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National Aquatic Resource Surveys

What is the National Rivers and Streams Assessment?

The National Rivers and Streams Assessment (NRSA) is a national probability-based survey of rivers and streams based on physical, chemical and biological data collected and analyzed using standardized field and laboratory methods.

The goals of the NRSA are to determine the extent to which rivers and streams support a healthy biological condition and the extent of major stressors that affect them. The survey supports a longer-term goal: to determine whether our rivers and streams are getting cleaner and how we might best invest in protecting and restoring them. Additionally, the survey compares the condition of streams to those of an earlier study that focused on small streams (the Wadeable Streams Assessment or WSA) conducted by the U.S. Environmental Protection Agency and its partners in 2004.

NRSA sampling field seasons were conducted in 2008-2009, 2013-2014, and 2018-2019.

Survey Design

Sampling locations are selected using a modern survey design approach called a probability-based sample design. In such a design, every element in the population has a known probability of being selected for sampling. This important feature ensures that the results of the survey reflect the full range in character and variation among flowing waters across the U.S. Site selection rules included weighting to provide balance in the number of river and stream sites from each of the size classes. Site selection is also controlled for spatial distribution to make sure sample sites are distributed across the U.S.

The NRSA assesses the ecological condition of the full range of flowing waters in the conterminous U.S. (lower 48 states). The target population includes the Great Rivers (such as the Mississippi and the Missouri), small perennial streams, and urban and non-urban rivers. Run-of-the-river ponds and pools are included, along with tidally influenced streams and rivers up to the leading edge of dilute sea water.

View more details about the survey design in the design documents.

The use of standardized field and laboratory protocols for sampling across all 48 states included in the NRSA is a key feature of the survey. It allows the data to be combined to produce a nationally consistent assessment.


National Rivers and Streams Assessment (NRSA) collects data on key indicators of biological, chemical and physical condition. These indicators are used to assess ecological and condition and to examine conditions that may negatively influence or affect stream condition (i.e. stressors). The NRSA uses fish and benthic macroinvertebrates (insects and other small animals such as snails and crayfish) as biological indicators of ecological condition. Stressors are the chemical, physical, and biological components of the ecosystem that have the potential to degrade biological integrity.  Some of these are naturally occurring, some result only from human activities, but most come from both sources.  Examples include chemical indicators such as nutrients and physical indicators such as streambed sediments or instream fish habitat.  The NRSA also assesses indicators related to human health, specifically mercury levels in fish tissue and the fecal indicator enterococci. 

The table below shows the key indicators evaluated for the NRSA 2013-2014.  More information on these indicators specific to rivers and streams can be found in the NRSA report and other documents such as related manuals.  Note, a smaller set of indicators was used in the Wadeable Streams Assessment.

Indicators Evaluated for the NRSA 2013-2014
Biological Chemical Physical Human Health