> Generally, liquids get denser as they get cooler. In the summer time
> the sun warms up the surface and the lake stays cooler at the bottom.
> But in the fall the surface gets colder and denser until the top
> layer is heavy enough to fall through the middle layers and sink to
> the bottom. This creates mild convection currents that disturb the
> lake bottom stirring up mud and rotting organic material.

That makes sense now (some).

> In the spring snow melt and rain causes streams to flood, increasing
> the current's velocity so that they carry a larger than average amount
> of suspended solids which get dumped into the lake making it dirty
> until the junk settles out.

My notion was high volume means low density, so I'd
imagined it should stay mainly on top. Of course as
the water supply intakes are probably fairly close
to the surface, the effect will be noticeable.
 
> It affects fishing too so outdoorsmen are keenly aware of spring
> runoff and the fall turnover.

Different species appearing at various depths
depending on the time of year?
 
Coriander Chutney
categories: Indianish, sauce, snack
yield: 2 c

1 c chopped coriander leaves
2 md garlic cloves
2 md ripe tomatoes
green chillies to taste (as much as you can take)
salt to taste

Blend everything except the salt to a fine paste. Add salt just
before serving. Serve with rice and curry or with dholl pooris.

http://www.users.bigpond.com/Clancy/recipe.htm