Governments may alter the flow or course of an urban stream to prevent localized flooding by river engineering: lining stream beds with concrete or other hardscape materials, diverting the stream into culverts and storm sewers, or other means. Some urban streams, such as the subterranean rivers of London, run completely underground. These modifications have often reduced habitat for fish and other species, caused downstream flooding due to alterations of flood plains, and worsened water quality.