Streams and valleys provide the cheapest media for unwanted run-off and waste, which degrade the water quality
Originally, the water from rain/snow was absorbed to the ground or entered to the smaller streams which banks were covered by trees. This way the temperature of the water was kept cool. Today, this water flows over the land surface with lesser or no forest cover; this way it is being warmed up by the sun, it also picks up the garbage, pollutants, and sediments on its way to our streams lowering their water quality. Cool and clean water is important for salmon/trout survival. Warm and polluted water helps to spread bacteria; some of the pollutants get into our drinking water and they cannot be removed.