With the commissioning of Roxburgh, the sediment which had previously flowed down the Clutha river became trapped behind the dam. Regular surveys commenced in 1961 to monitor this sediment. By 1979 the average river bed level downstream of the Alexandra bridge has increased by 3.6 metres since the lake was created in 1956. The completion of the Clyde Power Station in 1992 reduced the sediment inflows from the Clutha River, leaving the Manuherikia River as the principal source. Floods in 1979, 1987, 1994 and 1995 have led many residents of Alexandra to put pressure on the owners of Roxburgh power station to better manage the sediment build-up. A major flood in 1999 caused flooding of large parts of the main business area of Alexandra. This led to Contact Energy and the government purchasing flood affected properties and flood easements over others as well as constructing a flood bank. Contact Energy has also introduced a program of drawing down the lake level during floods in an attempt to move flush sediment downstream.