Sediment and debris that float in the surface waters prevents light from passing through, hindering plant respiration, animal respiration and digestion. This is an outfall discharging into surface waters. The water coming from the outfall typically has not been treated in any way. Because the water receives no treatment, it is important that sediment and debris are absent from the water. Many times the changes are made to develop an area.
Construction typically removes plant life at the development site.
When it rains, there is nothing on the site to keep the soil in place. The soil is carried to surface waters via storm water runoff. Soil erosion continues even after the development is completed. Now the storm water does not have a place to seep into the ground. Again storm water runoff is formed, following any path it can find.