: Coastal zones, where freshwater and marine systems meet and co-exist are regions of great ecological diversity. Many are undergoing dramatic change. Deltas and estuaries in particular are affected by changes in river regimes, sediment and pollution loads often the result of upstream human activities. Sea level rise in combination with increased occurrence and intensity of storm surges further effects changes in ecosystems. Many coastal areas exhibit extreme concentration of human settlement and activity. Many of the world's megacities are in coastal zones and several include areas that are below sea level. Densities of rural settlements in coastal zones also tend to be higher than rural densities inland. Economic development in many countries is intimately dependent on activities in coastal regions. Yet these same regions are under increasing threat from both the sea (storms, tsunamis, etc) and from land-based pollution.