Because settlements, industries, transport facilities and recreational developments are so concentrated on coasts, the pressures placed on coastal landforms are often acute. While most areas are subject to some degree of natural erosion and accretion, the balance can be upset by human activity in a whole range of different ways. Many soft coastlines in the world are actively eroding at the present time. Many deltas have been formed or have grown considerably in the wake of human interventions that liberated large amounts of sediments in their catchments. However, where sediment loads are reduced through the construction of large reservoirs and sediment accumulation within them, delta erosion may result. Coral reefs are another coastal landform type that is undergoing profound change at the present time in response to a number of stressors. Estuaries have also been subjected to a range of human influences. Many salt marshes have been much modified by human actions, including draining and ditching. In the future, rising sea levels, will occur. These will impact upon low-lying shorelines, including islands, wetlands, mangrove swamps, muddy coasts, coral reefs, lagoons, deltas, barrier islands and beaches. In addition, phenomena like storm surges may increase if hurricane activity becomes more severe. Coral reefs face a suite of potential threats from climate change, ocean acidification and sea level rise.