How do sewers contribute to pollution in water?

If raw or improperly treated sewage is discharged to a stream on purpose or by misadventure (pipe breaks, leaks etc.) it can pollute the water by: * Increasing the burden of fecal organisms * Adding chemical contaminants * Depleting the oxygen levels in the water * Making the water aesthetically unappealing (foams, solids and odours) * Covering the stream bed with solids