Vertical Thermocline?

[Starscream]

In the lakes where we swim there is a very pronounced thermocline, where you can swim in warm water near the surface but heaven help you if you try to tread water with your legs in the deeper, cold water. I understand the physics when talking about a "barrier" between different depths.

I took this photo the other day of the line between the waters of the St. Lawrence and Ottawa Rivers, where they combine to the southeast of Ile Perrot. The line is is probably 10 miles long: the waters just don't mix. You can see the dirty brown, natural water of the Ottawa river and the clear (zebra-mussel filtered) blue-green water of the St. Lawrence. The transition from one to the other happens in the space of about two feet, usually with a thick line of floating seaweed trapped along the barrier line.

Does anyone know why this happens? Is it just the temperature/density difference? There is usually about a 1F differential between the two waters.

[K9Kampers]

I've observed something like this while kayaking tidal rivers. Noticed a peculiar 'line' across the water, featuring different turbidities and smooth vs rippled surface water. Figured it to be where the salt water meets the fresh water.

[Jimmyt]

Looks like laminar flow.

[Todd]

I saw a similar line when I took a sailing class at Saint Andrew Bay Florida-- we popped out into the Gulf of Mexico and there was a distinct line between the fresh and sea water.