Sevan's level began to drop significantly, at a rate over per year. The water was used for irrigation and the Sevan–Hrazdan Cascade of six hydroelectric power stations on Hrazdan River. Effects During the second half of the 20th century, the ecological condition of Lake Sevan underwent tangible changes and vast degradation due to reduced water level, increased eutrophication, and detrimental impact of human activity on the biological diversity of the lake. According to Babayan et al. the lake level dropped by by 2002, while the volume decreased by 43.8% (from ). Due to the water level decrease, the quality of the water deteriorated, natural habitats were destroyed that meant loss of biodiversity. Vardanian wrote that drop of the lake level and the economic development in the basin brought about the change in hydro-chemical regime of the lake. The quality of the water deteriorated, water turbidity increased. The inner circulation of the water constituents as well as the circulation of the biological substances altered. Reversal and recovery According to Babayan et al. "by the 1950s it had become evident that the ecological and economic consequences of extensive exploitation of the water of Lake Sevan were too undesirable to continue in the same way." Arpa–Sevan tunnel In 1964 a project began to divert the Arpa River (from a reservoir near Kechut) through a long tunnel to the lake near Artsvanist. The tunnel, called Arpa–Sevan, was completed in 1981. It brings up to of water to Sevan per year. It is estimated, that due to climate change by year 2030, the outflow of the Arpa river will decrease by 22%. Vorotan–Arpa tunnel Since the water level in the lake did not rise as fast and as much, on 20 April 1981 the Council of Ministers of the Soviet Union made a decision for the construction of the Vorotan–Arpa tunnel. This long tunnel was begun from the Spandarian Reservoir on the Vorotan River further south from Kechut. Due to the Nagorno-Karabakh conflict and the 1988 earthquake in northwestern Armenia construction was halted. The tunnel was inaugurated on 26 April 2004. The Vorotan–Arpa tunnel brings an additional to the lake annually. Increase in water level After the construction of the two tunnels the water level began rising significantly since the mid-2000s. In 2007 it was reported that the water level had risen by in the previous six years. It reached in October 2010. The government