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They arise mainly due to the action of rainwater. Part of them infiltrates through the most permeable areas and another part runs down the surface towards the lower ground, forming small rivulets that, as they come together, create larger threads, small streams and, finally, rivers. The water that infiltrates the ground penetrates to the lower layers, formed by impermeable stones, and there continues to move underground according to the inclination of the rock layer. “Further on, then, it returns to the surface, also feeding the rivers”, says mining engineer José Renato B. de Lima, from USP. New rivers can appear when, due to the action of earthquakes or volcanoes, the relief of a landscape is altered, creating new elevations or depressions – and, with them, alternative paths for water to flow. There are also rivers that originate from the melting of snow accumulated on the tops of mountains: this is the case of the Amazon, formed by melted snow from the peaks of the Andes.