Almost three quarters of the world's 33 major river deltas are sinking, according to new research. Results indicate that the sinking is worsened by the impacts of human activity, such as upstream sediment collection caused by reservoirs, dams, accelerated sediment compaction, and control of river channels. About 500 million people live in or near river deltas, which are formed when rivers deposit sediment as they flow into the sea. The 2007 Intergovernmental Panel on Climate Change report concluded that many deltas will experience sea level rises due to climate change. The effects of twentieth century development and population growth will also increase the risk of deltas flooding. The research analysed high resolution satellite data, historical maps and infrared images of 33 world river deltas. Four of these were EU deltas, in Italy, France, Poland and Romania. The results indicated that 85 per cent of deltas experienced severe flooding in the past decade, causing 260,000 km2 of land to be temporarily submerged. The study investigated the possible role of compaction of sediment in the increase in flooding, particularly compaction caused by human activities, such as removal of gas and water, trapping of sediments upstream in reservoirs and floodplain engineering. For example, the Po Delta in Italy subsided 3.7 metres in the twentieth century; 81 per cent of this is attributed to methane mining. This research is the first to estimate the volume of sediment delivered to the deltas both before and after substantial human activity. The results demonstrated that sediment delivery has been reduced or eliminated at the majority of the deltas. Much of this can be attributed to upstream damming, eg in the Ganges (India) and the Mekong (Vietnam). Another factor is the reduction in the number of side channels. The number of distributary channels has dropped for 13 of the major deltas, including the Vistula (Poland) and the Nile (Egypt) which both suffered a 70 to 80 per cent reduction in distributary channels. This reduction in sediment delivery has caused deltas to sink and makes them more vulnerable to flooding from sea level rises. A few deltas have remained largely unchanged over the twentieth century, such as the Amazon (Brazil) and the Congo (Western Africa).