It’s just us and our detritus.

People walking and biking along the Cherry Creek on this rainy Tuesday noticed that the water looked sudsy or soapy. It’s not unusual to see foam on the water when it’s been raining, but what, exactly, is it? We sent this picture over to Denver’s Department of Environmental Health to find out.

Rain washes contaminants from the streets, spokeswoman Kerra Jones said. This is mostly oil and gas from vehicles, but urban runoff also includes fertilizers, herbicides, pet waste and trash. If it’s been a long time since it rained, there will be more contaminants built up on the streets and more pollution will enter the water at once when it finally does rain.

Those substances decrease the surface tension in the water, allowing bubbles to form and creating the soapy look.

Decomposing plant matter and soil erosion can also contribute to foam in the water.

Jones said it’s unlikely the suds are from an illegal discharge. When surfactants are more concentrated, the bubbles mound up in foam and sometimes there is a detergent-like smell.