Because as far as we know, no cow has yet been toilet trained.
In other words, if cows are allowed to roam in shallow streams or even along the banks, they can…uh…make a mess right in the water.
Manure from cattle and other animals contain microorganisms--microscopic creatures that can pollute water. This pollution can make people (and even the cows) sick if they come into contact with these streams.
Another big problem is that cows can damage the streambank, knocking soil into the water.
Cattle wander through a stream, causing pollution. The farmer of this land later changed things, fencing cattle away from the stream.
Photo: USDA NRCS
By providing a good water supply, cattle can be kept away from streams.
Photo: USDA NRCS