There are many dams for hydroelectric power. The Lewis River is impounded by three dams for hydroelectric power generation. Electricity is generated mainly by thermal and hydroelectric power plants. Pico hydro is a term used for hydroelectric power generation of under . Electricity used in Vancouver is generated using sustainable resources such as hydroelectric power. In the 1930s reversible hydroelectric turbines became available. Many lakes have been used as reservoirs for hydroelectric projects. An electrochemical gradient is analogous to the water pressure across a hydroelectric dam. Norway produces 96% of its electricity from hydroelectric sources. Generation of hydroelectric power changes the downstream river environment. Domestic oil and gas along with hydroelectric power facilities provide power. Methods for testing hydroelectric turbines are given in IEC standard 41. The Alps provide lowland Europe with drinking water, irrigation, and hydroelectric power. There are five power plants on the Sulak River providing hydroelectric power. Canada's many rivers have afforded extensive development of hydroelectric power.