Storm drain gutters on your street move rain water untreated into local waterways including the San Lorenzo River and Monterey Bay. The City of Santa Cruz sweeps streets and vacuums out storm drains to make sure they’re functioning before winter storms arrive. During dry weather, these same storm drains need to be kept dry.

Water can enter the storm drain system during dry weather months because of human activity. When this happens the water coming from humans is called incidental flows. Incidental flow is a water quality nerd term for human-caused water, clean or polluted, that runs off your property and into the street. Incidental flows can come from car-washing or cleaning off the grill in your driveway, washing the dog, or overwatering a garden.

When water enters the storm drain system during dry weather months, there’s not enough flow to flush it through the system, so it stagnates, and bacteria known as biofilm can grow in the underground pipe. Then, when first major rain storms of the season occur the actual stormwater or rainwater flushes the pipes and the high loads of bacteria wash into the river.

You can do your part to ensure only clean stormwater reaches your street gutters. Pick up trash, dispose of household chemicals properly and stop water from going down the storm drain by washing your car at a car wash or tending to a leaky sprinkler. If there has not been rain, then gutters should be dry. If you notice there is water flowing down your gutter, take a look at where it may be coming from. If it is from a human source, share what you know about incidental flows. You can use this Incidental Flows song, composed by CWC Technical Advisor Armand Ruby, to communicate with friends about biofilm and more! What the debut performance at the State of the San Lorenzo River Series in March 2017 and sing along using the lyrics below!

Incidental Flows 


This is a bit about bugs

[All] Yeah!

Little tiny ones

Microscopic ones!

What do we call em?


[All] Yeah!

[All] Bacteria!


You know they live in your guts

[All] Yeah!

We like ‘em there

We need ‘em there

But there’s a limit


[All] Yeah!

[All] In the river!


[music shifts; quicker rap]

So listen up

[All] Yo

Because there’s somethin’ you should know

[All] Yo

And it’s somethin’ ‘bout flow!

[All] Oh!


[music shifts, bouncier rap]

If you leave your sprinkler running

And it’s running down the street

If you’re cleaning off your car

And it’s running down the street

If you’re washing off your dog

And it’s running down the street

That’s what we call nuisance flow!

[All] Feel the beat!

[2 bar dance break]


The problem with the nuisance flow

A-Runnin’ down the street

Is when it hits the storm drain

And travels underneath

A biofilm can grow there

And fester in the heat

Then bacteria can multiply

[All] Not so sweet!

[2 bar dance break]


[Chorus; sung several times]

Now that you know

Where that water goes

Please do your part

To stop incidental flows



Learn About More Actions You Can Take for a Healthier River

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