# What are those 'streaks' in lakes when it rains?

**URL:** <https://boards.straightdope.com/t/what-are-those-streaks-in-lakes-when-it-rains/252067>\
**Category:** Factual Questions\
**Created:** [June 24, 2004, 7:32pm UTC](https://boards.straightdope.com/t/what-are-those-streaks-in-lakes-when-it-rains/252067 "2004-06-24T19:32:35Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![Rikster](https://avatars.discourse-cdn.com/v4/letter/r/e9a140/32.png) [@Rikster](https://boards.straightdope.com/u/Rikster)\
**Post date:** [June 27, 2004, 5:46am UTC](https://boards.straightdope.com/t/what-are-those-streaks-in-lakes-when-it-rains/252067/21 "2004-06-27T05:46:47Z")

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Not quite a highjack because boat trails were also referenced above as being similar phenomena. This is a really cool link with pictures from the space shuttle; boat trails kilometers long and have to be hours old.  
But back to the rain marks as I stated before I near the Delaware River and have seen these pools move up or down with the tidal flow almost like they are debris so I wouldn’t think springs have anything to do with them. What about PH could that cause an effect? Rain verses river water. I always thought it was temperature variances

[http://www.madsci.org/posts/archives/jul2001/994688283.Ph.r.html](http://www.madsci.org/posts/archives/jul2001/994688283.Ph.r.html)

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**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\
**Post date:** [June 27, 2004, 9:46am UTC](https://boards.straightdope.com/t/what-are-those-streaks-in-lakes-when-it-rains/252067/22 "2004-06-27T09:46:41Z")

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> [@Bruce\_Daddy](#):
>
> Yeah, that’s it! The last picture is really representative of what I’m talking about, especially on the left in the bay. And you’re right, the streaks are usually smoother than the surrounding water.
> 
> So what about boats and rain cause this?

Those streaks are oil slicks.

It only takes a few molecules thick of oil-film to have a drastic effect on the surface friction… so the wind-spawned ripples are different where oil exists.

The old “Oil on troubled waters” really does work, it really does make wind-ripples smaller.

Speedboats leave these trails when they’re exhaust outflow is into the water. The oil in the exhaust makes an oil film. I assume the trails slowly become wiggly as the flowing water carries the oil film along.

I’ve always wanted to try pouring a cup of vegetable oil into the middle of a pond when the wind is blowing, then paddling quickly back to shore to watch what happens.

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**Author:** ![Bruce\_Daddy](https://avatars.discourse-cdn.com/v4/letter/b/85f322/32.png) [@Bruce\_Daddy](https://boards.straightdope.com/u/Bruce_Daddy)\
**Post date:** [June 27, 2004, 5:49pm UTC](https://boards.straightdope.com/t/what-are-those-streaks-in-lakes-when-it-rains/252067/23 "2004-06-27T17:49:35Z")

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> [@Casey1505](#):
>
> I’m liking **mangetout** ’s theroy of water temperature. Not so silly after all, **Bruce**. Is this lake spring fed? Could it have anything to do with the temperature and/or purity of the spring water in relation to the rest of the lake?

Some springs, mostly river fed though.

> [@bbeaty](#):
>
> Those streaks are oil slicks.
> 
> It only takes a few molecules thick of oil-film to have a drastic effect on the surface friction… so the wind-spawned ripples are different where oil exists.
> 
> The old “Oil on troubled waters” really does work, it really does make wind-ripples smaller.
> 
> Speedboats leave these trails when they’re exhaust outflow is into the water. The oil in the exhaust makes an oil film. I assume the trails slowly become wiggly as the flowing water carries the oil film along.
> 
> I’ve always wanted to try pouring a cup of vegetable oil into the middle of a pond when the wind is blowing, then paddling quickly back to shore to watch what happens.

I’ll buy that but what about the rain makes oil trails? Perhaps there is always a small coating of oil on the water and during the rain there is some water on top of the original oil surface that eventually sorts itself out to oil on top?

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