Erosion- why it should be avoided.

When walking along Mill Creek, we noticed that there were these large walls constructed along the side of the recreational trail. They were lots of rocks, covered in what appeared to be chicken wire. It is obvious that these were put there to keep the trail from eroding into Mill Creek.
This led us to question the following:

Erosion seems like a pretty normal, natural process. Why are environmentalists trying to avoid erosion into waterways?

So, we did a little research. When the soil is very arid, it tends to be picked up by wind. In the case of Mill Creek, the light dirt from the path would have easily just slipped down into the water because it was higher up. When extra soil/sediment is in the water, there are more loose particles. The turbidity has increased.
This ends up causing a problem because, as the water has a higher level of turbidity, less sunlight can get down to the bottom. A lack of sunlight causes a problem for the plants in the river that need it for photosynthesis.
From a recreational standpoint, turbidity is also a problem because it is less appealing. Not as many people are likely to jump into a nice, dirty river to take a swim. People tend to be more drawn towards cleaner sources of water, and erosion makes the water dirtier.

-Marra

Here is a photo of the wall of rocks which was constructed alongside Mill Creek. This photo was taken between sites 2 and 3, near Walla Walla Community College. 

Sources:

Impacts of erosion on soil, air, and water quality. In (2013).Plant and Soil Sciences eLibrary. Davis: USDA National Institute of Food and Agriculture. Retrieved from http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1086025423&topicorder=4&maxto=7