Stains and discoloration on the sides of the channel walls indicate a discharge of a toxic chemical or caustic substance of some nature.
Another example of used oil disposal from a neighborhood do-it-yourself mechanic.

Less than 5 miles downstream from where this photo was taken, lies the entrance to the Santa Monica Bay.

These two photos clearly illustrate the consequences of urban runoff. A Santa Monica Bay beach just after a rainstorm.

 

Every year, approximately 40 tons of trash is picked up off of Los Angeles County beaches at a cost of $1.3 million.