# The Biological Assessment and Rehabilitation of the World's Rivers: An Overview
## 1 Introduction  
Since the beginning of human civilization, we have used, exploited, and degraded freshwater ecosystems, beginning with agriculture in the fertile lands around rivers, then by industrialization, and in the second half of the 20 th century by urbanization [1; 2; 3]. The consequences are manifold, from water pollution to highly modified channels, riparian zones, and flow regimes [4; 5; 6]. This has resulted in catastrophic and accelerated loss of freshwater biodiversity, increased prevalence of invasive non-native species, and altered ecosystem functioning [4; 7]. Thus, the necessity of assessing the ecological condition of rivers arose, first to prevent public health problems and more recently to understand the extent of damage in the ecosystems and plan cost-effective recovery actions [8; 9]. Ecological assessments assume that rivers should be viewed as ecosystems, composed of different biological assemblages that interact among themselves and with the abiotic conditions, all contributing to complex ecosystem functions. The biological elements integrate the alterations and pollution of the ecosystem over time and space and are therefore good indicators of river ecological condition [9; 10]. The first biotic indices for assessing rivers appeared in the middle of the 20 th century [10]. Initially, they were based on the microbial effects of organic pollution [11], leading to the development of saprobic indices. Later, indices were developed for diatoms and macroinvertebrates [12; 13]. For fish, one of the first widely implemented approaches was the Index of Biotic Integrity (IBI) that responded to the goals of the Water Pollution Control Act of 1972 in the USA [14; 15]. The use of biological assemblages (i.e., benthic invertebrates, fish, algae, or other plants) as indicators was integrated into other pieces of legislation such as the Resource Management Act (1991) in New Zealand, the European Water Framework Directive (WFD; [16]), or CONAMA--Conselho Nacional do Meio Ambiente 357/2005 in Brazil. However, despite the large number of existing methods [10; 17], the extent to which the biological assessments of rivers and streams are performed worldwide is unclear, as information is largely scattered. Theoretically, the assessment of river or stream conditions is just an intermediate step that could lead to rehabilitating rivers that fail to meet the quality criteria. Yet, how much has been done to improve the quality of rivers at a global scale is even more obscure. In addition, restoration or rehabilitation of rivers may happen for different number reasons, often independent of their ecological condition. Common reasons for rehabilitation are the will to improve the aesthetics of an urban area near a river, implement ecological flows to comply with existent legislation, prevent floods, or facilitate fish passage at barriers. Here we use the word rehabilitation instead of restoration because nearly all recovery efforts focus on rehabilitating river ecosystem services for humans in small river reaches--not restoring entire river ecosystems to some pristine or natural state [18]. Regardless of the reasons for rehabilitation projects, often there is no associated ecosystem monitoring, and therefore the amelioration of ecological quality is unknown. This prevents learning about the most effective measures to restore rivers. In addition, the ideal measures may not be feasible because of high costs that may be a barrier to implementation. Additionally, different nations sharing the same catchments may differ widely in the degree to which they protect the environment and use natural resources, which hinders continental and global improvement. Yet, the aims of the United Nations Agenda 2030 for sustainable development clearly state the need to decrease pollution and guarantee access to safe drinking water for all and protect the freshwater aquatic ecosystems and biodiversity.