Made of a rugged truck-tarp material and loaded with features, the Banjo Brothers Whitey Von-Backpack is avialable in 1500 and 2000 cubic inch varieties.  A two-layer design keeps a tough outer shell to soak up the abuse and a 100% waterproof liner to keep things bone dry. You can take a look at  Banjo Brothers entire range of cycling specific backpacks here.