There's basically four options to keep your pack contents dry during a rainstorm.
>1. use a pack coverThis is a fine solution, although you will notice that the cover doesn't "cover" the whole pack. There is a possibility for rain to still get in from a certain angle, and pack covers also don't protect your pack contents should your pack get submerged in a stream crossing.
>2. use a pack linerThis is potentially the most inexpensive solution, as you can use a regular heavy duty garbage can liner. A more durable pack liner can be found made by companies like Sea 2 Summit and made out of silnylon. I consider this the most perfect solution to keeping your pack contents dry, since you will be able to dunk the whole thing in a river and keep the contents dry, but the outside of your pack will get wet so I hope you weren't hoping to bring your backpack inside your tent at night.
>3. use a ponchoAs
>>73943 not-so-eloquently put it, you can use a poncho to drape over your body, head, and backpack, and it will do double-duty as a pack cover - better than a dedicated pack cover because it's going to form a better seal against the elements, and be lighter overall (no need to pack dedicated rain gear). Additionally, you can buy something called a "poncho tarp" by companies like Six Moon Designs, Golite, MLD, and a variety of others that will do *triple duty* as rain gear, a pack cover, and a shelter (see pic related).
>4. Buy a truly waterproof backpackThere are a few companies that sell either a) a harness system for a dry bag, or b) a seam-sealed waterproof backpack (see: Arc'Teryx). These products tend to be heavy, and they tend to be very expensive for the good ones, but they're out there.
