These two images show the town of Cromwell at the junction of the Kawarau and Clutha rivers in 1903 (top) and 2005. Built in the late 1980s, the Clyde Dam impounded the two rivers, producing Lake Dunstan in 1993. Hydroelectric schemes have altered many New Zealand river catchments. Dams remain one of the major threats to freshwater ecosystems. They obstruct the pathways of fish, and few invertebrates can cope with the fluctuations in river levels downstream (caused by releasing more water to generate more power, when demand surges in the evenings).