors to calculate the RBS.
Because streambed characteristics vary geographically, streams were divided into nine broad ecoregions (U.S. EPA, 2006b), which were defined by the WSA based on groupings of EPA Level III ecoregions (Omernik, 1987; U.S. EPA, 2007). In each ecoregion, a set of relatively undisturbed sites was sampled in order to determine the range of RBS values that would be expected among “least disturbed” streams. Next, the RBS for every site was compared to the distribution of RBS values among the ecoregion’s reference sites. If the observed RBS for a sample site was below the 5th or the 10th percentile of the regional reference distribution (depending on the ecoregion), the site was classified as “most disturbed.” This threshold was used because it offers a high degree of confidence that the observed condition is statistically different from the “least disturbed” reference condition. Any stream with an RBS above the 25th percentile of the reference range was labeled “least disturbed,” indicating a high probability that the site is similar to the relatively undisturbed reference sites. Streams falling between the 5th and 25th percentiles were classified as “moderately disturbed.” Note that the “least disturbed” category may include some streams with higher-than-expected RBS values, which represent overly stable streambeds. Because it is more difficult to determine whether overly stable streambeds are “natural” or result from anthropogenic factors, this indicator only measures the prevalence of unstable streambeds (i.e., excess sedimentation).
Roughly 50 percent of wadeable stream miles are classified as “least disturbed” with respect to streambed condition; that is, their streambed stability is close to or greater than what would be expected (Exhibit 3-6). Conversely, 25 percent of the nation’s wadeable streambeds are significantly less stable than regional reference conditions for streambed stability (“most disturbed”), and an additional 20 percent are classified as “moderately disturbed.” Approximately 5 percent of the nation’s stream length could not be assessed because of missing or inadequate sample data.
- Samples were taken one time from each sampling location during the index period (April-November). Although the probability sampling design results in unbiased estimates for relative streambed stability in wadeable streams during the study period, RBS values may be different during other seasons and years because of variations in hydrology.
- Trend data are unavailable because this is the first time that a survey on this broad scale has been conducted, and the survey design does not allow trends to be calculated within a single sampling period (2000-2004). These data will serve as a baseline for future surveys.
Aggregate data for this indicator were provided by EPA’s Wadeable Streams Assessment (U.S. EPA, 2006b). Data from individual stream sites can be obtained from EPA’s STORET database (U.S. EPA, 2006a) (http://www.epa.gov/owow/streamsurvey/web_data.html).
Kaufmann, P.R., P. Levine, E.G. Robison, C. Seeliger, and D. Peck. 1999. Quantifying physical habitat in wadeable streams. EPA/620/R-99/003. Washington, DC: U.S. Environmental Protection Agency. http://www.epa.gov/emap/html/pubs/docs/groupdocs/surfwatr/field/phyhab.pdf
Omernik, J.M. 1987. Ecoregions of the conterminous United States. Map (scale 1:7,500,000). Ann. Assoc. Am. Geog. 77(1):118-125.
Strahler, A.N. 1952. Dynamic basis of geomorphology. Geol. Soc. Am. Bull. 63:923-938.
U.S. EPA (United States Environmental Protection Agency). 2007. Level III ecoregions of the conterminous United States. Accessed November 2007. http://www.epa.gov/wed/pages/ecoregions/level_iii.htm
U.S. EPA. 2006a. Data from the Wadeable Streams Assessment. Accessed 2006. http://www.epa.gov/owow/streamsurvey/web_data.html
U.S. EPA. 2006b. Wadeable Streams Assessment: A collaborative survey of the nation’s streams. EPA/841/B-06/002. http://www.epa.gov/owow/streamsurvey/pdf/WSA_Assessment_May2007.pdf
U.S. EPA. 2004. Wadeable Streams Assessment: Field operations manual. EPA/841/B-04/004. http://www.epa.gov/owow/monitoring/wsa/wsa_fulldocument.pdf
|Streambed Stability in Wadeable Streams|
|2.||ROE Question(s) This Indicator Helps to Answer|
|This indicator is used to help answer two ROE questions: "What are the trends in extent and condition of fresh surface waters and their effects on human health and the environment?" and "What are the trends in the critical physical and chemical attributes of the Nation's ecological systems?"|
This indicator describes the streambed stability of wadeable streams nationwide as surveyed from 2000 to 2004. Streambed stability—which may be influenced by human activities—affects the quality of aquatic habitats and is therefore a factor in the ecological health of wadeable streams.
The indicator is based on data collected for EPA's Wadeable Streams Assessment (WSA).
