How damaging is oil when it hits the ground or enters a river? how many barrels before it's a complete catastrophe?

Oil will kill much of the biology that it comes in contact with. Often this is because it will block respiration. Oil also contains substances that are toxic.

Pollution levels depend upon concentrations. A barrel of oil in an ocean of water will do far less harm than that same barrel in a tiny pond. So the damage done depends upon the size of what the pollution is mixed with. At one time it was thought that all of the oceans or all of the air were too big a "sink" for pollution levels to become toxic. We know now that this was naive.