So I have a fender and a rain jacket, but is there anything that I can do to keep my backpack and its contents dry? I know that a lot of backpacking backpacks come with a plastic bag attached to them that you can pull over the bag when it rains. Does anything like that exist for regular backpacks? Are some backpacks better at keeping the rain out than others?

Search for Rain Cover at https://www.mec.ca/. They had a number of different covers for backpacks. I have the pannier covers myself and really like them.

They're Canadian, so if you're in the US you'll have to pay a bit more for shipping. Or, like I did, take a nice trip up to Vancouver!

For the rare times that I am wearing my backpack when it is actually pouring the rain I just use a trash bag with some holes cut in it. For the light drizzle I don't worry.
But I use a backpack about 1 in 10 rides and it doesn't rain that often here during the hours I am on the big. I think Momma Nature likes me.

yes, there are better backpacks for rain resistance.

my current fave is a very lightweight Outdoor Research model. rolltop closure like a drybag, sealed seams, completely waterproof fabric. you could also sport an Ortlieb messenger bag (all ortlieb messenger bags are backpack style) Arxteryx also makes some rolltop daypacks.

you can pick up a daypack sized rain cover for your existing backpack at any good outdoor supply house like REI, Campmor, etc. Jansport makes them, Dana Design makes them, etc.

REI should sell a variety of backpack rain covers.

I recently abandoned my garment bag that hooks onto my rack because even with the rain cover on it my stuff got wet. I think it's partially because the rain cover had holes in it for some reason that I could never figure out (manufactured that way - holes on the sides yet no drain holes on the bottom - wtf?) and partially because the bag was so low to the ground that whenever I hit a puddle, it would get soaked.
Now I use a Chrome Metropolis messenger bag and NOTHING gets wet - it's rained almost every day since I started using it 2 weeks ago and I ride every day. That bag is amazing!! I'm thoroughly impressed and would highly recommend it.

Hello All

I use one of the plastic bags,you get a the neighorhood
w-mart. or the grocery store. They fit over my Trans-it
back rack bag, and also will cover my backpack.
I always carry about 4 in the rack, bag.In a pinch
they make handy foot covers in the rain, or inside
gloves, if you happen to get caught, out and the
weather turns cold.

Maybe you could put the backpack on before the raincoat? I have done this in the past with a poncho and it worked great.

Yes that does work well and I've done it myself both with backpacks and shoulder bags; - the only downside is that you can endup looking like you've got some really terrible skeletal malformation.

Get some sea 2 summit dry bags from a camping store and wrap them around the contents of your pannier/backpack. Seriously, I've ridden in some tropical downpours, and even the occasional flood and they've never failed me.

I've tried rain covers before and they just didn't work. All the water collected in the bottom of the cover after running down the inside of the pannier and soaked whatever I'd packed at the bottom.

The one in Portland has like 5 sizes. I have one for my daypack and love it.

I keep a trash bag or 2 stuffed in the bottom of my backpack and panniers. When it might rain, I put all my stuff in it. For my more valuable stuff (phone, wallet, etc.), I keep a big zip lock bag also. The backpack and panniers get wet, but the stuff inside stay dry.

-- John

Pack covers are OK for hiking, but when you're zipping along on a bike they turn into a drag chute (which fills up with water at the bottom) no matter how well they're fitted to the pack.

Don't worry about the bag getting wet, worry about the contents. Get a dry bag and use it for a pack liner. Venture Sports and Sea-to-Summit are the two best I've seen. Venture Sports dry bags are vinyl, so they're heavier and more puncture/abuse resistant. S-t-S bags are SilNylon and very lightweight, but are more prone to getting torn if you overstuff them or carry sharp things.