Lake of/with green water.

[99bottles]

I saw a lake of/with green water.

Which preposition is better?

 

[Tarheel]

Perhaps:

I saw the lake up close. The water was green with algae.

 

[emsr2d2]

I saw a green lake.

You need to mention something like Tarheel's algae only if you know that's why the water is green.

 

[Tarheel]

That is true, but that's the only reason I can think of that the lake might be green.

 

[99bottles]

That is true, but that's the only reason I can think of that the lake might be green.

Can it be green because it's filthy?

 

[Skrej]

Possibly, but green does not automatically mean dirty or even opaque. Various natural non-toxic chemical reactions can turn water green.

See this link for a list of naturally occurring green waters, none of which I'd consider dirty. Some are quite clear and stunningly beautiful.