Why are lakes freshwater and oceans saltwater? (Smithsonian.com)
Why are lakes freshwater and oceans saltwater?
Lexile

It has something to do with this leaky faucet. You see, rain falls on the land as fresh water. As this water flows downward, it ever so slightly erodes the rocks and soil. This causes an ever so slight amount of salt and minerals to dissolve in the water and continue downstream. Eventually, all the dissolved minerals get dumped into the ocean.

Meanwhile water on the ocean's surface evaporates into the air, ditching all those salts and minerals. It then falls back on land as fresh water, replenishes the lakes and rivers, and the cycle continues. In lakes and rivers, theres enough turnover that the water stays fresh. But in the oceans, the salts pretty much stay there for millions of years.

Worldwide, it's estimated that rivers dump about 4 billion tons of salts into the oceans each year. But even with all that new salt, the oceans are so big, that their salinity stays fairly constant, at about 3.5%.

Critical thinking challenge: How is the water cycle like a bicycle?


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COMMENTS (13)
  • KAYLIEC-ZDE
    6/02/2014 - 11:05 a.m.

    I never actually thought why there was a difference. Its actually really interesting. Many wouldn't think that the way the water evaporates could effect the salt in our mastic oceans. Very interesting am i am excited to share my new found information with others.

  • JohnD-ZDE
    6/02/2014 - 03:47 p.m.

    The lakes have fresh water and the ocean is salty because as the water flows down to the ocean, then it erodes the rock and soil which make the water salty along the way. Water is like a cycle because it goes through different phases. The water evaporates into the clouds. As the clouds get full, they turn gray and are ready to rain down and water the crops of the farmers and do many more wonderful things. Water is a great part of nature. Without water, we probably would have been dead million of years ago!

  • 2AlexL
    6/04/2014 - 09:51 a.m.

    Rain falls as fresh clean water onto land. This water then drains through the ground and into lakes and rivers. As the water drains it erodes the dirt and minerals. This also happens when it travels through streams. As it travels through the rivers the dirt picks up some of the minerals from the broken down soil such as salt. In most streams there isn't enough salt to be noticeable, but as the streams empty out into the oceans the salt percentage is much higher. Water then evaporates off the ocean to become clouds and rain. The salt is left behind in the ocean and the cycle starts all over again.

  • JACKELINEC-ZDE
    6/04/2014 - 06:33 p.m.

    This article can easily grab a person's attention because people know that lakes are freshwater and oceans are saltwater but they may not know why. When a person is taught about the water cycle they usually learn about precipitation, evaporation, and condensation, but they don't learn about what the water does once it precipitates to lead to the ocean or a lake. In a way the water cycle is like a bicycle because just like its wheels it continues to go in a circle where the same things occur.

  • RMansaray123
    6/05/2014 - 08:42 a.m.

    You see, I kind of knew that the rocks and minerals had something to do with salting the ocean, but what I didn't know was how lakes were formed without attracting salt from other rocks and minerals. I understand the whole concept of the water cycle and I know that rain is fresh water, but if it falls to the ground wouldn't the rocks and minerals add salt to it too? Is it because of this why lakes don't form in rocky area's. If so then that make a really good point.

  • 9nickb
    6/05/2014 - 03:23 p.m.

    the oceans on the earth are salt water because of the river flow and natural minerals dissolving and flowing in to the oceans. the fact that lakes and rivers are fresh water factors in the water cycle. where the fresh water in lakes and rivers flows picking up minerals from soil and sand and bring them down stream to the ocean and then the cycle continues and the water evaporates leaving behind what minerals it did carry such as salt.

  • 8aaronr
    6/06/2014 - 07:29 a.m.

    This is really interesting. I never knew how the oceans got salty. Four billion tons of salt dumped into the oceans by rivers is crazy. I wonder what four billion ton block of salt would look like? I bet that the edges of the oceans are more Dense with salt because the water is pushing the sait that way.

  • DelilahG-3
    6/09/2014 - 09:53 p.m.

    Oceans are saltwater because of the lake's freshwater. Rain falls on the land as freshwater and this water then flows down and while it is doing that, it erodes rocks and soil. This causes rock and minerals to dissolve in water which makes salt water. That salt water continues down stream and eventually flows all the way into the ocean. In the world total, the rivers dump an estimated amount of 4 billion tons of salt into the ocean each year. The ocean's salinity though keep their salinity to about 3.5% because the ocean is so large. I think this is very interesting because I never knew why the ocean was salt water and that always confused me so it was nice to clarify why. I also think that it is a very complicating process and it is hard to understand how they figured the whole process out.

  • PhoebeG-2
    6/09/2014 - 09:58 p.m.

    You might be wondering why oceans have saltwater and lakes have freshwater? Well, when it rains the water is fresh. So as the water rolls downward into the ocean it has picked up salt and minerals from the dirt and earth. The water left on the surface later evaporates. When it evaporates it takes out all the salt and minerals and falls into the lakes. Since the lakes have enough turnover they can keep their water fresh unlike the salt that stays in the oceans for millions of years. I find this very interesting because it explains how both are water, but one you van drink and the other could kill you. I think that this cycle is very important because if this didn't happen than we wouldn't be where we are today.

  • TrevorC42
    6/11/2014 - 12:18 a.m.

    That is so crazy I never knew how that worked. I wonder if you can remove water from salt out of the ocean. I love the ocean in Oregon it is so beautiful! I wish though it was a surfing type of coast like California. I would like too travel the world one day. :)

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