Data were provided by EPA's WSA. The aggregate results shown in this indicator have also been published in the WSA report (U.S. EPA, 2006). Data from individual stream sites can be obtained from EPA's online STORET database at http://www.epa.gov/owow/streamsurvey/web_data.html.
This indicator is based on measurements collected from wadeable streams, which are streams, creeks, and small rivers that are shallow enough to be sampled by methods that involve wading. They typically include waters classified as 1st through 4th order in the Strahler Stream Order classification system (Strahler, 1952). For a simple diagram of Strahler's classification scheme, see http://www.racac.nsw.gov.au/rfa/pdf/Sth_Schematic_diagram.pdf (1 p, 5.6 K, About PDF)
This indicator is based on measurements of the stream substrate particle size and specific physical characteristics of the stream, such as the stream channel and gradient dimensions, the extent of woody debris, and the presence of pools. These measurements were used to determine the "critical diameter"—the largest particle size the stream can move as bedload during stormflows. Trained field crews used a standardized set of field equipment provided by EPA, and followed protocols for collecting these physical habitat data. As described in Chapter 7 of the WSA Field Operations Manual (U.S. EPA, 2004a), crews measured the substrate particle size at 105 systematically-spaced locations within each sample reach.
Samples were collected as part of EPA's WSA, which is based on a probabilistic survey design meant to represent the condition of wadeable streams across the nation. The statistically valid survey ensures spatial dispersion within the target population, and all types of natural streams and associated ecosystem characteristics have a known probability of inclusion in the sample. Highly unusual or unique ecosystems, however, have a lower probability of being sampled due to the sparse nature of their location and the broad geographic scale of the sampling design. More information about the probabilistic survey design and implementation can be found on EPA's Web site at http://www.epa.gov/nheerl/arm/index.htm. Additional documentation is provided in Diaz-Ramos et al. (1996) and Stevens and Olsen (1999).
Crews sampled a total of 1,392 randomly selected sites throughout the contiguous U.S.—841 sites in the western states (sampled between 2000 and 2004) and 551 sites were sampled in the eastern and central states (sampled in 2004). Site selection was based on a randomized design described in the WSA Quality Assurance Project Plan (QAPP) (U.S. EPA, 2004b; see section 10) and on EPA's Aquatic Resources Monitoring Web site (http://www.epa.gov/nheerl/arm/index.htm). A map depicting the location of the sampling sites can be found in the 2006 WSA final report (U.S. EPA, 2006).
Approximately 20 reference sites representing the "least disturbed" condition were selected from each of nine ecoregions (see "Indicator Derivation" for more information about ecoregions). These sites allowed EPA to develop regionally specific reference conditions that test results could be compared against. These sites were selected based on recommendations from states and cooperators and from reference sites used in the U.S. Geological Survey's (USGS) Status and Trends Program and Hydrologic Benchmark Network. Some reference sites were chosen from among the 1,392 probability sample sites; other supplemental reference sites that met specific criteria were provided by federal and state agencies and cooperators, such as state monitoring agencies, USGS's NAQWA program, Regional Environmental Monitoring and Assessment Program (REMAP), and Utah State University's Science to Achieve Results (STAR) grant program for the Western states. All candidate reference sites were carefully evaluated using a variety of factors to assure that they represented the “least disturbed” set of sites in their ecoregion.
Each site was sampled once during an index period from mid-April to mid-November. For consistency, the index period (sampling window) is aimed not at a specific calendar period, but a specific hydrologic one. For the WSA, the goal was to collect samples during the base flow period (late spring through summer). This results in an index period that starts in April in the arid Southwest, does not begin until August in some of the high elevation areas, and may expand into the fall in parts of the East, where summer can be fairly wet.
All survey design, sampling, and analytical procedures are documented in EPA publications and available to the public. See U.S. EPA (2004a, 2006), http://www.epa.gov/nheerl/arm/index.htm, and http://www.epa.gov/owow/monitoring/wsa/materials.html.
This indicator is based on Relative Bed Stability (RBS), which is calculated as the logarithm of the ratio of the observed mean streambed particle diameter to the "critical diameter," the largest particle size the stream can move as bedload during stormflows. The critical diameter is calculated from field measurements of the size, slope, and other physical characteristics of the stream channel (Kaufmann et al., 1999). The concept of RBS is well-established in the scientific literature, although its application to synoptic survey data is relatively recent. Methods for calculating the RBS ratio are documented in Kaufmann et al. (1999), and analytical procedures are also described in the 2006 WSA final report (U.S. EPA, 2006). Several additional resources are accessible online at http://www.epa.gov/owow/monitoring/wsa/materials.html.
