I'm licensed part 107. I want to shoot some photos at Lake Hugo State Park, Oklahoma. Looking at the VFR, I'm noticing the wooded area surrounding the lake have horizontal dashed blue lines. What do they represent? I cannot find any reference to them in the legend. Any ideas?

I think these lines are the symbol for swamps...

[Edit]
Wiki says it is wetlands/marsh

It is a hydrography feature and it's a symbol for "Land Subject To Inundation" -  which means it may or may not be flooded or overflowing from a nearby source of water such as a lake or stream/river.

Found it in the "Aeronautical Chart User’s Guide" page 39
https://www.faa.gov/air_traffic/ ... ns/cug-complete.pdf

David Eberhardt, ATP, MEI, UAS

Eberhardt is correct and gave you a very useful manual for future reference.  
You should plan for that area to be swampy...however, if there has been an extended drought then some of the areas are likely dry and stable making it passable.  Otherwise, you should plan for at least some muck & mud making transit impossible in places, if it is flooded then it will require a john boat or flat bottom.  Either way you would prefer to not loose your drone in there.

Oops! I was going to give you a link to the chart users guide but I see someone beat me to it.  Sectional Charts contain a wealth of information once you become familiar with them.