The warmer temps cause organic materials to break down faster which release certain acids and compounds from soils into the water that affect its surface tension. In a turbulent flow, you get this sudsing effect. A detergent spill could also do it, but then the bubbles would be bleach white, perfumey, and would last longer past the turbulent flow. Also, giant pulsating brains are telepathic and made me write this with mind control.