You probably don't have to worry about the chasm unless you decide to start throwing trash in it. And even then you don't have to worry about it if you pour magma in it. Not like there's any hurry, though. Eventually you'll give in to your dwarvish tendencies and dig deeper for the beautiful deep metals and gems.
You've got your support poles placed awfully close together in your forest-to-be. You can have six spaces between each pole without problems.
Your forest is pretty small. At that size, it'll probably yield 4-6 trees per year, starting 4 years after the first tower caps sprout.
By the by, a neat way to water your tree farms without exposing your dwarves to flood dangers is to use fortifications. Just dig as close to the river as you can without breaking through, smooth the last blocks between the river and your farm, and (d)esignate the smoothed walls to be fortifications. Natural floods pour right through fortifications, muddying your farm, but the floods can't wash dwarves or valuable metals into the river. And you don't have to worry about river monsters surfacing in your tree farm and ambushing your woodchoppers later. And you don't have to worry about setting running the floodgates every so often.
If you can spare a log for a wooden cage, you can stuff all those excess animals in there and clear out your halls a bit. Even non-war trained animals do distract and attack intruders a bit, so it's up to you. There's a rumor on these boards and on the wiki that caging animals halts animal births, but I've never managed to reproduce that particular bug. Get it? Reproduce? Ahem.
I don't see a well, but I do see stockpiles full of barrels and more barrels pouring out of a still. If this means what I think it means, congratulations on attaining a completely alcohol fueled fortress! Drinking water is for chumps!