# The Biological Assessment and Rehabilitation of the World's Rivers: An Overview
## 2 Biological Assessment of Rivers
### North America  
The policy foundation for USA lytic ecosystem monitoring and assessment is the Federal Water Pollution Control Act (Clean Water Act; CWA, 1972). The objective of the CWA \"is to restore and maintain the chemical, physical, and biological integrity of the Nation's waters.\" More specifically it aims to \"provide for the protection and propagation of fish, shellfish, and wildlife and recreation in and on the water.\" Each state must provide a biennial report on the water quality of all navigable waters, including the extent to which they meet the objectives and goals of the CWA, and including \"the nature and extent of nonpoint sources of pollutants.\"  
Because they lacked standard monitoring, analysis and reporting protocols, those State reports could not be used to assess the ecological status or trends in all USA waters [207]. As a result, USEPA initiated the Environmental Monitoring and Assessment Program (EMAP), which initially focused on a set of methodological studies (e.g., [208; 209; 210]). Those studies were followed by regional demonstration projects (e.g., [211; 212; 213]). The demonstration projects led to a wadeable stream assessment (WSA) across the conterminous USA [214; 215]. The success of the WSA led to the establishment of the National Rivers and Streams Assessment (NRSA; [58]). NRSA surveys were conducted in 2008-2009, 2013-2014, and 2018-2019 during the summer base flow period through use of standard protocols [216; 217]. To date, nearly 5000 sites have been sampled, over 1000 of which have been resampled to refine trend and sampling error assessments. The NRSA, like all NARS programs, is based on a probability (dispersed, randomized) survey design for site selection [218; 219] and a systematic site sampling design [220]. At each wadeable stream and boatable river site, a standard suite of biological, physical, and chemical condition indicators is sampled. The raw physical habitat data are converted into many quantitative metrics [221], including measures of habitat condition, relative bed stability [222; 223] and hydrologic retention [224]. Biological elements include fish, macroinvertebrate and periprbity diatom assemblages. The accuracy and precision of fish field and macroinvertebrate and diatom taxonomic identifications have been assessed and improved in independent studies [225; 226; 227; 228]. The raw biological data are converted into indices (MMI, observed/expected models) for reporting at state, ecoregional and national spatial extents [213; 215; 229]. Ecological assessments are based on ecoregional reference site conditions [230; 231] and models [232]. The macroinvertebrate metrics include taxonomic composition, diversity and richness, as well as feeding, habit, and tolerance guilds. Fish MMIs are based on trophic, habitat, reproductive, life history and tolerance guilds, as well as taxonomic composition and non-native species. The physical and chemical indicators are used in conducting biological risk assessments [233]. Because all sites are sampled with standard methods and randomly selected, the results can be inferred to all target waters in the conterminous USA, which are important strengths of the NRSA. The NRSA data are publicly available at [[https://www.epa.gov/national-aquatic-resource-surveys/what-national-rivers-and-streams-assessment#tab-2](https://www.epa.gov/national-aquatic-resource-surveys/what-national-rivers-and-streams-assessment#tab-2)]([https://www.epa.gov/national-aquatic-resource-surveys/what-national-rivers-and-streams-assessment#tab-2](https://www.epa.gov/national-aquatic-resource-surveys/what-national-rivers-and-streams-assessment#tab-2)). The NRSA has produced 11 key findings [216; 217; 229]. (1) The biological condition of conterminous USA stream and river length is 30% good, 28% fair, and 44% poor based on macroinvertebrate multimetric index (MMI) scores. (2) This varies regionally, with 64% and 26% in poor condition in the Coastal Plains and the Southern Plains, respectively.