Why Is Sea Water Salty But River Water Fresh? 7 Interesting Facts You Didn't Know

Sea water is salty while rivers are mostly fresh because minerals accumulate in oceans over time. Explore how rain, rocks, evaporation and the water cycle explain the difference.

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Why Is Sea Water Salty?

Oceans contain dissolved salts and minerals accumulated over long periods. Unlike freshwater rivers, they retain much of the salt delivered by flowing water, making seawater noticeably salty.

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Rivers Contain Minerals Too

River water is not completely free of dissolved salts. As rainwater travels over rocks and soil, it collects small quantities of minerals. Their concentration is generally much lower than in seawater.

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Rainwater Slowly Breaks Down Rocks

Rainwater absorbs carbon dioxide and becomes slightly acidic. As it passes over rocks, it gradually dissolves minerals that enter streams and rivers before eventually reaching the sea.

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Evaporation Leaves Salt Behind

Heat from the Sun turns surface seawater into water vapour. Dissolved salts do not evaporate with the water, so they remain in the ocean and contribute to its salinity.

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Rivers Carry Dissolved Salts Into Oceans

Rivers transport minerals from land into the sea throughout the year. Although the water may taste fresh, it carries dissolved substances that contribute to the ocean's salt content over time.

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Salt Also Comes From Beneath The Seafloor

Rivers are not the only source of ocean minerals. Hydrothermal vents and reactions between seawater and rocks beneath the ocean floor also influence the chemical composition of seawater.

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The Water Cycle Helps Keep Rivers Fresh

Rain and other freshwater sources continually replenish rivers, while flowing water carries dissolved minerals downstream. This movement prevents salts from accumulating in most rivers to the concentrations found in oceans.