I think that, if managed properly, cattle are an exceptable part of an ecosystem...
You can't paint all open range cattle operations with the same brush.

http://www.cowsandfish.org/

I believe that allowing low density-grazing when the ground is dry of frozen is fine for a stream system.
Of course riparian pastures need to be fenced off so you can manage them, and off-stream watering systems set up.

I think people need to slow down when driving in open range country... just as liable to hit a deer or small fuzzy animal than a cow.