According to Brazilian law, (regulation number 1469,29 December 2000), all of the studied reaches of Rio das Velhas are inappropriate for human consumption
without prior treatment and inappropriate for primary contact, aquatic sports, and bathing. The Rio das Velhas has very low potential for recreational use without further treatment by the sanitary company (COPASAMG).Most Brazilian cities have experienced uncontrolled development and increased occupation of river
margins and their floodplains. Many cities are trying to solve problems of sanitation and flooding with river canalization. Canalization increases water velocities, reducing residence time and accelerating domestic effluent and pollutant transport. Such changes also lower richness and diversity in the Rio das Velhas downstream
of MRBH, highlighting the effects of sewage discharge on the main channel. However, urbanization costs are even higher in the small water courses draining the metropolitan area. Besides the poor benthos assemblage and water quality, pollution and canalization of MRBH streams have eliminated most native fish species. Only a
few well-preserved headwater reaches, protected as natural reserves or water supply sources, support typical headwater fishes, such as Trichomycterus and Astyanax.
Urbanization has also changed fish assemblages in the natural lakes of the MRBH. Lagoa Santa is a shallow permanent lake located north of Belo Horizonte, inside
the metropolitan region. Between 1850 and 1856,Reinhardt collected fish in Lagoa Santa (Lütken 2001).Comparisons between the historical and recent data showed that at least 70% of the original fish fauna is now extinct (Pompeu and Alves 2003). The reasons were directly related to urbanization: the obstruction and canalization of the natural connection between Lagoa Santa and the Rio das Velhas, the elimination of littoral (Cyperaceae) and submerged (Characeae) vegetation,and the introduction of alien fish species.