agriculture amazon river aran islands asia calcium carthage deforestation desalination desertification dorset england erosion euphrates europe firth of clyde flood control ganges geography great depression isle of arran israel latin levee madagascar magnesium middle ages mississippi river nile nomad nutrient pesticide photosynthesis plough rainforest reverse osmosis rhine roman republic salting the earth scotland shifting cultivation silt slash and burn sodium starvation tigris urban sprawl yellow river