Cattle are a problem when they graze too close to a stream. They can cause disease in the water and trample vegetation. Fencing cattle away from rivers keeps rivers clean and vegetation growing. Poor fishing practices can be a problem, also. Gill netting can kill more than just fish.
Gill netting is now against the law in most areas.
Along with gill netting, poaching is also illegal. Poaching is when people fish out of season. Dumping waste or garbage improperly can kill fish, too. If garbage is buried too close to rivers, rain can make it come out of the ground and blow it into rivers. However, now man has learned many ways to reduce, reuse, and recycle garbage.