For this analysis, the 48 contiguous states were divided into nine broad ecoregions based on groupings of EPA Level III ecoregion groupings (see http://www.epa.gov/wed/pages/ecoregions/level_iii.htm). After sampling, the RBS ratio for each stream was calculated and compared with a distribution of RBS ratios for "least disturbed" reference sites within the corresponding ecoregion. The ecoregion approach is recognized as a scientifically valid approach to account for the range of natural variability that might be expected in streambed stability (e.g., streambeds may naturally be sandy, gravelly, bedrock-dominated, etc.).
This indicator classifies the streambed stability of the nation's wadeable stream miles using three categories, which are based on the difference between the observed RBS and the regional reference conditions. For more information on how sites were classified, see "Reference Points."
|9.||Quality Assurance and Quality Control|
An extensive QA/QC procedure was an integral part of the WSA. Full documentation of the QA/QC procedures can be found in the WSA QAPP (U.S. EPA, 2004c). This Quality Assurance Project Plan (QAPP) was reviewed by an independent EPA team with members from the Office of Research and Development (ORD), the Office of Water (OW), and the Office of Environmental Information (OEI). It is available to the public at http://www.epa.gov/owow/monitoring/wsa/QAPP-August18.pdf (20 pp, 742K). QA/QC procedures in the field included training all crew members in WSA methods, conducting a thorough field audit of all crews, and completing extensive chain of custody documentation. All information on the field audits and training sessions are currently housed at the OW's Monitoring Branch and can be distributed on request.
For this indicator, observations from sampling sites were compared with a distribution of "least disturbed" reference sites within each ecoregion, which reflect the least amount of human disturbance. See "Data Collection" for more information on selecting reference sites.
Thresholds were determined using the 5th and 25th percentiles of the reference distribution in the Southern Appalachian ecoregion. In the other eight ecoregions, where reference sites were less numerous and more disturbed, thresholds were based on the 10th and 25th percentiles. Streams whose RBS fell below the lower threshold (5th or 10th percentile, depending on the ecogregion) were classified as "most disturbed." Streams with an RBS between the 5th/10th and 25th percentiles were classified as "moderately disturbed." Streams with an RBS above the 25th percentile were classified as "least disturbed."
|11.||Comparability Over Time and Space|
Although data were collected over four different years, the index period was consistently selected based on a specific hydrologic window (see "Data Collection"), which reduces the impact of yearly and seasonal variability. The same sampling and analytical methods were used at all locations. No corrections have been made to adjust for temporal or spatial biases.
|12.||Sources of Uncertainty|
Content under review.
|13.||Sources of Variability|
For any given sample site, there may be some variability over the sampling season and within a several-year time period. However, this variability is substantially less than the variability among streams within ecoregions, and consequently, the conclusions are not impacted by variability around the indicator. Overall, the WSA focuses on indicators that do not vary much over the index period.
Trend data are unavailable because this is the first time that a survey on this broad scale has been conducted, and the survey design does not allow trends to be calculated within a single sampling period (2000-2004). These data will serve as a baseline for future surveys. Trend analysis will not be available until future assessments are conducted.
Limitations to this indicator include the following:
Diaz-Ramos, S., D.L. Stevens, Jr., and A.R. Olsen. 1996. EMAP statistical methods manual. EPA/620/R-96/002. Corvallis, OR.
Kaufmann, P.R., P. Levine, E.G. Robison, C. Seeliger, and D. Peck. 1999. Quantifying physical habitat in wadeable streams. EPA/620/R-99/003. http://www.epa.gov/emap/html/pubs/docs/groupdocs/surfwatr/field/phyhab.html.
Stevens, D.L., Jr., and A.R. Olsen. 1999. Spatially restricted surveys over time for aquatic resources. J. Ag. Biol. Envir. Stat. 4:415-428.
Strahler, A.N. 1952. Dynamic basis of geomorphology. Geol. Soc. Am. Bull. 63:923-938
U.S. EPA (United States Environmental Protection Agency). 2006. Wadeable Streams Assessment: A collaborative survey of the nation's streams. EPA/841/B-06/002. http://www.epa.gov/owow/streamsurvey/pdf/WSA_Assessment_May2007.pdf (113 pp, 13.4MB).
U.S. EPA. 2004a. Wadeable Streams Assessment: Field operations manual. EPA/841/B-04/004. http://www.epa.gov/owow/monitoring/wsa/wsa_fulldocument.pdf (191 pp, 14MB).
U.S. EPA. 2004b. Wadeable Streams Assessment: Quality assurance project plan. EPA/841/B-04/005. http://www.epa.gov/owow/monitoring/wsa/QAPP-August18.pdf (85 pp, 742K).