The Yamato River used to be a familiar river widely known as a place of river play until the 1950s. However, along with the high economic growth, the development of the watershed developed as a residential area in the Keihanshin metropolitan area. When the industry was concentrated, the water quality deteriorated sharply due to household drainage, factory drainage, and livestock drainage. Since the 1960s, the Yamato River's water quality had a poor condition making it unsuitable for the use of water and other hydrophilic and living creatures. 70% of the causes of pollution in the Yamato River were domestic wastewater. At that time, the sewerage coverage rate of households in the Yamato River basin was less than 20%. This means that domestic drainage flowed into the river without any treatment .
A typical organism affected by this deterioration in water quality was sweetfish (ayu; Plecoglossus altivelis in its scientific name ). Before the war and shortly after the war, sweetfish's upswimming had been confirmed and reported in the Yamato river basin. However, the number of natural sweetfish's upswimming started to decline sharply from around 1965. On the other hand, the BOD value of the f Yamato River was excessive (see Project Details) in 1970, becoming the dirtiest water quality in observation history. From around the time, it was not possible to confirm the upswimming of sweetfish. Then, the Yamato River became one of the dirtiest rivers in the country .
In response to this situation, the relevant organizations of the basin, the residents, etc. began to mobilize with their efforts. Since the cause of pollution was from domestic wastewater, they conducted steady activities such as the review of wastewater and cleaning activities of the river . On the administration side, four major efforts have been made so far. In March 1991, the Yamato River Water Quality Improvement Emergency Five-Year Plan (commonly known as Aqua Road Yamato River Project), which aimed to improve the water quality suitable for water play, was compiled . Furthermore, in November 1993, in order to improve the water quality, as a comprehensive measure that the relevant organizations of the basin cooperated, the Yamatogawa Kiryu Renaissance Council was established to make plans such as river business, sewerage business, spread enlightenment activities, and to carry out various efforts . One year later, the Water Environment Improvement Emergency Plan (commonly known as Yamato River Clean Flow Renaissance 21) and the second phase Water Environment Improvement Emergency Action Plan (Yamato River Clean Flow Renaissance II) were formulated in October 2002. The Yamato River Water Environment Council was established in September 2005, and the C Project Plan 2006 (Water Environment Edition) was formulated in September of the following year, following the declaration of the Yamato River Water Environment Summit . The objectives of this plan were: (i) securement of a water environment that can be playful in summer; (ii) creation of a safe and comfortable waterfront environment; (iii) conservation of a water environment where various animals and plants can live and grow; (iv) creation of river landscapes so the Yamato River; and (v) Early achievement of the environmental standards . The Yamato River Water Environment Improvement Plan has been formulated as the latest policy, which set the directionality of water environment improvement . In addition, various efforts have been carried out such as promoting connection to sewers and promoting conversion to merger septic tanks, social experiments to improve water quality in priority rivers, and partial revision of the septic tank maintenance assistance system .
Through these efforts, the government began maintaining environmental standards established by the country from around 2009 . In 2010 and 2011, the Yamato River ranked first in the nation as "a river whose water quality has been significantly improved over the past 10 years" among the first-class rivers managed by the country . In 2015, the water quality of the Yamato River became 2.6 mg/l (BOD 75% value) and recorded the most beautiful water quality in the observation history of the river . This is about nine times lower than that in 1970, when water quality deterioration peaked. As a result of measuring the elements contained in the "otolith" of sweetfish collected in the lower reaches of the Yamato River in 2004 and 2005, the upswimming of natural sweetfish was confirmed . In 2007, egg laying of sweetfish was confirmed for the first time after pollution .