Rainproof pack covers are great, but sometimes you need something totally waterproof. Canoe camping comes to mind. The North Face, ever so considerate, has thought of you in that regard. Say hello to The North Face Waterproof Daypack (34.5 Litres).
Waterproof and super resilient doesn't mean accidents don't happen with pointy objects in the wild. The fabric is constructed à la ripstop, meaning that if disaster strikes, you can repair the damage more easily. Because those who fail to plan, plan to fail.