Having grown up in the area, I'll tell you what I remember.........though I don't claim this to be absolute fact; just my recollection.

The lake was originally created for freshwater recreation. It used to be substantially deeper and was even dredged periodically. When the lake was deeper, the water stayed cooler and weeds were much less of an issue. I can remember cutting my teeth on trolling for kings on the south lake (nearest I-5) back in high school and experiencing 10-15 ft depths.......now your lucky to see 5 feet.

New factors came into play through the years:

1) Dredging was stopped because of cost.

2) Siltation from winter floods quickly shallowed the
lake thus allowing the establishment of weeds.

3) Warm water pollution from the then Olympia Brewery
not only contributed to warmer water but increased
the nutrient load which weeds love.

With the exception of the brewery shutting down, not much has changed. personnally, I'd like to see the whole lake returned to tidal flats as it used to be. Saltwater would take care of the problem.