Does anyone have a general rule of thumb for estimating how deep the grass will be in natural northern lakes? Would be good to know when looking over a map prior to fishing. 

 

I know it can vary, but also know that light penetration plays the biggest role, would be nice if there was a formula like 1.5 times water clarity is deepest weeds will grow.

In the Northern lakes I fish, Michigan, Wisconsin it depends on a few things.  First and formost, the type of "weed" or grass that is growing.  Cabbage, milfoil, sand grass, eel grass, and a few more species all exist at different depths from 15 feet to 2 feet.  The only way to know for sure is to get out and graph it or throw a bottom dragging bait and see what you pull up.  

There are some nice studies out there on this subject showing good correlation between light penetration and maximum growing depth of vegetation. While there will always be one-off examples and outliers, the studies generally show that submerged vegetation requires a minimum of 10% incident light for growth. Using Secchi Depth transparency, about 90% of the time the max depth of growth will be 3X water clarity, with around 60% of that falling within 2X water clarity.

... and water clarity can change significantly throughout the year, so using clarity as a gauge can be "murky" at best.

 

oe