In Ace of Spades, I've hopped in a river with hopes of crossing it, but I can't jump out because the banks are too high! How do I get out of the water without dying?
You'll need to do what's referred to as a 'Crouch Jump'. The technique if fairly common in Valve games but has seen increasing use in most modern first-person shooters.
To perform a crouch jump you'll need to jump and then hold the crouch button while in midair. This should allow you to jump slightly higher and get out of the river areas.